// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
/* Copyright (c) 2019 Facebook */

#ifdef __KERNEL__
#include <linux/bpf.h>
#include <linux/btf.h>
#include <linux/string.h>
#include <linux/bpf_verifier.h>
#include "relo_core.h"

static const char *btf_kind_str(const struct btf_type *t)
{
        return btf_type_str(t);
}

static bool is_ldimm64_insn(struct bpf_insn *insn)
{
        return insn->code == (BPF_LD | BPF_IMM | BPF_DW);
}

static const struct btf_type *
skip_mods_and_typedefs(const struct btf *btf, u32 id, u32 *res_id)
{
        return btf_type_skip_modifiers(btf, id, res_id);
}

static const char *btf__name_by_offset(const struct btf *btf, u32 offset)
{
        return btf_name_by_offset(btf, offset);
}

static s64 btf__resolve_size(const struct btf *btf, u32 type_id)
{
        const struct btf_type *t;
        int size;

        t = btf_type_by_id(btf, type_id);
        t = btf_resolve_size(btf, t, &size);
        if (IS_ERR(t))
                return PTR_ERR(t);
        return size;
}

enum libbpf_print_level {
        LIBBPF_WARN,
        LIBBPF_INFO,
        LIBBPF_DEBUG,
};

#undef pr_warn
#undef pr_info
#undef pr_debug
#define pr_warn(fmt, log, ...)  bpf_log((void *)log, fmt, "", ##__VA_ARGS__)
#define pr_info(fmt, log, ...)  bpf_log((void *)log, fmt, "", ##__VA_ARGS__)
#define pr_debug(fmt, log, ...) bpf_log((void *)log, fmt, "", ##__VA_ARGS__)
#define libbpf_print(level, fmt, ...)   bpf_log((void *)prog_name, fmt, ##__VA_ARGS__)
#else
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <ctype.h>
#include <linux/err.h>

#include "libbpf.h"
#include "bpf.h"
#include "btf.h"
#include "libbpf_internal.h"
#endif

static bool is_flex_arr(const struct btf *btf,
                        const struct bpf_core_accessor *acc,
                        const struct btf_array *arr)
{
        const struct btf_type *t;

        /* not a flexible array, if not inside a struct or has non-zero size */
        if (!acc->name || arr->nelems > 0)
                return false;

        /* has to be the last member of enclosing struct */
        t = btf_type_by_id(btf, acc->type_id);
        return acc->idx == btf_vlen(t) - 1;
}

static const char *core_relo_kind_str(enum bpf_core_relo_kind kind)
{
        switch (kind) {
        case BPF_CORE_FIELD_BYTE_OFFSET: return "byte_off";
        case BPF_CORE_FIELD_BYTE_SIZE: return "byte_sz";
        case BPF_CORE_FIELD_EXISTS: return "field_exists";
        case BPF_CORE_FIELD_SIGNED: return "signed";
        case BPF_CORE_FIELD_LSHIFT_U64: return "lshift_u64";
        case BPF_CORE_FIELD_RSHIFT_U64: return "rshift_u64";
        case BPF_CORE_TYPE_ID_LOCAL: return "local_type_id";
        case BPF_CORE_TYPE_ID_TARGET: return "target_type_id";
        case BPF_CORE_TYPE_EXISTS: return "type_exists";
        case BPF_CORE_TYPE_MATCHES: return "type_matches";
        case BPF_CORE_TYPE_SIZE: return "type_size";
        case BPF_CORE_ENUMVAL_EXISTS: return "enumval_exists";
        case BPF_CORE_ENUMVAL_VALUE: return "enumval_value";
        default: return "unknown";
        }
}

static bool core_relo_is_field_based(enum bpf_core_relo_kind kind)
{
        switch (kind) {
        case BPF_CORE_FIELD_BYTE_OFFSET:
        case BPF_CORE_FIELD_BYTE_SIZE:
        case BPF_CORE_FIELD_EXISTS:
        case BPF_CORE_FIELD_SIGNED:
        case BPF_CORE_FIELD_LSHIFT_U64:
        case BPF_CORE_FIELD_RSHIFT_U64:
                return true;
        default:
                return false;
        }
}

static bool core_relo_is_type_based(enum bpf_core_relo_kind kind)
{
        switch (kind) {
        case BPF_CORE_TYPE_ID_LOCAL:
        case BPF_CORE_TYPE_ID_TARGET:
        case BPF_CORE_TYPE_EXISTS:
        case BPF_CORE_TYPE_MATCHES:
        case BPF_CORE_TYPE_SIZE:
                return true;
        default:
                return false;
        }
}

static bool core_relo_is_enumval_based(enum bpf_core_relo_kind kind)
{
        switch (kind) {
        case BPF_CORE_ENUMVAL_EXISTS:
        case BPF_CORE_ENUMVAL_VALUE:
                return true;
        default:
                return false;
        }
}

int __bpf_core_types_are_compat(const struct btf *local_btf, __u32 local_id,
                                const struct btf *targ_btf, __u32 targ_id, int level)
{
        const struct btf_type *local_type, *targ_type;
        int depth = 32; /* max recursion depth */

        /* caller made sure that names match (ignoring flavor suffix) */
        local_type = btf_type_by_id(local_btf, local_id);
        targ_type = btf_type_by_id(targ_btf, targ_id);
        if (!btf_kind_core_compat(local_type, targ_type))
                return 0;

recur:
        depth--;
        if (depth < 0)
                return -EINVAL;

        local_type = skip_mods_and_typedefs(local_btf, local_id, &local_id);
        targ_type = skip_mods_and_typedefs(targ_btf, targ_id, &targ_id);
        if (!local_type || !targ_type)
                return -EINVAL;

        if (!btf_kind_core_compat(local_type, targ_type))
                return 0;

        switch (btf_kind(local_type)) {
        case BTF_KIND_UNKN:
        case BTF_KIND_STRUCT:
        case BTF_KIND_UNION:
        case BTF_KIND_ENUM:
        case BTF_KIND_FWD:
        case BTF_KIND_ENUM64:
                return 1;
        case BTF_KIND_INT:
                /* just reject deprecated bitfield-like integers; all other
                 * integers are by default compatible between each other
                 */
                return btf_int_offset(local_type) == 0 && btf_int_offset(targ_type) == 0;
        case BTF_KIND_PTR:
                local_id = local_type->type;
                targ_id = targ_type->type;
                goto recur;
        case BTF_KIND_ARRAY:
                local_id = btf_array(local_type)->type;
                targ_id = btf_array(targ_type)->type;
                goto recur;
        case BTF_KIND_FUNC_PROTO: {
                struct btf_param *local_p = btf_params(local_type);
                struct btf_param *targ_p = btf_params(targ_type);
                __u32 local_vlen = btf_vlen(local_type);
                __u32 targ_vlen = btf_vlen(targ_type);
                int i, err;

                if (local_vlen != targ_vlen)
                        return 0;

                for (i = 0; i < local_vlen; i++, local_p++, targ_p++) {
                        if (level <= 0)
                                return -EINVAL;

                        skip_mods_and_typedefs(local_btf, local_p->type, &local_id);
                        skip_mods_and_typedefs(targ_btf, targ_p->type, &targ_id);
                        err = __bpf_core_types_are_compat(local_btf, local_id, targ_btf, targ_id,
                                                          level - 1);
                        if (err <= 0)
                                return err;
                }

                /* tail recurse for return type check */
                skip_mods_and_typedefs(local_btf, local_type->type, &local_id);
                skip_mods_and_typedefs(targ_btf, targ_type->type, &targ_id);
                goto recur;
        }
        default:
                pr_warn("unexpected kind %s relocated, local [%d], target [%d]\n",
                        btf_kind_str(local_type), local_id, targ_id);
                return 0;
        }
}

