Documentation/vDSO/vdso_standalone_test_x86.c

   1 /*
   2  * vdso_test.c: Sample code to test parse_vdso.c on x86
   3  * Copyright (c) 2011-2014 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 -lgcc_s
   9  *      vdso_standalone_test_x86.c parse_vdso.c
  10  * to generate a small binary.  On x86_64, you can omit -lgcc_s
  11  * if you want the binary to be completely standalone.
  12  */
  13
  14 #include <sys/syscall.h>
  15 #include <sys/time.h>
  16 #include <unistd.h>
  17 #include <stdint.h>
  18
  19 extern void *vdso_sym(const char *version, const char *name);
  20 extern void vdso_init_from_sysinfo_ehdr(uintptr_t base);
  21 extern void vdso_init_from_auxv(void *auxv);
  22
  23 /* We need a libc functions... */
  24 int strcmp(const char *a, const char *b)
  25 {
  26         /* This implementation is buggy: it never returns -1. */
  27         while (*a || *b) {
  28                 if (*a != *b)
  29                         return 1;
  30                 if (*a == 0 || *b == 0)
  31                         return 1;
  32                 a++;
  33                 b++;
  34         }
  35
  36         return 0;
  37 }
  38
  39 /* ...and two syscalls.  This is x86-specific. */
  40 static inline long x86_syscall3(long nr, long a0, long a1, long a2)
  41 {
  42         long ret;
  43 #ifdef __x86_64__
  44         asm volatile ("syscall" : "=a" (ret) : "a" (nr),
  45                       "D" (a0), "S" (a1), "d" (a2) :
  46                       "cc", "memory", "rcx",
  47                       "r8", "r9", "r10", "r11" );
  48 #else
  49         asm volatile ("int $0x80" : "=a" (ret) : "a" (nr),
  50                       "b" (a0), "c" (a1), "d" (a2) :
  51                       "cc", "memory" );
  52 #endif
  53         return ret;
  54 }
  55
  56 static inline long linux_write(int fd, const void *data, size_t len)
  57 {
  58         return x86_syscall3(__NR_write, fd, (long)data, (long)len);
  59 }
  60
  61 static inline void linux_exit(int code)
  62 {
  63         x86_syscall3(__NR_exit, code, 0, 0);
  64 }
  65
  66 void to_base10(char *lastdig, time_t n)
  67 {
  68         while (n) {
  69                 *lastdig = (n % 10) + '0';
  70                 n /= 10;
  71                 lastdig--;
  72         }
  73 }
  74
  75 __attribute__((externally_visible)) void c_main(void **stack)
  76 {
  77         /* Parse the stack */
  78         long argc = (long)*stack;
  79         stack += argc + 2;
  80
  81         /* Now we're pointing at the environment.  Skip it. */
  82         while(*stack)
  83                 stack++;
  84         stack++;
  85
  86         /* Now we're pointing at auxv.  Initialize the vDSO parser. */
  87         vdso_init_from_auxv((void *)stack);
  88
  89         /* Find gettimeofday. */
  90         typedef long (*gtod_t)(struct timeval *tv, struct timezone *tz);
  91         gtod_t gtod = (gtod_t)vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
  92
  93         if (!gtod)
  94                 linux_exit(1);
  95
  96         struct timeval tv;
  97         long ret = gtod(&tv, 0);
  98
  99         if (ret == 0) {
 100                 char buf[] = "The time is                     .000000\n";
 101                 to_base10(buf + 31, tv.tv_sec);
 102                 to_base10(buf + 38, tv.tv_usec);
 103                 linux_write(1, buf, sizeof(buf) - 1);
 104         } else {
 105                 linux_exit(ret);
 106         }
 107
 108         linux_exit(0);
 109 }
 110
 111 /*
 112  * This is the real entry point.  It passes the initial stack into
 113  * the C entry point.
 114  */
 115 asm (
 116         ".text\n"
 117         ".global _start\n"
 118         ".type _start,@function\n"
 119         "_start:\n\t"
 120 #ifdef __x86_64__
 121         "mov %rsp,%rdi\n\t"
 122         "jmp c_main"
 123 #else
 124         "push %esp\n\t"
 125         "call c_main\n\t"
 126         "int $3"
 127 #endif
 128         );