diff options
author | Andy Lutomirski <luto@mit.edu> | 2011-07-13 09:24:16 -0400 |
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committer | H. Peter Anvin <hpa@linux.intel.com> | 2011-07-14 17:57:09 -0700 |
commit | 98eedc3a9dbf90cecb91093d2a7fa083942b7d13 (patch) | |
tree | 0ed9320faed2d62caea337b978d5216a7fea55a8 /Documentation/vDSO/vdso_test.c | |
parent | 574c44fa8fa6262ffd5939789ef51a6e98ed62d7 (diff) |
Document the vDSO and add a reference parser
It turns out that parsing the vDSO is nontrivial if you don't already
have an ELF dynamic loader around. So document it in Documentation/ABI
and add a reference CC0-licenced parser.
This code is dedicated to Go issue 1933:
http://code.google.com/p/go/issues/detail?id=1933
Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/a315a9514cd71bcf29436cc31e35aada21a5ff21.1310563276.git.luto@mit.edu
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Diffstat (limited to 'Documentation/vDSO/vdso_test.c')
-rw-r--r-- | Documentation/vDSO/vdso_test.c | 111 |
1 files changed, 111 insertions, 0 deletions
diff --git a/Documentation/vDSO/vdso_test.c b/Documentation/vDSO/vdso_test.c new file mode 100644 index 00000000000..fff633432df --- /dev/null +++ b/Documentation/vDSO/vdso_test.c @@ -0,0 +1,111 @@ +/* + * 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\n"; + 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\n" + ".global _start\n" + ".type _start,@function\n" + "_start:\n\t" + "mov %rsp,%rdi\n\t" + "jmp c_main" + ); |