/*
 * Turn bpf_core_relo into a low- and high-level spec representation,
 * validating correctness along the way, as well as calculating resulting
 * field bit offset, specified by accessor string. Low-level spec captures
 * every single level of nestedness, including traversing anonymous
 * struct/union members. High-level one only captures semantically meaningful
 * "turning points": named fields and array indicies.
 * E.g., for this case:
 *
 *   struct sample {
 *       int __unimportant;
 *       struct {
 *           int __1;
 *           int __2;
 *           int a[7];
 *       };
 *   };
 *
 *   struct sample *s = ...;
 *
 *   int x = &s->a[3]; // access string = '0:1:2:3'
 *
 * Low-level spec has 1:1 mapping with each element of access string (it's
 * just a parsed access string representation): [0, 1, 2, 3].
 *
 * High-level spec will capture only 3 points:
 *   - initial zero-index access by pointer (&s->... is the same as &s[0]...);
 *   - field 'a' access (corresponds to '2' in low-level spec);
 *   - array element #3 access (corresponds to '3' in low-level spec).
 *
 * Type-based relocations (TYPE_EXISTS/TYPE_MATCHES/TYPE_SIZE,
 * TYPE_ID_LOCAL/TYPE_ID_TARGET) don't capture any field information. Their
 * spec and raw_spec are kept empty.
 *
 * Enum value-based relocations (ENUMVAL_EXISTS/ENUMVAL_VALUE) use access
 * string to specify enumerator's value index that need to be relocated.
 */
int bpf_core_parse_spec(const char *prog_name, const struct btf *btf,
                        const struct bpf_core_relo *relo,
                        struct bpf_core_spec *spec)
{
        int access_idx, parsed_len, i;
        struct bpf_core_accessor *acc;
        const struct btf_type *t;
        const char *name, *spec_str;
        __u32 id, name_off;
        __s64 sz;

        spec_str = btf__name_by_offset(btf, relo->access_str_off);
        if (str_is_empty(spec_str) || *spec_str == ':')
                return -EINVAL;

        memset(spec, 0, sizeof(*spec));
        spec->btf = btf;
        spec->root_type_id = relo->type_id;
        spec->relo_kind = relo->kind;

        /* type-based relocations don't have a field access string */
        if (core_relo_is_type_based(relo->kind)) {
                if (strcmp(spec_str, "0"))
                        return -EINVAL;
                return 0;
        }

        /* parse spec_str="0:1:2:3:4" into array raw_spec=[0, 1, 2, 3, 4] */
        while (*spec_str) {
                if (*spec_str == ':')
                        ++spec_str;
                if (sscanf(spec_str, "%d%n", &access_idx, &parsed_len) != 1)
                        return -EINVAL;
                if (access_idx < 0)
                        return -EINVAL;
                if (spec->raw_len == BPF_CORE_SPEC_MAX_LEN)
                        return -E2BIG;
                spec_str += parsed_len;
                spec->raw_spec[spec->raw_len++] = access_idx;
        }

        if (spec->raw_len == 0)
                return -EINVAL;

        t = skip_mods_and_typedefs(btf, relo->type_id, &id);
        if (!t)
                return -EINVAL;

        access_idx = spec->raw_spec[0];
        acc = &spec->spec[0];
        acc->type_id = id;
        acc->idx = access_idx;
        spec->len++;

        if (core_relo_is_enumval_based(relo->kind)) {
                if (!btf_is_any_enum(t) || spec->raw_len > 1 || access_idx >= btf_vlen(t))
                        return -EINVAL;

                /* record enumerator name in a first accessor */
                name_off = btf_is_enum(t) ? btf_enum(t)[access_idx].name_off
                                          : btf_enum64(t)[access_idx].name_off;
                acc->name = btf__name_by_offset(btf, name_off);
                return 0;
        }

        if (!core_relo_is_field_based(relo->kind))
                return -EINVAL;

        sz = btf__resolve_size(btf, id);
        if (sz < 0)
                return sz;
        spec->bit_offset = access_idx * sz * 8;

        for (i = 1; i < spec->raw_len; i++) {
                t = skip_mods_and_typedefs(btf, id, &id);
                if (!t)
                        return -EINVAL;

                access_idx = spec->raw_spec[i];
                acc = &spec->spec[spec->len];

                if (btf_is_composite(t)) {
                        const struct btf_member *m;
                        __u32 bit_offset;

                        if (access_idx >= btf_vlen(t))
                                return -EINVAL;

                        bit_offset = btf_member_bit_offset(t, access_idx);
                        spec->bit_offset += bit_offset;

                        m = btf_members(t) + access_idx;
                        if (m->name_off) {
                                name = btf__name_by_offset(btf, m->name_off);
                                if (str_is_empty(name))
                                        return -EINVAL;

                                acc->type_id = id;
                                acc->idx = access_idx;
                                acc->name = name;
                                spec->len++;
                        }

                        id = m->type;
                } else if (btf_is_array(t)) {
                        const struct btf_array *a = btf_array(t);
                        bool flex;

                        t = skip_mods_and_typedefs(btf, a->type, &id);
                        if (!t)
                                return -EINVAL;

                        flex = is_flex_arr(btf, acc - 1, a);
                        if (!flex && access_idx >= a->nelems)
                                return -EINVAL;

                        spec->spec[spec->len].type_id = id;
                        spec->spec[spec->len].idx = access_idx;
                        spec->len++;

                        sz = btf__resolve_size(btf, id);
                        if (sz < 0)
                                return sz;
                        spec->bit_offset += access_idx * sz * 8;
                } else {
                        pr_warn("prog '%s': relo for [%u] %s (at idx %d) captures type [%d] of unexpected kind %s\n",
                                prog_name, relo->type_id, spec_str, i, id, btf_kind_str(t));
                        return -EINVAL;
                }
        }

        return 0;
}

/* Check two types for compatibility for the purpose of field access
 * relocation. const/volatile/restrict and typedefs are skipped to ensure we
 * are relocating semantically compatible entities:
 *   - any two STRUCTs/UNIONs are compatible and can be mixed;
 *   - any two FWDs are compatible, if their names match (modulo flavor suffix);
 *   - any two PTRs are always compatible;
 *   - for ENUMs, names should be the same (ignoring flavor suffix) or at
 *     least one of enums should be anonymous;
 *   - for ENUMs, check sizes, names are ignored;
 *   - for INT, size and signedness are ignored;
 *   - any two FLOATs are always compatible;
 *   - for ARRAY, dimensionality is ignored, element types are checked for
 *     compatibility recursively;
 *   - everything else shouldn't be ever a target of relocation.
 * These rules are not set in stone and probably will be adjusted as we get
 * more experience with using BPF CO-RE relocations.
 */
static int bpf_core_fields_are_compat(const struct btf *local_btf,
                                      __u32 local_id,
                                      const struct btf *targ_btf,
                                      __u32 targ_id)
{
        const struct btf_type *local_type, *targ_type;

recur:
        local_type = skip_mods_and_typedefs(local_btf, local_id, &local_id);
        targ_type = skip_mods_and_typedefs(targ_btf, targ_id, &targ_id);
        if (!local_type || !targ_type)
                return -EINVAL;

        if (btf_is_composite(local_type) && btf_is_composite(targ_type))
                return 1;
        if (!btf_kind_core_compat(local_type, targ_type))
                return 0;

        switch (btf_kind(local_type)) {
        case BTF_KIND_PTR:
        case BTF_KIND_FLOAT:
                return 1;
        case BTF_KIND_FWD:
        case BTF_KIND_ENUM64:
        case BTF_KIND_ENUM: {
                const char *local_name, *targ_name;
                size_t local_len, targ_len;

                local_name = btf__name_by_offset(local_btf,
                                                 local_type->name_off);
                targ_name = btf__name_by_offset(targ_btf, targ_type->name_off);
                local_len = bpf_core_essential_name_len(local_name);
                targ_len = bpf_core_essential_name_len(targ_name);
                /* one of them is anonymous or both w/ same flavor-less names */
                return local_len == 0 || targ_len == 0 ||
                       (local_len == targ_len &&
                        strncmp(local_name, targ_name, local_len) == 0);
        }
        case BTF_KIND_INT:
                /* just reject deprecated bitfield-like integers; all other
                 * integers are by default compatible between each other
                 */
                return btf_int_offset(local_type) == 0 &&
                       btf_int_offset(targ_type) == 0;
        case BTF_KIND_ARRAY:
                local_id = btf_array(local_type)->type;
                targ_id = btf_array(targ_type)->type;
                goto recur;
        default:
                return 0;
        }
}

