// SPDX-License-Identifier: GPL-2.0-only
/*
 * fsgsbase_restore.c, test ptrace vs fsgsbase
 * Copyright (c) 2020 Andy Lutomirski
 *
 * This test case simulates a tracer redirecting tracee execution to
 * a function and then restoring tracee state using PTRACE_GETREGS and
 * PTRACE_SETREGS.  This is similar to what gdb does when doing
 * 'p func()'.  The catch is that this test has the called function
 * modify a segment register.  This makes sure that ptrace correctly
 * restores segment state when using PTRACE_SETREGS.
 *
 * This is not part of fsgsbase.c, because that test is 64-bit only.
 */

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <err.h>
#include <sys/user.h>
#include <asm/prctl.h>
#include <sys/prctl.h>
#include <asm/ldt.h>
#include <sys/mman.h>
#include <stddef.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <stdint.h>

#define EXPECTED_VALUE 0x1337f00d

#ifdef __x86_64__
# define SEG "%gs"
#else
# define SEG "%fs"
#endif

/*
 * Defined in clang_helpers_[32|64].S, because unlike gcc, clang inline asm does
 * not support segmentation prefixes.
 */
unsigned int dereference_seg_base(void);

static void init_seg(void)
{
        unsigned int *target = mmap(
                NULL, sizeof(unsigned int),
                PROT_READ | PROT_WRITE,
                MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0);
        if (target == MAP_FAILED)
                err(1, "mmap");

        *target = EXPECTED_VALUE;

        printf("\tsegment base address = 0x%lx\n", (unsigned long)target);

        struct user_desc desc = {
                .entry_number    = 0,
                .base_addr       = (unsigned int)(uintptr_t)target,
                .limit           = sizeof(unsigned int) - 1,
                .seg_32bit       = 1,
                .contents        = 0, /* Data, grow-up */
                .read_exec_only  = 0,
                .limit_in_pages  = 0,
                .seg_not_present = 0,
                .useable         = 0
        };
        if (syscall(SYS_modify_ldt, 1, &desc, sizeof(desc)) == 0) {
                printf("\tusing LDT slot 0\n");
                asm volatile ("mov %0, %" SEG :: "rm" ((unsigned short)0x7));
        } else {
                /* No modify_ldt for us (configured out, perhaps) */

                struct user_desc *low_desc = mmap(
                        NULL, sizeof(desc),
                        PROT_READ | PROT_WRITE,
                        MAP_PRIVATE | MAP_ANONYMOUS | MAP_32BIT, -1, 0);
                memcpy(low_desc, &desc, sizeof(desc));

                low_desc->entry_number = -1;

                /* 32-bit set_thread_area */
                long ret;
                asm volatile ("int $0x80"
                              : "=a" (ret), "+m" (*low_desc)
                              : "a" (243), "b" (low_desc)
#ifdef __x86_64__
                              : "r8", "r9", "r10", "r11"
#endif
                        );
                memcpy(&desc, low_desc, sizeof(desc));
                munmap(low_desc, sizeof(desc));

                if (ret != 0) {
                        printf("[NOTE]\tcould not create a segment -- can't test anything\n");
                        exit(0);
                }
                printf("\tusing GDT slot %d\n", desc.entry_number);

                unsigned short sel = (unsigned short)((desc.entry_number << 3) | 0x3);
                asm volatile ("mov %0, %" SEG :: "rm" (sel));
        }
}

static void tracee_zap_segment(void)
{
        /*
         * The tracer will redirect execution here.  This is meant to
         * work like gdb's 'p func()' feature.  The tricky bit is that
         * we modify a segment register in order to make sure that ptrace
         * can correctly restore segment registers.
         */
        printf("\tTracee: in tracee_zap_segment()\n");

        /*
         * Write a nonzero selector with base zero to the segment register.
         * Using a null selector would defeat the test on AMD pre-Zen2
         * CPUs, as such CPUs don't clear the base when loading a null
         * selector.
         */
        unsigned short sel;
        asm volatile ("mov %%ss, %0\n\t"
                      "mov %0, %" SEG
                      : "=rm" (sel));

        pid_t pid = getpid(), tid = syscall(SYS_gettid);

        printf("\tTracee is going back to sleep\n");
        syscall(SYS_tgkill, pid, tid, SIGSTOP);

