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... */
strcmp(const char * a,const char * b)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. */
linux_write(int fd,const void * data,size_t len)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
linux_exit(int code)50 static inline void linux_exit(int code)
51 {
52 asm volatile ("syscall" : : "a" (__NR_exit), "D" (code));
53 }
54
to_base10(char * lastdig,uint64_t n)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
c_main(void ** stack)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 );
112