/*
 * Given single high-level named field accessor in local type, find
 * corresponding high-level accessor for a target type. Along the way,
 * maintain low-level spec for target as well. Also keep updating target
 * bit offset.
 *
 * Searching is performed through recursive exhaustive enumeration of all
 * fields of a struct/union. If there are any anonymous (embedded)
 * structs/unions, they are recursively searched as well. If field with
 * desired name is found, check compatibility between local and target types,
 * before returning result.
 *
 * 1 is returned, if field is found.
 * 0 is returned if no compatible field is found.
 * <0 is returned on error.
 */
static int bpf_core_match_member(const struct btf *local_btf,
                                 const struct bpf_core_accessor *local_acc,
                                 const struct btf *targ_btf,
                                 __u32 targ_id,
                                 struct bpf_core_spec *spec,
                                 __u32 *next_targ_id)
{
        const struct btf_type *local_type, *targ_type;
        const struct btf_member *local_member, *m;
        const char *local_name, *targ_name;
        __u32 local_id;
        int i, n, found;

        targ_type = skip_mods_and_typedefs(targ_btf, targ_id, &targ_id);
        if (!targ_type)
                return -EINVAL;
        if (!btf_is_composite(targ_type))
                return 0;

        local_id = local_acc->type_id;
        local_type = btf_type_by_id(local_btf, local_id);
        local_member = btf_members(local_type) + local_acc->idx;
        local_name = btf__name_by_offset(local_btf, local_member->name_off);

        n = btf_vlen(targ_type);
        m = btf_members(targ_type);
        for (i = 0; i < n; i++, m++) {
                __u32 bit_offset;

                bit_offset = btf_member_bit_offset(targ_type, i);

                /* too deep struct/union/array nesting */
                if (spec->raw_len == BPF_CORE_SPEC_MAX_LEN)
                        return -E2BIG;

                /* speculate this member will be the good one */
                spec->bit_offset += bit_offset;
                spec->raw_spec[spec->raw_len++] = i;

                targ_name = btf__name_by_offset(targ_btf, m->name_off);
                if (str_is_empty(targ_name)) {
                        /* embedded struct/union, we need to go deeper */
                        found = bpf_core_match_member(local_btf, local_acc,
                                                      targ_btf, m->type,
                                                      spec, next_targ_id);
                        if (found) /* either found or error */
                                return found;
                } else if (strcmp(local_name, targ_name) == 0) {
                        /* matching named field */
                        struct bpf_core_accessor *targ_acc;

                        targ_acc = &spec->spec[spec->len++];
                        targ_acc->type_id = targ_id;
                        targ_acc->idx = i;
                        targ_acc->name = targ_name;

                        *next_targ_id = m->type;
                        found = bpf_core_fields_are_compat(local_btf,
                                                           local_member->type,
                                                           targ_btf, m->type);
                        if (!found)
                                spec->len--; /* pop accessor */
                        return found;
                }
                /* member turned out not to be what we looked for */
                spec->bit_offset -= bit_offset;
                spec->raw_len--;
        }

        return 0;
}

/*
 * Try to match local spec to a target type and, if successful, produce full
 * target spec (high-level, low-level + bit offset).
 */
static int bpf_core_spec_match(struct bpf_core_spec *local_spec,
                               const struct btf *targ_btf, __u32 targ_id,
                               struct bpf_core_spec *targ_spec)
{
        const struct btf_type *targ_type;
        const struct bpf_core_accessor *local_acc;
        struct bpf_core_accessor *targ_acc;
        int i, sz, matched;
        __u32 name_off;

        memset(targ_spec, 0, sizeof(*targ_spec));
        targ_spec->btf = targ_btf;
        targ_spec->root_type_id = targ_id;
        targ_spec->relo_kind = local_spec->relo_kind;

        if (core_relo_is_type_based(local_spec->relo_kind)) {
                if (local_spec->relo_kind == BPF_CORE_TYPE_MATCHES)
                        return bpf_core_types_match(local_spec->btf,
                                                    local_spec->root_type_id,
                                                    targ_btf, targ_id);
                else
                        return bpf_core_types_are_compat(local_spec->btf,
                                                         local_spec->root_type_id,
                                                         targ_btf, targ_id);
        }

        local_acc = &local_spec->spec[0];
        targ_acc = &targ_spec->spec[0];

        if (core_relo_is_enumval_based(local_spec->relo_kind)) {
                size_t local_essent_len, targ_essent_len;
                const char *targ_name;

                /* has to resolve to an enum */
                targ_type = skip_mods_and_typedefs(targ_spec->btf, targ_id, &targ_id);
                if (!btf_is_any_enum(targ_type))
                        return 0;

                local_essent_len = bpf_core_essential_name_len(local_acc->name);

                for (i = 0; i < btf_vlen(targ_type); i++) {
                        if (btf_is_enum(targ_type))
                                name_off = btf_enum(targ_type)[i].name_off;
                        else
                                name_off = btf_enum64(targ_type)[i].name_off;

                        targ_name = btf__name_by_offset(targ_spec->btf, name_off);
                        targ_essent_len = bpf_core_essential_name_len(targ_name);
                        if (targ_essent_len != local_essent_len)
                                continue;
                        if (strncmp(local_acc->name, targ_name, local_essent_len) == 0) {
                                targ_acc->type_id = targ_id;
                                targ_acc->idx = i;
                                targ_acc->name = targ_name;
                                targ_spec->len++;
                                targ_spec->raw_spec[targ_spec->raw_len] = targ_acc->idx;
                                targ_spec->raw_len++;
                                return 1;
                        }
                }
                return 0;
        }

        if (!core_relo_is_field_based(local_spec->relo_kind))
                return -EINVAL;

        for (i = 0; i < local_spec->len; i++, local_acc++, targ_acc++) {
                targ_type = skip_mods_and_typedefs(targ_spec->btf, targ_id,
                                                   &targ_id);
                if (!targ_type)
                        return -EINVAL;

                if (local_acc->name) {
                        matched = bpf_core_match_member(local_spec->btf,
                                                        local_acc,
                                                        targ_btf, targ_id,
                                                        targ_spec, &targ_id);
                        if (matched <= 0)
                                return matched;
                } else {
                        /* for i=0, targ_id is already treated as array element
                         * type (because it's the original struct), for others
                         * we should find array element type first
                         */
                        if (i > 0) {
                                const struct btf_array *a;
                                bool flex;

                                if (!btf_is_array(targ_type))
                                        return 0;

                                a = btf_array(targ_type);
                                flex = is_flex_arr(targ_btf, targ_acc - 1, a);
                                if (!flex && local_acc->idx >= a->nelems)
                                        return 0;
                                if (!skip_mods_and_typedefs(targ_btf, a->type,
                                                            &targ_id))
                                        return -EINVAL;
                        }

                        /* too deep struct/union/array nesting */
                        if (targ_spec->raw_len == BPF_CORE_SPEC_MAX_LEN)
                                return -E2BIG;

                        targ_acc->type_id = targ_id;
                        targ_acc->idx = local_acc->idx;
                        targ_acc->name = NULL;
                        targ_spec->len++;
                        targ_spec->raw_spec[targ_spec->raw_len] = targ_acc->idx;
                        targ_spec->raw_len++;

