/*
 * parse_vdso.c: Linux reference vDSO parser
 * Written by Andrew Lutomirski, 2011-2014.
 *
 * This code is meant to be linked in to various programs that run on Linux.
 * As such, it is available with as few restrictions as possible.  This file
 * is licensed under the Creative Commons Zero License, version 1.0,
 * available at http://creativecommons.org/publicdomain/zero/1.0/legalcode
 *
 * The vDSO is a regular ELF DSO that the kernel maps into user space when
 * it starts a program.  It works equally well in statically and dynamically
 * linked binaries.
 *
 * This code is tested on x86.  In principle it should work on any
 * architecture that has a vDSO.
 */

#include <stdbool.h>
#include <stdint.h>
#include <string.h>
#include <limits.h>
#include <linux/auxvec.h>
#include <linux/elf.h>

#include "parse_vdso.h"

/* And here's the code. */
#ifndef ELF_BITS
# if __SIZEOF_LONG__ >= 8
#  define ELF_BITS 64
# else
#  define ELF_BITS 32
# endif
#endif

#define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
#define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
#define ELF(x) ELF_BITS_XFORM(ELF_BITS, x)

#ifdef __s390x__
#define ELF_HASH_ENTRY ELF(Xword)
#else
#define ELF_HASH_ENTRY ELF(Word)
#endif

static struct vdso_info
{
        bool valid;

        /* Load information */
        uintptr_t load_addr;
        uintptr_t load_offset;  /* load_addr - recorded vaddr */

        /* Symbol table */
        ELF(Sym) *symtab;
        const char *symstrings;
        ELF(Word) *gnu_hash, *gnu_bucket;
        ELF_HASH_ENTRY *bucket, *chain;
        ELF_HASH_ENTRY nbucket, nchain;

        /* Version table */
        ELF(Versym) *versym;
        ELF(Verdef) *verdef;
} vdso_info;

/*
 * Straight from the ELF specification...and then tweaked slightly, in order to
 * avoid a few clang warnings.
 */
static unsigned long elf_hash(const char *name)
{
        unsigned long h = 0, g;
        const unsigned char *uch_name = (const unsigned char *)name;

        while (*uch_name)
        {
                h = (h << 4) + *uch_name++;
                g = h & 0xf0000000;
                if (g)
                        h ^= g >> 24;
                h &= ~g;
        }
        return h;
}

static uint32_t gnu_hash(const char *name)
{
        const unsigned char *s = (void *)name;
        uint32_t h = 5381;

        for (; *s; s++)
                h += h * 32 + *s;
        return h;
}

void vdso_init_from_sysinfo_ehdr(uintptr_t base)
{
        size_t i;
        bool found_vaddr = false;

        vdso_info.valid = false;

        vdso_info.load_addr = base;

        ELF(Ehdr) *hdr = (ELF(Ehdr)*)base;
        if (hdr->e_ident[EI_CLASS] !=
            (ELF_BITS == 32 ? ELFCLASS32 : ELFCLASS64)) {
                return;  /* Wrong ELF class -- check ELF_BITS */
        }

        ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff);
        ELF(Dyn) *dyn = 0;

        /*
         * We need two things from the segment table: the load offset
         * and the dynamic table.
         */
        for (i = 0; i < hdr->e_phnum; i++)
        {
                if (pt[i].p_type == PT_LOAD && !found_vaddr) {
                        found_vaddr = true;
                        vdso_info.load_offset = base
                                + (uintptr_t)pt[i].p_offset
                                - (uintptr_t)pt[i].p_vaddr;
                } else if (pt[i].p_type == PT_DYNAMIC) {
                        dyn = (ELF(Dyn)*)(base + pt[i].p_offset);
                }
        }

        if (!found_vaddr || !dyn)
                return;  /* Failed */

        /*
         * Fish out the useful bits of the dynamic table.
         */
        ELF_HASH_ENTRY *hash = 0;
        vdso_info.symstrings = 0;
        vdso_info.gnu_hash = 0;
        vdso_info.symtab = 0;
        vdso_info.versym = 0;
        vdso_info.verdef = 0;
        for (i = 0; dyn[i].d_tag != DT_NULL; i++) {
                switch (dyn[i].d_tag) {
                case DT_STRTAB:
                        vdso_info.symstrings = (const char *)
                                ((uintptr_t)dyn[i].d_un.d_ptr
                                 + vdso_info.load_offset);
                        break;
                case DT_SYMTAB:
                        vdso_info.symtab = (ELF(Sym) *)
                                ((uintptr_t)dyn[i].d_un.d_ptr
                                 + vdso_info.load_offset);
                        break;
                case DT_HASH:
                        hash = (ELF_HASH_ENTRY *)
                                ((uintptr_t)dyn[i].d_un.d_ptr
                                 + vdso_info.load_offset);
                        break;
                case DT_GNU_HASH:
                        vdso_info.gnu_hash =
                                (ELF(Word) *)((uintptr_t)dyn[i].d_un.d_ptr +
                                              vdso_info.load_offset);
                        break;
                case DT_VERSYM:
                        vdso_info.versym = (ELF(Versym) *)
                                ((uintptr_t)dyn[i].d_un.d_ptr
                                 + vdso_info.load_offset);
                        break;
                case DT_VERDEF:
                        vdso_info.verdef = (ELF(Verdef) *)
                                ((uintptr_t)dyn[i].d_un.d_ptr
                                 + vdso_info.load_offset);
                        break;
                }
        }
        if (!vdso_info.symstrings || !vdso_info.symtab ||
            (!hash && !vdso_info.gnu_hash))
                return;  /* Failed */