        /* Should not get here. */
        while (true) {
                printf("[FAIL]\tTracee hit unreachable code\n");
                pause();
        }
}

int main()
{
        printf("\tSetting up a segment\n");
        init_seg();

        unsigned int val = dereference_seg_base();
        if (val != EXPECTED_VALUE) {
                printf("[FAIL]\tseg[0] == %x; should be %x\n", val, EXPECTED_VALUE);
                return 1;
        }
        printf("[OK]\tThe segment points to the right place.\n");

        pid_t chld = fork();
        if (chld < 0)
                err(1, "fork");

        if (chld == 0) {
                prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0, 0);

                if (ptrace(PTRACE_TRACEME, 0, 0, 0) != 0)
                        err(1, "PTRACE_TRACEME");

                pid_t pid = getpid(), tid = syscall(SYS_gettid);

                printf("\tTracee will take a nap until signaled\n");
                syscall(SYS_tgkill, pid, tid, SIGSTOP);

                printf("\tTracee was resumed.  Will re-check segment.\n");

                val = dereference_seg_base();
                if (val != EXPECTED_VALUE) {
                        printf("[FAIL]\tseg[0] == %x; should be %x\n", val, EXPECTED_VALUE);
                        exit(1);
                }

                printf("[OK]\tThe segment points to the right place.\n");
                exit(0);
        }

        int status;

        /* Wait for SIGSTOP. */
        if (waitpid(chld, &status, 0) != chld || !WIFSTOPPED(status))
                err(1, "waitpid");

        struct user_regs_struct regs;

        if (ptrace(PTRACE_GETREGS, chld, NULL, &regs) != 0)
                err(1, "PTRACE_GETREGS");

#ifdef __x86_64__
        printf("\tChild GS=0x%lx, GSBASE=0x%lx\n", (unsigned long)regs.gs, (unsigned long)regs.gs_base);
#else
        printf("\tChild FS=0x%lx\n", (unsigned long)regs.xfs);
#endif

        struct user_regs_struct regs2 = regs;
#ifdef __x86_64__
        regs2.rip = (unsigned long)tracee_zap_segment;
        regs2.rsp -= 128;       /* Don't clobber the redzone. */
#else
        regs2.eip = (unsigned long)tracee_zap_segment;
#endif

        printf("\tTracer: redirecting tracee to tracee_zap_segment()\n");
        if (ptrace(PTRACE_SETREGS, chld, NULL, &regs2) != 0)
                err(1, "PTRACE_GETREGS");
        if (ptrace(PTRACE_CONT, chld, NULL, NULL) != 0)
                err(1, "PTRACE_GETREGS");

        /* Wait for SIGSTOP. */
        if (waitpid(chld, &status, 0) != chld || !WIFSTOPPED(status))
                err(1, "waitpid");

        printf("\tTracer: restoring tracee state\n");
        if (ptrace(PTRACE_SETREGS, chld, NULL, &regs) != 0)
                err(1, "PTRACE_GETREGS");
        if (ptrace(PTRACE_DETACH, chld, NULL, NULL) != 0)
                err(1, "PTRACE_GETREGS");

        /* Wait for SIGSTOP. */
        if (waitpid(chld, &status, 0) != chld)
                err(1, "waitpid");

        if (WIFSIGNALED(status)) {
                printf("[FAIL]\tTracee crashed\n");
                return 1;
        }

        if (!WIFEXITED(status)) {
                printf("[FAIL]\tTracee stopped for an unexpected reason: %d\n", status);
                return 1;
        }

        int exitcode = WEXITSTATUS(status);
        if (exitcode != 0) {
                printf("[FAIL]\tTracee reported failure\n");
                return 1;
        }

        printf("[OK]\tAll is well.\n");
        return 0;
}