                        sz = btf__resolve_size(targ_btf, targ_id);
                        if (sz < 0)
                                return sz;
                        targ_spec->bit_offset += local_acc->idx * sz * 8;
                }
        }

        return 1;
}

static int bpf_core_calc_field_relo(const char *prog_name,
                                    const struct bpf_core_relo *relo,
                                    const struct bpf_core_spec *spec,
                                    __u64 *val, __u32 *field_sz, __u32 *type_id,
                                    bool *validate)
{
        const struct bpf_core_accessor *acc;
        const struct btf_type *t;
        __u32 byte_off, byte_sz, bit_off, bit_sz, field_type_id, elem_id;
        const struct btf_member *m;
        const struct btf_type *mt;
        bool bitfield;
        __s64 sz;

        *field_sz = 0;

        if (relo->kind == BPF_CORE_FIELD_EXISTS) {
                *val = spec ? 1 : 0;
                return 0;
        }

        if (!spec)
                return -EUCLEAN; /* request instruction poisoning */

        acc = &spec->spec[spec->len - 1];
        t = btf_type_by_id(spec->btf, acc->type_id);

        /* a[n] accessor needs special handling */
        if (!acc->name) {
                if (relo->kind == BPF_CORE_FIELD_BYTE_OFFSET) {
                        *val = spec->bit_offset / 8;
                        /* remember field size for load/store mem size;
                         * note, for arrays we care about individual element
                         * sizes, not the overall array size
                         */
                        t = skip_mods_and_typedefs(spec->btf, acc->type_id, &elem_id);
                        while (btf_is_array(t))
                                t = skip_mods_and_typedefs(spec->btf, btf_array(t)->type, &elem_id);
                        sz = btf__resolve_size(spec->btf, elem_id);
                        if (sz < 0)
                                return -EINVAL;
                        *field_sz = sz;
                        *type_id = acc->type_id;
                } else if (relo->kind == BPF_CORE_FIELD_BYTE_SIZE) {
                        sz = btf__resolve_size(spec->btf, acc->type_id);
                        if (sz < 0)
                                return -EINVAL;
                        *val = sz;
                } else {
                        pr_warn("prog '%s': relo %d at insn #%d can't be applied to array access\n",
                                prog_name, relo->kind, relo->insn_off / 8);
                        return -EINVAL;
                }
                if (validate)
                        *validate = true;
                return 0;
        }

        m = btf_members(t) + acc->idx;
        mt = skip_mods_and_typedefs(spec->btf, m->type, &field_type_id);
        bit_off = spec->bit_offset;
        bit_sz = btf_member_bitfield_size(t, acc->idx);

        bitfield = bit_sz > 0;
        if (bitfield) {
                byte_sz = mt->size;
                byte_off = bit_off / 8 / byte_sz * byte_sz;
                /* figure out smallest int size necessary for bitfield load */
                while (bit_off + bit_sz - byte_off * 8 > byte_sz * 8) {
                        if (byte_sz >= 8) {
                                /* bitfield can't be read with 64-bit read */
                                pr_warn("prog '%s': relo %d at insn #%d can't be satisfied for bitfield\n",
                                        prog_name, relo->kind, relo->insn_off / 8);
                                return -E2BIG;
                        }
                        byte_sz *= 2;
                        byte_off = bit_off / 8 / byte_sz * byte_sz;
                }
        } else {
                sz = btf__resolve_size(spec->btf, field_type_id);
                if (sz < 0)
                        return -EINVAL;
                byte_sz = sz;
                byte_off = spec->bit_offset / 8;
                bit_sz = byte_sz * 8;
        }

        /* for bitfields, all the relocatable aspects are ambiguous and we
         * might disagree with compiler, so turn off validation of expected
         * value, except for signedness
         */
        if (validate)
                *validate = !bitfield;

        switch (relo->kind) {
        case BPF_CORE_FIELD_BYTE_OFFSET:
                *val = byte_off;
                if (!bitfield) {
                        /* remember field size for load/store mem size;
                         * note, for arrays we care about individual element
                         * sizes, not the overall array size
                         */
                        t = skip_mods_and_typedefs(spec->btf, field_type_id, &elem_id);
                        while (btf_is_array(t))
                                t = skip_mods_and_typedefs(spec->btf, btf_array(t)->type, &elem_id);
                        sz = btf__resolve_size(spec->btf, elem_id);
                        if (sz < 0)
                                return -EINVAL;
                        *field_sz = sz;
                        *type_id = field_type_id;
                }
                break;
        case BPF_CORE_FIELD_BYTE_SIZE:
                *val = byte_sz;
                break;
        case BPF_CORE_FIELD_SIGNED:
                *val = (btf_is_any_enum(mt) && BTF_INFO_KFLAG(mt->info)) ||
                       (btf_is_int(mt) && (btf_int_encoding(mt) & BTF_INT_SIGNED));
                if (validate)
                        *validate = true; /* signedness is never ambiguous */
                break;
        case BPF_CORE_FIELD_LSHIFT_U64:
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
                *val = 64 - (bit_off + bit_sz - byte_off  * 8);
#else
                *val = (8 - byte_sz) * 8 + (bit_off - byte_off * 8);
#endif
                break;
        case BPF_CORE_FIELD_RSHIFT_U64:
                *val = 64 - bit_sz;
                if (validate)
                        *validate = true; /* right shift is never ambiguous */
                break;
        case BPF_CORE_FIELD_EXISTS:
        default:
                return -EOPNOTSUPP;
        }

        return 0;
}

static int bpf_core_calc_type_relo(const struct bpf_core_relo *relo,
                                   const struct bpf_core_spec *spec,
                                   __u64 *val, bool *validate)
{
        __s64 sz;

        /* by default, always check expected value in bpf_insn */
        if (validate)
                *validate = true;

        /* type-based relos return zero when target type is not found */
        if (!spec) {
                *val = 0;
                return 0;
        }

        switch (relo->kind) {
        case BPF_CORE_TYPE_ID_TARGET:
                *val = spec->root_type_id;
                /* type ID, embedded in bpf_insn, might change during linking,
                 * so enforcing it is pointless
                 */
                if (validate)
                        *validate = false;
                break;
        case BPF_CORE_TYPE_EXISTS:
        case BPF_CORE_TYPE_MATCHES:
                *val = 1;
                break;
        case BPF_CORE_TYPE_SIZE:
                sz = btf__resolve_size(spec->btf, spec->root_type_id);
                if (sz < 0)
                        return -EINVAL;
                *val = sz;
                break;
        case BPF_CORE_TYPE_ID_LOCAL:
        /* BPF_CORE_TYPE_ID_LOCAL is handled specially and shouldn't get here */
        default:
                return -EOPNOTSUPP;
        }

        return 0;
}

static int bpf_core_calc_enumval_relo(const struct bpf_core_relo *relo,
                                      const struct bpf_core_spec *spec,
                                      __u64 *val)
{
        const struct btf_type *t;

        switch (relo->kind) {
        case BPF_CORE_ENUMVAL_EXISTS:
                *val = spec ? 1 : 0;
                break;
        case BPF_CORE_ENUMVAL_VALUE:
                if (!spec)
                        return -EUCLEAN; /* request instruction poisoning */
                t = btf_type_by_id(spec->btf, spec->spec[0].type_id);
                if (btf_is_enum(t))
                        *val = btf_enum(t)[spec->spec[0].idx].val;
                else
                        *val = btf_enum64_value(btf_enum64(t) + spec->spec[0].idx);
                break;
        default:
                return -EOPNOTSUPP;
        }

