blob: 8627db24a7f6a25993bc3cb5a53157cab7c7df1a [file] [log] [blame]
* vdso2c - A vdso image preparation tool
* Copyright (c) 2014 Andy Lutomirski and others
* Licensed under the GPL v2
* vdso2c requires stripped and unstripped input. It would be trivial
* to fully strip the input in here, but, for reasons described below,
* we need to write a section table. Doing this is more or less
* equivalent to dropping all non-allocatable sections, but it's
* easier to let objcopy handle that instead of doing it ourselves.
* If we ever need to do something fancier than what objcopy provides,
* it would be straightforward to add here.
* We're keep a section table for a few reasons:
* The Go runtime had a couple of bugs: it would read the section
* table to try to figure out how many dynamic symbols there were (it
* shouldn't have looked at the section table at all) and, if there
* were no SHT_SYNDYM section table entry, it would use an
* uninitialized value for the number of symbols. An empty DYNSYM
* table would work, but I see no reason not to write a valid one (and
* keep full performance for old Go programs). This hack is only
* needed on x86_64.
* The bug was introduced on 2012-08-31 by:
* and was fixed on 2014-06-13 by:
* Binutils has issues debugging the vDSO: it reads the section table to
* find SHT_NOTE; it won't look at PT_NOTE for the in-memory vDSO, which
* would break build-id if we removed the section table. Binutils
* also requires that shstrndx != 0. See:
* elfutils might not look for PT_NOTE if there is a section table at
* all. I don't know whether this matters for any practical purpose.
* For simplicity, rather than hacking up a partial section table, we
* just write a mostly complete one. We omit non-dynamic symbols,
* though, since they're rather large.
* Once binutils gets fixed, we might be able to drop this for all but
* the 64-bit vdso, since build-id only works in kernel RPMs, and
* systems that update to new enough kernel RPMs will likely update
* binutils in sync. build-id has never worked for home-built kernel
* RPMs without manual symlinking, and I suspect that no one ever does
* that.
#include <inttypes.h>
#include <stdint.h>
#include <unistd.h>
#include <stdarg.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <err.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <tools/le_byteshift.h>
#include <linux/elf.h>
#include <linux/types.h>
const char *outfilename;
/* Symbols that we need in vdso2c. */
enum {
const int special_pages[] = {
struct vdso_sym {
const char *name;
bool export;
struct vdso_sym required_syms[] = {
[sym_vvar_start] = {"vvar_start", true},
[sym_vvar_page] = {"vvar_page", true},
[sym_hpet_page] = {"hpet_page", true},
{"VDSO32_NOTE_MASK", true},
{"__kernel_vsyscall", true},
{"__kernel_sigreturn", true},
{"__kernel_rt_sigreturn", true},
__attribute__((format(printf, 1, 2))) __attribute__((noreturn))
static void fail(const char *format, ...)
va_list ap;
va_start(ap, format);
fprintf(stderr, "Error: ");
vfprintf(stderr, format, ap);
if (outfilename)
* Evil macros for little-endian reads and writes
#define GLE(x, bits, ifnot) \
__builtin_choose_expr( \
(sizeof(*(x)) == bits/8), \
(__typeof__(*(x)))get_unaligned_le##bits(x), ifnot)
extern void bad_get_le(void);
#define LAST_GLE(x) \
__builtin_choose_expr(sizeof(*(x)) == 1, *(x), bad_get_le())
#define GET_LE(x) \
GLE(x, 64, GLE(x, 32, GLE(x, 16, LAST_GLE(x))))
#define PLE(x, val, bits, ifnot) \
__builtin_choose_expr( \
(sizeof(*(x)) == bits/8), \
put_unaligned_le##bits((val), (x)), ifnot)
extern void bad_put_le(void);
#define LAST_PLE(x, val) \
__builtin_choose_expr(sizeof(*(x)) == 1, *(x) = (val), bad_put_le())
#define PUT_LE(x, val) \
PLE(x, val, 64, PLE(x, val, 32, PLE(x, val, 16, LAST_PLE(x, val))))
#define NSYMS (sizeof(required_syms) / sizeof(required_syms[0]))
#define BITSFUNC3(name, bits, suffix) name##bits##suffix
#define BITSFUNC2(name, bits, suffix) BITSFUNC3(name, bits, suffix)
#define BITSFUNC(name) BITSFUNC2(name, ELF_BITS, )
#define INT_BITS BITSFUNC2(int, ELF_BITS, _t)
#define ELF_BITS_XFORM2(bits, x) Elf##bits##_##x
#define ELF_BITS_XFORM(bits, x) ELF_BITS_XFORM2(bits, x)
#define ELF_BITS 64
#include "vdso2c.h"
#undef ELF_BITS
#define ELF_BITS 32
#include "vdso2c.h"
#undef ELF_BITS
static void go(void *raw_addr, size_t raw_len,
void *stripped_addr, size_t stripped_len,
FILE *outfile, const char *name)
Elf64_Ehdr *hdr = (Elf64_Ehdr *)raw_addr;
if (hdr->e_ident[EI_CLASS] == ELFCLASS64) {
go64(raw_addr, raw_len, stripped_addr, stripped_len,
outfile, name);
} else if (hdr->e_ident[EI_CLASS] == ELFCLASS32) {
go32(raw_addr, raw_len, stripped_addr, stripped_len,
outfile, name);
} else {
fail("unknown ELF class\n");
static void map_input(const char *name, void **addr, size_t *len, int prot)
off_t tmp_len;
int fd = open(name, O_RDONLY);
if (fd == -1)
err(1, "%s", name);
tmp_len = lseek(fd, 0, SEEK_END);
if (tmp_len == (off_t)-1)
err(1, "lseek");
*len = (size_t)tmp_len;
*addr = mmap(NULL, tmp_len, prot, MAP_PRIVATE, fd, 0);
if (*addr == MAP_FAILED)
err(1, "mmap");
int main(int argc, char **argv)
size_t raw_len, stripped_len;
void *raw_addr, *stripped_addr;
FILE *outfile;
char *name, *tmp;
int namelen;
if (argc != 4) {
printf("Usage: vdso2c RAW_INPUT STRIPPED_INPUT OUTPUT\n");
return 1;
* Figure out the struct name. If we're writing to a .so file,
* generate raw output insted.
name = strdup(argv[3]);
namelen = strlen(name);
if (namelen >= 3 && !strcmp(name + namelen - 3, ".so")) {
name = NULL;
} else {
tmp = strrchr(name, '/');
if (tmp)
name = tmp + 1;
tmp = strchr(name, '.');
if (tmp)
*tmp = '\0';
for (tmp = name; *tmp; tmp++)
if (*tmp == '-')
*tmp = '_';
map_input(argv[1], &raw_addr, &raw_len, PROT_READ);
map_input(argv[2], &stripped_addr, &stripped_len, PROT_READ);
outfilename = argv[3];
outfile = fopen(outfilename, "w");
if (!outfile)
err(1, "%s", argv[2]);
go(raw_addr, raw_len, stripped_addr, stripped_len, outfile, name);
munmap(raw_addr, raw_len);
munmap(stripped_addr, stripped_len);
return 0;