Documentation/vDSO/vdso_test.c

   1 /*
   2  * vdso_test.c: Sample code to test parse_vdso.c on x86_64
   3  * Copyright (c) 2011 Andy Lutomirski
   4  * Subject to the GNU General Public License, version 2
   5  *
   6  * You can amuse yourself by compiling with:
   7  * gcc -std=gnu99 -nostdlib
   8  *     -Os -fno-asynchronous-unwind-tables -flto
   9  *      vdso_test.c parse_vdso.c -o vdso_test
  10  * to generate a small binary with no dependencies at all.
  11  */
  12
  13 #include <sys/syscall.h>
  14 #include <sys/time.h>
  15 #include <unistd.h>
  16 #include <stdint.h>
  17
  18 extern void *vdso_sym(const char *version, const char *name);
  19 extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
  20 extern void vdso_init_from_auxv(void *auxv);
  21
  22 /* We need a libc functions... */
  23 int strcmp(const char *a, const char *b)
  24 {
  25         /* This implementation is buggy: it never returns -1. */
  26         while (*a || *b) {
  27                 if (*a != *b)
  28                         return 1;
  29                 if (*a == 0 || *b == 0)
  30                         return 1;
  31                 a++;
  32                 b++;
  33         }
  34
  35         return 0;
  36 }
  37
  38 /* ...and two syscalls.  This is x86_64-specific. */
  39 static inline long linux_write(int fd, const void *data, size_t len)
  40 {
  41
  42         long ret;
  43         asm volatile ("syscall" : "=a" (ret) : "a" (__NR_write),
  44                       "D" (fd), "S" (data), "d" (len) :
  45                       "cc", "memory", "rcx",
  46                       "r8", "r9", "r10", "r11" );
  47         return ret;
  48 }
  49
  50 static inline void linux_exit(int code)
  51 {
  52         asm volatile ("syscall" : : "a" (__NR_exit), "D" (code));
  53 }
  54
  55 void to_base10(char *lastdig, uint64_t n)
  56 {
  57         while (n) {
  58                 *lastdig = (n % 10) + '0';
  59                 n /= 10;
  60                 lastdig--;
  61         }
  62 }
  63
  64 __attribute__((externally_visible)) void c_main(void **stack)
  65 {
  66         /* Parse the stack */
  67         long argc = (long)*stack;
  68         stack += argc + 2;
  69
  70         /* Now we're pointing at the environment.  Skip it. */
  71         while(*stack)
  72                 stack++;
  73         stack++;
  74
  75         /* Now we're pointing at auxv.  Initialize the vDSO parser. */
  76         vdso_init_from_auxv((void *)stack);
  77
  78         /* Find gettimeofday. */
  79         typedef long (*gtod_t)(struct timeval *tv, struct timezone *tz);
  80         gtod_t gtod = (gtod_t)vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
  81
  82         if (!gtod)
  83                 linux_exit(1);
  84
  85         struct timeval tv;
  86         long ret = gtod(&tv, 0);
  87
  88         if (ret == 0) {
  89                 char buf[] = "The time is                     .000000\n";
  90                 to_base10(buf + 31, tv.tv_sec);
  91                 to_base10(buf + 38, tv.tv_usec);
  92                 linux_write(1, buf, sizeof(buf) - 1);
  93         } else {
  94                 linux_exit(ret);
  95         }
  96
  97         linux_exit(0);
  98 }
  99
 100 /*
 101  * This is the real entry point.  It passes the initial stack into
 102  * the C entry point.
 103  */
 104 asm (
 105         ".text\n"
 106         ".global _start\n"
 107         ".type _start,@function\n"
 108         "_start:\n\t"
 109         "mov %rsp,%rdi\n\t"
 110         "jmp c_main"
 111         );