        return 0;
}

/* Calculate original and target relocation values, given local and target
 * specs and relocation kind. These values are calculated for each candidate.
 * If there are multiple candidates, resulting values should all be consistent
 * with each other. Otherwise, libbpf will refuse to proceed due to ambiguity.
 * If instruction has to be poisoned, *poison will be set to true.
 */
static int bpf_core_calc_relo(const char *prog_name,
                              const struct bpf_core_relo *relo,
                              int relo_idx,
                              const struct bpf_core_spec *local_spec,
                              const struct bpf_core_spec *targ_spec,
                              struct bpf_core_relo_res *res)
{
        int err = -EOPNOTSUPP;

        res->orig_val = 0;
        res->new_val = 0;
        res->poison = false;
        res->validate = true;
        res->fail_memsz_adjust = false;
        res->orig_sz = res->new_sz = 0;
        res->orig_type_id = res->new_type_id = 0;

        if (core_relo_is_field_based(relo->kind)) {
                err = bpf_core_calc_field_relo(prog_name, relo, local_spec,
                                               &res->orig_val, &res->orig_sz,
                                               &res->orig_type_id, &res->validate);
                err = err ?: bpf_core_calc_field_relo(prog_name, relo, targ_spec,
                                                      &res->new_val, &res->new_sz,
                                                      &res->new_type_id, NULL);
                if (err)
                        goto done;
                /* Validate if it's safe to adjust load/store memory size.
                 * Adjustments are performed only if original and new memory
                 * sizes differ.
                 */
                res->fail_memsz_adjust = false;
                if (res->orig_sz != res->new_sz) {
                        const struct btf_type *orig_t, *new_t;

                        orig_t = btf_type_by_id(local_spec->btf, res->orig_type_id);
                        new_t = btf_type_by_id(targ_spec->btf, res->new_type_id);

                        /* There are two use cases in which it's safe to
                         * adjust load/store's mem size:
                         *   - reading a 32-bit kernel pointer, while on BPF
                         *   size pointers are always 64-bit; in this case
                         *   it's safe to "downsize" instruction size due to
                         *   pointer being treated as unsigned integer with
                         *   zero-extended upper 32-bits;
                         *   - reading unsigned integers, again due to
                         *   zero-extension is preserving the value correctly.
                         *
                         * In all other cases it's incorrect to attempt to
                         * load/store field because read value will be
                         * incorrect, so we poison relocated instruction.
                         */
                        if (btf_is_ptr(orig_t) && btf_is_ptr(new_t))
                                goto done;
                        if (btf_is_int(orig_t) && btf_is_int(new_t) &&
                            btf_int_encoding(orig_t) != BTF_INT_SIGNED &&
                            btf_int_encoding(new_t) != BTF_INT_SIGNED)
                                goto done;

                        /* mark as invalid mem size adjustment, but this will
                         * only be checked for LDX/STX/ST insns
                         */
                        res->fail_memsz_adjust = true;
                }
        } else if (core_relo_is_type_based(relo->kind)) {
                err = bpf_core_calc_type_relo(relo, local_spec, &res->orig_val, &res->validate);
                err = err ?: bpf_core_calc_type_relo(relo, targ_spec, &res->new_val, NULL);
        } else if (core_relo_is_enumval_based(relo->kind)) {
                err = bpf_core_calc_enumval_relo(relo, local_spec, &res->orig_val);
                err = err ?: bpf_core_calc_enumval_relo(relo, targ_spec, &res->new_val);
        }

done:
        if (err == -EUCLEAN) {
                /* EUCLEAN is used to signal instruction poisoning request */
                res->poison = true;
                err = 0;
        } else if (err == -EOPNOTSUPP) {
                /* EOPNOTSUPP means unknown/unsupported relocation */
                pr_warn("prog '%s': relo #%d: unrecognized CO-RE relocation %s (%d) at insn #%d\n",
                        prog_name, relo_idx, core_relo_kind_str(relo->kind),
                        relo->kind, relo->insn_off / 8);
        }

        return err;
}

/*
 * Turn instruction for which CO_RE relocation failed into invalid one with
 * distinct signature.
 */
static void bpf_core_poison_insn(const char *prog_name, int relo_idx,
                                 int insn_idx, struct bpf_insn *insn)
{
        pr_debug("prog '%s': relo #%d: substituting insn #%d w/ invalid insn\n",
                 prog_name, relo_idx, insn_idx);
        insn->code = BPF_JMP | BPF_CALL;
        insn->dst_reg = 0;
        insn->src_reg = 0;
        insn->off = 0;
        /* if this instruction is reachable (not a dead code),
         * verifier will complain with the following message:
         * invalid func unknown#195896080
         */
        insn->imm = 195896080; /* => 0xbad2310 => "bad relo" */
}

static int insn_bpf_size_to_bytes(struct bpf_insn *insn)
{
        switch (BPF_SIZE(insn->code)) {
        case BPF_DW: return 8;
        case BPF_W: return 4;
        case BPF_H: return 2;
        case BPF_B: return 1;
        default: return -1;
        }
}

static int insn_bytes_to_bpf_size(__u32 sz)
{
        switch (sz) {
        case 8: return BPF_DW;
        case 4: return BPF_W;
        case 2: return BPF_H;
        case 1: return BPF_B;
        default: return -1;
        }
}

/*
 * Patch relocatable BPF instruction.
 *
 * Patched value is determined by relocation kind and target specification.
 * For existence relocations target spec will be NULL if field/type is not found.
 * Expected insn->imm value is determined using relocation kind and local
 * spec, and is checked before patching instruction. If actual insn->imm value
 * is wrong, bail out with error.
 *
 * Currently supported classes of BPF instruction are:
 * 1. rX = <imm> (assignment with immediate operand);
 * 2. rX += <imm> (arithmetic operations with immediate operand);
 * 3. rX = <imm64> (load with 64-bit immediate value);
 * 4. rX = *(T *)(rY + <off>), where T is one of {u8, u16, u32, u64};
 * 5. *(T *)(rX + <off>) = rY, where T is one of {u8, u16, u32, u64};
 * 6. *(T *)(rX + <off>) = <imm>, where T is one of {u8, u16, u32, u64}.
 */
int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
                        int insn_idx, const struct bpf_core_relo *relo,
                        int relo_idx, const struct bpf_core_relo_res *res)
{
        __u64 orig_val, new_val;
        __u8 class;

        class = BPF_CLASS(insn->code);

        if (res->poison) {
poison:
                /* poison second part of ldimm64 to avoid confusing error from
                 * verifier about "unknown opcode 00"
                 */
                if (is_ldimm64_insn(insn))
                        bpf_core_poison_insn(prog_name, relo_idx, insn_idx + 1, insn + 1);
                bpf_core_poison_insn(prog_name, relo_idx, insn_idx, insn);
                return 0;
        }

        orig_val = res->orig_val;
        new_val = res->new_val;

        switch (class) {
        case BPF_ALU:
        case BPF_ALU64:
                if (BPF_SRC(insn->code) != BPF_K)
                        return -EINVAL;
                if (res->validate && insn->imm != orig_val) {
                        pr_warn("prog '%s': relo #%d: unexpected insn #%d (ALU/ALU64) value: got %u, exp %llu -> %llu\n",
                                prog_name, relo_idx,
                                insn_idx, insn->imm, (unsigned long long)orig_val,
                                (unsigned long long)new_val);
                        return -EINVAL;
                }
                orig_val = insn->imm;
                insn->imm = new_val;
                pr_debug("prog '%s': relo #%d: patched insn #%d (ALU/ALU64) imm %llu -> %llu\n",
                         prog_name, relo_idx, insn_idx,
                         (unsigned long long)orig_val, (unsigned long long)new_val);
                break;
        case BPF_LDX:
        case BPF_ST:
        case BPF_STX:
                if (res->validate && insn->off != orig_val) {
                        pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDX/ST/STX) value: got %u, exp %llu -> %llu\n",
                                prog_name, relo_idx, insn_idx, insn->off, (unsigned long long)orig_val,
                                (unsigned long long)new_val);
                        return -EINVAL;
                }
                if (new_val > SHRT_MAX) {
                        pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) value too big: %llu\n",
                                prog_name, relo_idx, insn_idx, (unsigned long long)new_val);
                        return -ERANGE;
                }
                if (res->fail_memsz_adjust) {
                        pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) accesses field incorrectly. "
                                "Make sure you are accessing pointers, unsigned integers, or fields of matching type and size.\n",
                                prog_name, relo_idx, insn_idx);
                        goto poison;
                }

                orig_val = insn->off;
                insn->off = new_val;
                pr_debug("prog '%s': relo #%d: patched insn #%d (LDX/ST/STX) off %llu -> %llu\n",
                         prog_name, relo_idx, insn_idx, (unsigned long long)orig_val,
                         (unsigned long long)new_val);

                if (res->new_sz != res->orig_sz) {
                        int insn_bytes_sz, insn_bpf_sz;

