1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * vdso_test.c: Sample code to test parse_vdso.c on x86
4  * Copyright (c) 2011-2014 Andy Lutomirski
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 #include "parse_vdso.h"
20 
21 /* We need a libc functions... */
22 int strcmp(const char *a, const char *b)
23 {
24 	/* This implementation is buggy: it never returns -1. */
25 	while (*a || *b) {
26 		if (*a != *b)
27 			return 1;
28 		if (*a == 0 || *b == 0)
29 			return 1;
30 		a++;
31 		b++;
32 	}
33 
34 	return 0;
35 }
36 
37 /* ...and two syscalls.  This is x86-specific. */
38 static inline long x86_syscall3(long nr, long a0, long a1, long a2)
39 {
40 	long ret;
41 #ifdef __x86_64__
42 	asm volatile ("syscall" : "=a" (ret) : "a" (nr),
43 		      "D" (a0), "S" (a1), "d" (a2) :
44 		      "cc", "memory", "rcx",
45 		      "r8", "r9", "r10", "r11" );
46 #else
47 	asm volatile ("int $0x80" : "=a" (ret) : "a" (nr),
48 		      "b" (a0), "c" (a1), "d" (a2) :
49 		      "cc", "memory" );
50 #endif
51 	return ret;
52 }
53 
54 static inline long linux_write(int fd, const void *data, size_t len)
55 {
56 	return x86_syscall3(__NR_write, fd, (long)data, (long)len);
57 }
58 
59 static inline void linux_exit(int code)
60 {
61 	x86_syscall3(__NR_exit, code, 0, 0);
62 }
63 
64 void to_base10(char *lastdig, time_t n)
65 {
66 	while (n) {
67 		*lastdig = (n % 10) + '0';
68 		n /= 10;
69 		lastdig--;
70 	}
71 }
72 
73 __attribute__((externally_visible)) void c_main(void **stack)
74 {
75 	/* Parse the stack */
76 	long argc = (long)*stack;
77 	stack += argc + 2;
78 
79 	/* Now we're pointing at the environment.  Skip it. */
80 	while(*stack)
81 		stack++;
82 	stack++;
83 
84 	/* Now we're pointing at auxv.  Initialize the vDSO parser. */
85 	vdso_init_from_auxv((void *)stack);
86 
87 	/* Find gettimeofday. */
88 	typedef long (*gtod_t)(struct timeval *tv, struct timezone *tz);
89 	gtod_t gtod = (gtod_t)vdso_sym("LINUX_2.6", "__vdso_gettimeofday");
90 
91 	if (!gtod)
92 		linux_exit(1);
93 
94 	struct timeval tv;
95 	long ret = gtod(&tv, 0);
96 
97 	if (ret == 0) {
98 		char buf[] = "The time is                     .000000\n";
99 		to_base10(buf + 31, tv.tv_sec);
100 		to_base10(buf + 38, tv.tv_usec);
101 		linux_write(1, buf, sizeof(buf) - 1);
102 	} else {
103 		linux_exit(ret);
104 	}
105 
106 	linux_exit(0);
107 }
108 
109 /*
110  * This is the real entry point.  It passes the initial stack into
111  * the C entry point.
112  */
113 asm (
114 	".text\n"
115 	".global _start\n"
116 	".type _start,@function\n"
117 	"_start:\n\t"
118 #ifdef __x86_64__
119 	"mov %rsp,%rdi\n\t"
120 	"jmp c_main"
121 #else
122 	"push %esp\n\t"
123 	"call c_main\n\t"
124 	"int $3"
125 #endif
126 	);
127