        if (!vdso_info.verdef)
                vdso_info.versym = 0;

        /* Parse the hash table header. */
        if (vdso_info.gnu_hash) {
                vdso_info.nbucket = vdso_info.gnu_hash[0];
                /* The bucket array is located after the header (4 uint32) and the bloom
                 * filter (size_t array of gnu_hash[2] elements).
                 */
                vdso_info.gnu_bucket = vdso_info.gnu_hash + 4 +
                                       sizeof(size_t) / 4 * vdso_info.gnu_hash[2];
        } else {
                vdso_info.nbucket = hash[0];
                vdso_info.nchain = hash[1];
                vdso_info.bucket = &hash[2];
                vdso_info.chain = &hash[vdso_info.nbucket + 2];
        }

        /* That's all we need. */
        vdso_info.valid = true;
}

static bool vdso_match_version(ELF(Versym) ver,
                               const char *name, ELF(Word) hash)
{
        /*
         * This is a helper function to check if the version indexed by
         * ver matches name (which hashes to hash).
         *
         * The version definition table is a mess, and I don't know how
         * to do this in better than linear time without allocating memory
         * to build an index.  I also don't know why the table has
         * variable size entries in the first place.
         *
         * For added fun, I can't find a comprehensible specification of how
         * to parse all the weird flags in the table.
         *
         * So I just parse the whole table every time.
         */

        /* First step: find the version definition */
        ver &= 0x7fff;  /* Apparently bit 15 means "hidden" */
        ELF(Verdef) *def = vdso_info.verdef;
        while(true) {
                if ((def->vd_flags & VER_FLG_BASE) == 0
                    && (def->vd_ndx & 0x7fff) == ver)
                        break;

                if (def->vd_next == 0)
                        return false;  /* No definition. */

                def = (ELF(Verdef) *)((char *)def + def->vd_next);
        }

        /* Now figure out whether it matches. */
        ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux);
        return def->vd_hash == hash
                && !strcmp(name, vdso_info.symstrings + aux->vda_name);
}

static bool check_sym(ELF(Sym) *sym, ELF(Word) i, const char *name,
                      const char *version, unsigned long ver_hash)
{
        /* Check for a defined global or weak function w/ right name. */
        if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
                return false;
        if (ELF64_ST_BIND(sym->st_info) != STB_GLOBAL &&
            ELF64_ST_BIND(sym->st_info) != STB_WEAK)
                return false;
        if (strcmp(name, vdso_info.symstrings + sym->st_name))
                return false;

        /* Check symbol version. */
        if (vdso_info.versym &&
            !vdso_match_version(vdso_info.versym[i], version, ver_hash))
                return false;

        return true;
}

void *vdso_sym(const char *version, const char *name)
{
        unsigned long ver_hash;
        if (!vdso_info.valid)
                return 0;

        ver_hash = elf_hash(version);
        ELF(Word) i;

        if (vdso_info.gnu_hash) {
                uint32_t h1 = gnu_hash(name), h2, *hashval;

                i = vdso_info.gnu_bucket[h1 % vdso_info.nbucket];
                if (i == 0)
                        return 0;
                h1 |= 1;
                hashval = vdso_info.gnu_bucket + vdso_info.nbucket +
                          (i - vdso_info.gnu_hash[1]);
                for (;; i++) {
                        ELF(Sym) *sym = &vdso_info.symtab[i];
                        h2 = *hashval++;
                        if (h1 == (h2 | 1) &&
                            check_sym(sym, i, name, version, ver_hash))
                                return (void *)(vdso_info.load_offset +
                                                sym->st_value);
                        if (h2 & 1)
                                break;
                }
        } else {
                i = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
                for (; i; i = vdso_info.chain[i]) {
                        ELF(Sym) *sym = &vdso_info.symtab[i];
                        if (sym->st_shndx != SHN_UNDEF &&
                            check_sym(sym, i, name, version, ver_hash))
                                return (void *)(vdso_info.load_offset +
                                                sym->st_value);
                }
        }

        return 0;
}