                        insn_bytes_sz = insn_bpf_size_to_bytes(insn);
                        if (insn_bytes_sz != res->orig_sz) {
                                pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) unexpected mem size: got %d, exp %u\n",
                                        prog_name, relo_idx, insn_idx, insn_bytes_sz, res->orig_sz);
                                return -EINVAL;
                        }

                        insn_bpf_sz = insn_bytes_to_bpf_size(res->new_sz);
                        if (insn_bpf_sz < 0) {
                                pr_warn("prog '%s': relo #%d: insn #%d (LDX/ST/STX) invalid new mem size: %u\n",
                                        prog_name, relo_idx, insn_idx, res->new_sz);
                                return -EINVAL;
                        }

                        insn->code = BPF_MODE(insn->code) | insn_bpf_sz | BPF_CLASS(insn->code);
                        pr_debug("prog '%s': relo #%d: patched insn #%d (LDX/ST/STX) mem_sz %u -> %u\n",
                                 prog_name, relo_idx, insn_idx, res->orig_sz, res->new_sz);
                }
                break;
        case BPF_LD: {
                __u64 imm;

                if (!is_ldimm64_insn(insn) ||
                    insn[0].src_reg != 0 || insn[0].off != 0 ||
                    insn[1].code != 0 || insn[1].dst_reg != 0 ||
                    insn[1].src_reg != 0 || insn[1].off != 0) {
                        pr_warn("prog '%s': relo #%d: insn #%d (LDIMM64) has unexpected form\n",
                                prog_name, relo_idx, insn_idx);
                        return -EINVAL;
                }

                imm = (__u32)insn[0].imm | ((__u64)insn[1].imm << 32);
                if (res->validate && imm != orig_val) {
                        pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDIMM64) value: got %llu, exp %llu -> %llu\n",
                                prog_name, relo_idx,
                                insn_idx, (unsigned long long)imm,
                                (unsigned long long)orig_val, (unsigned long long)new_val);
                        return -EINVAL;
                }

                insn[0].imm = new_val;
                insn[1].imm = new_val >> 32;
                pr_debug("prog '%s': relo #%d: patched insn #%d (LDIMM64) imm64 %llu -> %llu\n",
                         prog_name, relo_idx, insn_idx,
                         (unsigned long long)imm, (unsigned long long)new_val);
                break;
        }
        default:
                pr_warn("prog '%s': relo #%d: trying to relocate unrecognized insn #%d, code:0x%x, src:0x%x, dst:0x%x, off:0x%x, imm:0x%x\n",
                        prog_name, relo_idx, insn_idx, insn->code,
                        insn->src_reg, insn->dst_reg, insn->off, insn->imm);
                return -EINVAL;
        }

        return 0;
}

/* Output spec definition in the format:
 * [<type-id>] (<type-name>) + <raw-spec> => <offset>@<spec>,
 * where <spec> is a C-syntax view of recorded field access, e.g.: x.a[3].b
 */
int bpf_core_format_spec(char *buf, size_t buf_sz, const struct bpf_core_spec *spec)
{
        const struct btf_type *t;
        const char *s;
        __u32 type_id;
        int i, len = 0;

#define append_buf(fmt, args...)                                \
        ({                                                      \
                int r;                                          \
                r = snprintf(buf, buf_sz, fmt, ##args);         \
                len += r;                                       \
                if (r >= buf_sz)                                \
                        r = buf_sz;                             \
                buf += r;                                       \
                buf_sz -= r;                                    \
        })

        type_id = spec->root_type_id;
        t = btf_type_by_id(spec->btf, type_id);
        s = btf__name_by_offset(spec->btf, t->name_off);

        append_buf("<%s> [%u] %s %s",
                   core_relo_kind_str(spec->relo_kind),
                   type_id, btf_kind_str(t), str_is_empty(s) ? "<anon>" : s);

        if (core_relo_is_type_based(spec->relo_kind))
                return len;

        if (core_relo_is_enumval_based(spec->relo_kind)) {
                t = skip_mods_and_typedefs(spec->btf, type_id, NULL);
                if (btf_is_enum(t)) {
                        const struct btf_enum *e;
                        const char *fmt_str;

                        e = btf_enum(t) + spec->raw_spec[0];
                        s = btf__name_by_offset(spec->btf, e->name_off);
                        fmt_str = BTF_INFO_KFLAG(t->info) ? "::%s = %d" : "::%s = %u";
                        append_buf(fmt_str, s, e->val);
                } else {
                        const struct btf_enum64 *e;
                        const char *fmt_str;

                        e = btf_enum64(t) + spec->raw_spec[0];
                        s = btf__name_by_offset(spec->btf, e->name_off);
                        fmt_str = BTF_INFO_KFLAG(t->info) ? "::%s = %lld" : "::%s = %llu";
                        append_buf(fmt_str, s, (unsigned long long)btf_enum64_value(e));
                }
                return len;
        }

        if (core_relo_is_field_based(spec->relo_kind)) {
                for (i = 0; i < spec->len; i++) {
                        if (spec->spec[i].name)
                                append_buf(".%s", spec->spec[i].name);
                        else if (i > 0 || spec->spec[i].idx > 0)
                                append_buf("[%u]", spec->spec[i].idx);
                }

                append_buf(" (");
                for (i = 0; i < spec->raw_len; i++)
                        append_buf("%s%d", i == 0 ? "" : ":", spec->raw_spec[i]);

                if (spec->bit_offset % 8)
                        append_buf(" @ offset %u.%u)", spec->bit_offset / 8, spec->bit_offset % 8);
                else
                        append_buf(" @ offset %u)", spec->bit_offset / 8);
                return len;
        }

