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Documentation/vDSO/vdso_test.c
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Document the vDSO...
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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
";
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
"
".global _start
"
".type _start,@function
"
"_start:
\t"
"mov %rsp,%rdi
\t"
"jmp c_main"
);
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