vdso_test.c

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/*
 * vdso_test.c: Sample code to test parse_vdso.c on x86_64
 * Copyright (c) 2011 Andy Lutomirski
 * Subject to the GNU General Public License, version 2
 *
 * You can amuse yourself by compiling with:
 * gcc -std=gnu99 -nostdlib
 *     -Os -fno-asynchronous-unwind-tables -flto
 *      vdso_test.c parse_vdso.c -o vdso_test
 * to generate a small binary with no dependencies at all.
 */

#include <sys/syscall.h>
#include <sys/time.h>
#include <unistd.h>
#include <stdint.h>

extern void *vdso_sym(const char *version, const char *name);
extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
extern void vdso_init_from_auxv(void *auxv);

/* We need a libc functions... */
int strcmp(const char *a, const char *b)
{
    /* This implementation is buggy: it never returns -1. */
    while (*a || *b) {
        if (*a != *b)
            return 1;
        if (*a == 0 || *b == 0)
            return 1;
        a++;
        b++;
    }

    return 0;
}

/* ...and two syscalls.  This is x86_64-specific. */
static inline long linux_write(int fd, const void *data, size_t len)
{

    long ret;
    asm volatile ("syscall" : "=a" (ret) : "a" (__NR_write),
              "D" (fd), "S" (data), "d" (len) :
              "cc", "memory", "rcx",
              "r8", "r9", "r10", "r11" );
    return ret;
}

static inline void linux_exit(int code)
{
    asm volatile ("syscall" : : "a" (__NR_exit), "D" (code));
}

void to_base10(char *lastdig, uint64_t n)
{
    while (n) {
        *lastdig = (n % 10) + '0';
        n /= 10;
        lastdig--;
    }
}

__attribute__((externally_visible)) void c_main(void **stack)
{
    /* Parse the stack */
    long argc = (long)*stack;
    stack += argc + 2;

    /* Now we're pointing at the environment.  Skip it. */
    while(*stack)
        stack++;
    stack++;

    /* Now we're pointing at auxv.  Initialize the vDSO parser. */
    vdso_init_from_auxv((void *)stack);

    /* Find gettimeofday. */
    typedef long (*gtod_t)(struct timeval *tv, struct timezone *tz);
    gtod_t gtod = (gtod_t)vdso_sym("LINUX_2.6", "__vdso_gettimeofday");

    if (!gtod)
        linux_exit(1);

    struct timeval tv;
    long ret = gtod(&tv, 0);

    if (ret == 0) {
        char buf[] = "The time is                     .000000\n";
        to_base10(buf + 31, tv.tv_sec);
        to_base10(buf + 38, tv.tv_usec);
        linux_write(1, buf, sizeof(buf) - 1);
    } else {
        linux_exit(ret);
    }

    linux_exit(0);
}

/*
 * This is the real entry point.  It passes the initial stack into
 * the C entry point.
 */
asm (
    ".text\n"
    ".global _start\n"
        ".type _start,@function\n"
        "_start:\n\t"
        "mov %rsp,%rdi\n\t"
        "jmp c_main"
    );