        return len;
#undef append_buf
}

/*
 * Calculate CO-RE relocation target result.
 *
 * The outline and important points of the algorithm:
 * 1. For given local type, find corresponding candidate target types.
 *    Candidate type is a type with the same "essential" name, ignoring
 *    everything after last triple underscore (___). E.g., `sample`,
 *    `sample___flavor_one`, `sample___flavor_another_one`, are all candidates
 *    for each other. Names with triple underscore are referred to as
 *    "flavors" and are useful, among other things, to allow to
 *    specify/support incompatible variations of the same kernel struct, which
 *    might differ between different kernel versions and/or build
 *    configurations.
 *
 *    N.B. Struct "flavors" could be generated by bpftool's BTF-to-C
 *    converter, when deduplicated BTF of a kernel still contains more than
 *    one different types with the same name. In that case, ___2, ___3, etc
 *    are appended starting from second name conflict. But start flavors are
 *    also useful to be defined "locally", in BPF program, to extract same
 *    data from incompatible changes between different kernel
 *    versions/configurations. For instance, to handle field renames between
 *    kernel versions, one can use two flavors of the struct name with the
 *    same common name and use conditional relocations to extract that field,
 *    depending on target kernel version.
 * 2. For each candidate type, try to match local specification to this
 *    candidate target type. Matching involves finding corresponding
 *    high-level spec accessors, meaning that all named fields should match,
 *    as well as all array accesses should be within the actual bounds. Also,
 *    types should be compatible (see bpf_core_fields_are_compat for details).
 * 3. It is supported and expected that there might be multiple flavors
 *    matching the spec. As long as all the specs resolve to the same set of
 *    offsets across all candidates, there is no error. If there is any
 *    ambiguity, CO-RE relocation will fail. This is necessary to accommodate
 *    imperfection of BTF deduplication, which can cause slight duplication of
 *    the same BTF type, if some directly or indirectly referenced (by
 *    pointer) type gets resolved to different actual types in different
 *    object files. If such a situation occurs, deduplicated BTF will end up
 *    with two (or more) structurally identical types, which differ only in
 *    types they refer to through pointer. This should be OK in most cases and
 *    is not an error.
 * 4. Candidate types search is performed by linearly scanning through all
 *    types in target BTF. It is anticipated that this is overall more
 *    efficient memory-wise and not significantly worse (if not better)
 *    CPU-wise compared to prebuilding a map from all local type names to
 *    a list of candidate type names. It's also sped up by caching resolved
 *    list of matching candidates per each local "root" type ID, that has at
 *    least one bpf_core_relo associated with it. This list is shared
 *    between multiple relocations for the same type ID and is updated as some
 *    of the candidates are pruned due to structural incompatibility.
 */
int bpf_core_calc_relo_insn(const char *prog_name,
                            const struct bpf_core_relo *relo,
                            int relo_idx,
                            const struct btf *local_btf,
                            struct bpf_core_cand_list *cands,
                            struct bpf_core_spec *specs_scratch,
                            struct bpf_core_relo_res *targ_res)
{
        struct bpf_core_spec *local_spec = &specs_scratch[0];
        struct bpf_core_spec *cand_spec = &specs_scratch[1];
        struct bpf_core_spec *targ_spec = &specs_scratch[2];
        struct bpf_core_relo_res cand_res;
        const struct btf_type *local_type;
        const char *local_name;
        __u32 local_id;
        char spec_buf[256];
        int i, j, err;

        local_id = relo->type_id;
        local_type = btf_type_by_id(local_btf, local_id);
        local_name = btf__name_by_offset(local_btf, local_type->name_off);
        if (!local_name)
                return -EINVAL;

        err = bpf_core_parse_spec(prog_name, local_btf, relo, local_spec);
        if (err) {
                const char *spec_str;

                spec_str = btf__name_by_offset(local_btf, relo->access_str_off);
                pr_warn("prog '%s': relo #%d: parsing [%d] %s %s + %s failed: %d\n",
                        prog_name, relo_idx, local_id, btf_kind_str(local_type),
                        str_is_empty(local_name) ? "<anon>" : local_name,
                        spec_str ?: "<?>", err);
                return -EINVAL;
        }

        bpf_core_format_spec(spec_buf, sizeof(spec_buf), local_spec);
        pr_debug("prog '%s': relo #%d: %s\n", prog_name, relo_idx, spec_buf);

        /* TYPE_ID_LOCAL relo is special and doesn't need candidate search */
        if (relo->kind == BPF_CORE_TYPE_ID_LOCAL) {
                /* bpf_insn's imm value could get out of sync during linking */
                memset(targ_res, 0, sizeof(*targ_res));
                targ_res->validate = false;
                targ_res->poison = false;
                targ_res->orig_val = local_spec->root_type_id;
                targ_res->new_val = local_spec->root_type_id;
                return 0;
        }

        /* libbpf doesn't support candidate search for anonymous types */
        if (str_is_empty(local_name)) {
                pr_warn("prog '%s': relo #%d: <%s> (%d) relocation doesn't support anonymous types\n",
                        prog_name, relo_idx, core_relo_kind_str(relo->kind), relo->kind);
                return -EOPNOTSUPP;
        }

        for (i = 0, j = 0; i < cands->len; i++) {
                err = bpf_core_spec_match(local_spec, cands->cands[i].btf,
                                          cands->cands[i].id, cand_spec);
                if (err < 0) {
                        bpf_core_format_spec(spec_buf, sizeof(spec_buf), cand_spec);
                        pr_warn("prog '%s': relo #%d: error matching candidate #%d %s: %d\n",
                                prog_name, relo_idx, i, spec_buf, err);
                        return err;
                }

                bpf_core_format_spec(spec_buf, sizeof(spec_buf), cand_spec);
                pr_debug("prog '%s': relo #%d: %s candidate #%d %s\n", prog_name,
                         relo_idx, err == 0 ? "non-matching" : "matching", i, spec_buf);

                if (err == 0)
                        continue;

                err = bpf_core_calc_relo(prog_name, relo, relo_idx, local_spec, cand_spec, &cand_res);
                if (err)
                        return err;

                if (j == 0) {
                        *targ_res = cand_res;
                        *targ_spec = *cand_spec;
                } else if (cand_spec->bit_offset != targ_spec->bit_offset) {
                        /* if there are many field relo candidates, they
                         * should all resolve to the same bit offset
                         */
                        pr_warn("prog '%s': relo #%d: field offset ambiguity: %u != %u\n",
                                prog_name, relo_idx, cand_spec->bit_offset,
                                targ_spec->bit_offset);
                        return -EINVAL;
                } else if (cand_res.poison != targ_res->poison ||
                           cand_res.new_val != targ_res->new_val) {
                        /* all candidates should result in the same relocation
                         * decision and value, otherwise it's dangerous to
                         * proceed due to ambiguity
                         */
                        pr_warn("prog '%s': relo #%d: relocation decision ambiguity: %s %llu != %s %llu\n",
                                prog_name, relo_idx,
                                cand_res.poison ? "failure" : "success",
                                (unsigned long long)cand_res.new_val,
                                targ_res->poison ? "failure" : "success",
                                (unsigned long long)targ_res->new_val);
                        return -EINVAL;
                }

                cands->cands[j++] = cands->cands[i];
        }

        /*
         * For BPF_CORE_FIELD_EXISTS relo or when used BPF program has field
         * existence checks or kernel version/config checks, it's expected
         * that we might not find any candidates. In this case, if field
         * wasn't found in any candidate, the list of candidates shouldn't
         * change at all, we'll just handle relocating appropriately,
         * depending on relo's kind.
         */
        if (j > 0)
                cands->len = j;

        /*
         * If no candidates were found, it might be both a programmer error,
         * as well as expected case, depending whether instruction w/
         * relocation is guarded in some way that makes it unreachable (dead
         * code) if relocation can't be resolved. This is handled in
         * bpf_core_patch_insn() uniformly by replacing that instruction with
         * BPF helper call insn (using invalid helper ID). If that instruction
         * is indeed unreachable, then it will be ignored and eliminated by
         * verifier. If it was an error, then verifier will complain and point
         * to a specific instruction number in its log.
         */
        if (j == 0) {
                pr_debug("prog '%s': relo #%d: no matching targets found\n",
                         prog_name, relo_idx);

                /* calculate single target relo result explicitly */
                err = bpf_core_calc_relo(prog_name, relo, relo_idx, local_spec, NULL, targ_res);
                if (err)
                        return err;
        }

        return 0;
}

static bool bpf_core_names_match(const struct btf *local_btf, size_t local_name_off,
                                 const struct btf *targ_btf, size_t targ_name_off)
{
        const char *local_n, *targ_n;
        size_t local_len, targ_len;

        local_n = btf__name_by_offset(local_btf, local_name_off);
        targ_n = btf__name_by_offset(targ_btf, targ_name_off);

        if (str_is_empty(targ_n))
                return str_is_empty(local_n);

        targ_len = bpf_core_essential_name_len(targ_n);
        local_len = bpf_core_essential_name_len(local_n);

        return targ_len == local_len && strncmp(local_n, targ_n, local_len) == 0;
}

static int bpf_core_enums_match(const struct btf *local_btf, const struct btf_type *local_t,
                                const struct btf *targ_btf, const struct btf_type *targ_t)
{
        __u32 local_vlen = btf_vlen(local_t);
        __u32 targ_vlen = btf_vlen(targ_t);
        int i, j;

        if (local_t->size != targ_t->size)
                return 0;

        if (local_vlen > targ_vlen)
                return 0;

        /* iterate over the local enum's variants and make sure each has
         * a symbolic name correspondent in the target
         */
        for (i = 0; i < local_vlen; i++) {
                bool matched = false;
                __u32 local_n_off, targ_n_off;

                local_n_off = btf_is_enum(local_t) ? btf_enum(local_t)[i].name_off :
                                                     btf_enum64(local_t)[i].name_off;

                for (j = 0; j < targ_vlen; j++) {
                        targ_n_off = btf_is_enum(targ_t) ? btf_enum(targ_t)[j].name_off :
                                                           btf_enum64(targ_t)[j].name_off;

                        if (bpf_core_names_match(local_btf, local_n_off, targ_btf, targ_n_off)) {
                                matched = true;
                                break;
                        }
                }

                if (!matched)
                        return 0;
        }
        return 1;
}

static int bpf_core_composites_match(const struct btf *local_btf, const struct btf_type *local_t,
                                     const struct btf *targ_btf, const struct btf_type *targ_t,
                                     bool behind_ptr, int level)
{
        const struct btf_member *local_m = btf_members(local_t);
        __u32 local_vlen = btf_vlen(local_t);
        __u32 targ_vlen = btf_vlen(targ_t);
        int i, j, err;

        if (local_vlen > targ_vlen)
                return 0;

        /* check that all local members have a match in the target */
        for (i = 0; i < local_vlen; i++, local_m++) {
                const struct btf_member *targ_m = btf_members(targ_t);
                bool matched = false;

                for (j = 0; j < targ_vlen; j++, targ_m++) {
                        if (!bpf_core_names_match(local_btf, local_m->name_off,
                                                  targ_btf, targ_m->name_off))
                                continue;

                        err = __bpf_core_types_match(local_btf, local_m->type, targ_btf,
                                                     targ_m->type, behind_ptr, level - 1);
                        if (err < 0)
                                return err;
                        if (err > 0) {
                                matched = true;
                                break;
                        }
                }

                if (!matched)
                        return 0;
        }
        return 1;
}

/* Check that two types "match". This function assumes that root types were
 * already checked for name match.
 *
 * The matching relation is defined as follows:
 * - modifiers and typedefs are stripped (and, hence, effectively ignored)
 * - generally speaking types need to be of same kind (struct vs. struct, union
 *   vs. union, etc.)
 *   - exceptions are struct/union behind a pointer which could also match a
 *     forward declaration of a struct or union, respectively, and enum vs.
 *     enum64 (see below)
 * Then, depending on type:
 * - integers:
 *   - match if size and signedness match
 * - arrays & pointers:
 *   - target types are recursively matched
 * - structs & unions:
 *   - local members need to exist in target with the same name
 *   - for each member we recursively check match unless it is already behind a
 *     pointer, in which case we only check matching names and compatible kind
 * - enums:
 *   - local variants have to have a match in target by symbolic name (but not
 *     numeric value)
 *   - size has to match (but enum may match enum64 and vice versa)
 * - function pointers:
 *   - number and position of arguments in local type has to match target
 *   - for each argument and the return value we recursively check match
 */
int __bpf_core_types_match(const struct btf *local_btf, __u32 local_id, const struct btf *targ_btf,
                           __u32 targ_id, bool behind_ptr, int level)
{
        const struct btf_type *local_t, *targ_t;
        int depth = 32; /* max recursion depth */
        __u16 local_k, targ_k;

        if (level <= 0)
                return -EINVAL;

recur:
        depth--;
        if (depth < 0)
                return -EINVAL;

        local_t = skip_mods_and_typedefs(local_btf, local_id, &local_id);
        targ_t = skip_mods_and_typedefs(targ_btf, targ_id, &targ_id);
        if (!local_t || !targ_t)
                return -EINVAL;

        /* While the name check happens after typedefs are skipped, root-level
         * typedefs would still be name-matched as that's the contract with
         * callers.
         */
        if (!bpf_core_names_match(local_btf, local_t->name_off, targ_btf, targ_t->name_off))
                return 0;

        local_k = btf_kind(local_t);
        targ_k = btf_kind(targ_t);

        switch (local_k) {
        case BTF_KIND_UNKN:
                return local_k == targ_k;
        case BTF_KIND_FWD: {
                bool local_f = BTF_INFO_KFLAG(local_t->info);

                if (behind_ptr) {
                        if (local_k == targ_k)
                                return local_f == BTF_INFO_KFLAG(targ_t->info);

                        /* for forward declarations kflag dictates whether the
                         * target is a struct (0) or union (1)
                         */
                        return (targ_k == BTF_KIND_STRUCT && !local_f) ||
                               (targ_k == BTF_KIND_UNION && local_f);
                } else {
                        if (local_k != targ_k)
                                return 0;

                        /* match if the forward declaration is for the same kind */
                        return local_f == BTF_INFO_KFLAG(targ_t->info);
                }
        }
        case BTF_KIND_ENUM:
        case BTF_KIND_ENUM64:
                if (!btf_is_any_enum(targ_t))
                        return 0;

                return bpf_core_enums_match(local_btf, local_t, targ_btf, targ_t);
        case BTF_KIND_STRUCT:
        case BTF_KIND_UNION:
                if (behind_ptr) {
                        bool targ_f = BTF_INFO_KFLAG(targ_t->info);

                        if (local_k == targ_k)
                                return 1;

                        if (targ_k != BTF_KIND_FWD)
                                return 0;

                        return (local_k == BTF_KIND_UNION) == targ_f;
                } else {
                        if (local_k != targ_k)
                                return 0;

                        return bpf_core_composites_match(local_btf, local_t, targ_btf, targ_t,
                                                         behind_ptr, level);
                }
        case BTF_KIND_INT: {
                __u8 local_sgn;
                __u8 targ_sgn;

                if (local_k != targ_k)
                        return 0;

                local_sgn = btf_int_encoding(local_t) & BTF_INT_SIGNED;
                targ_sgn = btf_int_encoding(targ_t) & BTF_INT_SIGNED;

                return local_t->size == targ_t->size && local_sgn == targ_sgn;
        }
        case BTF_KIND_PTR:
                if (local_k != targ_k)
                        return 0;

                behind_ptr = true;

                local_id = local_t->type;
                targ_id = targ_t->type;
                goto recur;
        case BTF_KIND_ARRAY: {
                const struct btf_array *local_array = btf_array(local_t);
                const struct btf_array *targ_array = btf_array(targ_t);

                if (local_k != targ_k)
                        return 0;

                if (local_array->nelems != targ_array->nelems)
                        return 0;

                local_id = local_array->type;
                targ_id = targ_array->type;
                goto recur;
        }
        case BTF_KIND_FUNC_PROTO: {
                struct btf_param *local_p = btf_params(local_t);
                struct btf_param *targ_p = btf_params(targ_t);
                __u32 local_vlen = btf_vlen(local_t);
                __u32 targ_vlen = btf_vlen(targ_t);
                int i, err;

                if (local_k != targ_k)
                        return 0;

                if (local_vlen != targ_vlen)
                        return 0;

                for (i = 0; i < local_vlen; i++, local_p++, targ_p++) {
                        err = __bpf_core_types_match(local_btf, local_p->type, targ_btf,
                                                     targ_p->type, behind_ptr, level - 1);
                        if (err <= 0)
                                return err;
                }

                /* tail recurse for return type check */
                local_id = local_t->type;
                targ_id = targ_t->type;
                goto recur;
        }
        default:
                pr_warn("unexpected kind %s relocated, local [%d], target [%d]\n",
                        btf_kind_str(local_t), local_id, targ_id);
                return 0;
        }
}