[PATCH 2/3] doc,vdso: Make vDSO examples more portable

Andy Lutomirski
2014-06-12T18:48:22Z

This adds a new vdso_test.c that's written entirely in C. It also
makes all of the vDSO examples work on 32-bit x86.

Cc: Stefani Seibold <stefani@seibold.net>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
Documentation/vDSO/parse_vdso.c | 59 +++++++++++++++------------
Documentation/vDSO/vdso_standalone_test_x86.c | 45 +++++++++++++-------
Documentation/vDSO/vdso_test.c | 50 +++++++++++++++++++++++
3 files changed, 113 insertions(+), 41 deletions(-)
create mode 100644 Documentation/vDSO/vdso_test.c

diff --git a/Documentation/vDSO/parse_vdso.c b/Documentation/vDSO/parse_vdso.c
index 8587020..ca23f06 100644
--- a/Documentation/vDSO/parse_vdso.c
+++ b/Documentation/vDSO/parse_vdso.c
@@ -1,6 +1,6 @@
/*
* parse_vdso.c: Linux reference vDSO parser
- * Written by Andrew Lutomirski, 2011.
+ * Written by Andrew Lutomirski, 2011-2014.
*
* This code is meant to be linked in to various programs that run on Linux.
* As such, it is available with as few restrictions as possible. This file
@@ -11,7 +11,7 @@
* it starts a program. It works equally well in statically and dynamically
* linked binaries.
*
- * This code is tested on x86_64. In principle it should work on any 64-bit
+ * This code is tested on x86. In principle it should work on any
* architecture that has a vDSO.
*/

@@ -20,6 +20,8 @@
#include <string.h>
#include <elf.h>

+#include <asm/bitsperlong.h>
+
/*
* To use this vDSO parser, first call one of the vdso_init_* functions.
* If you've already parsed auxv, then pass the value of AT_SYSINFO_EHDR
@@ -45,11 +47,14 @@ extern void *vdso_sym(const char *version, const char *name);

/* And here's the code. */
-
-#ifndef __x86_64__
-# error Not yet ported to non-x86_64 architectures
+#ifndef __BITS_PER_LONG
+#error We need __BITS_PER_LONG
#endif

+#define ELF_BITS2(bits, x) Elf##bits##_##x
+#define ELF_BITS(bits, x) ELF_BITS2(bits, x)
+#define ELF(x) ELF_BITS(__BITS_PER_LONG, x)
+
static struct vdso_info
{
bool valid;
@@ -59,14 +64,14 @@ static struct vdso_info
uintptr_t load_offset; /* load_addr - recorded vaddr */

/* Symbol table */
- Elf64_Sym *symtab;
+ ELF(Sym) *symtab;
const char *symstrings;
- Elf64_Word *bucket, *chain;
- Elf64_Word nbucket, nchain;
+ ELF(Word) *bucket, *chain;
+ ELF(Word) nbucket, nchain;

/* Version table */
- Elf64_Versym *versym;
- Elf64_Verdef *verdef;
+ ELF(Versym) *versym;
+ ELF(Verdef) *verdef;
} vdso_info;

/* Straight from the ELF specification. */
@@ -92,9 +97,9 @@ void vdso_init_from_sysinfo_ehdr(uintptr_t base)

vdso_info.load_addr = base;

- Elf64_Ehdr *hdr = (Elf64_Ehdr*)base;
- Elf64_Phdr *pt = (Elf64_Phdr*)(vdso_info.load_addr + hdr->e_phoff);
- Elf64_Dyn *dyn = 0;
+ ELF(Ehdr) *hdr = (ELF(Ehdr)*)base;
+ ELF(Phdr) *pt = (ELF(Phdr)*)(vdso_info.load_addr + hdr->e_phoff);
+ ELF(Dyn) *dyn = 0;

/*
* We need two things from the segment table: the load offset
@@ -108,7 +113,7 @@ void vdso_init_from_sysinfo_ehdr(uintptr_t base)
+ (uintptr_t)pt[i].p_offset
- (uintptr_t)pt[i].p_vaddr;
} else if (pt[i].p_type == PT_DYNAMIC) {
- dyn = (Elf64_Dyn*)(base + pt[i].p_offset);
+ dyn = (ELF(Dyn)*)(base + pt[i].p_offset);
}
}

@@ -118,7 +123,7 @@ void vdso_init_from_sysinfo_ehdr(uintptr_t base)
/*
* Fish out the useful bits of the dynamic table.
*/
- Elf64_Word *hash = 0;
+ ELF(Word) *hash = 0;
vdso_info.symstrings = 0;
vdso_info.symtab = 0;
vdso_info.versym = 0;
@@ -131,22 +136,22 @@ void vdso_init_from_sysinfo_ehdr(uintptr_t base)
+ vdso_info.load_offset);
break;
case DT_SYMTAB:
- vdso_info.symtab = (Elf64_Sym *)
+ vdso_info.symtab = (ELF(Sym) *)
((uintptr_t)dyn[i].d_un.d_ptr
+ vdso_info.load_offset);
break;
case DT_HASH:
- hash = (Elf64_Word *)
+ hash = (ELF(Word) *)
((uintptr_t)dyn[i].d_un.d_ptr
+ vdso_info.load_offset);
break;
case DT_VERSYM:
- vdso_info.versym = (Elf64_Versym *)
+ vdso_info.versym = (ELF(Versym) *)
((uintptr_t)dyn[i].d_un.d_ptr
+ vdso_info.load_offset);
break;
case DT_VERDEF:
- vdso_info.verdef = (Elf64_Verdef *)
+ vdso_info.verdef = (ELF(Verdef) *)
((uintptr_t)dyn[i].d_un.d_ptr
+ vdso_info.load_offset);
break;
@@ -168,8 +173,8 @@ void vdso_init_from_sysinfo_ehdr(uintptr_t base)
vdso_info.valid = true;
}

-static bool vdso_match_version(Elf64_Versym ver,
- const char *name, Elf64_Word hash)
+static bool vdso_match_version(ELF(Versym) ver,
+ const char *name, ELF(Word) hash)
{
/*
* This is a helper function to check if the version indexed by
@@ -188,7 +193,7 @@ static bool vdso_match_version(Elf64_Versym ver,

/* First step: find the version definition */
ver &= 0x7fff; /* Apparently bit 15 means "hidden" */
- Elf64_Verdef *def = vdso_info.verdef;
+ ELF(Verdef) *def = vdso_info.verdef;
while(true) {
if ((def->vd_flags & VER_FLG_BASE) == 0
&& (def->vd_ndx & 0x7fff) == ver)
@@ -197,11 +202,11 @@ static bool vdso_match_version(Elf64_Versym ver,
if (def->vd_next == 0)
return false; /* No definition. */

- def = (Elf64_Verdef *)((char *)def + def->vd_next);
+ def = (ELF(Verdef) *)((char *)def + def->vd_next);
}

/* Now figure out whether it matches. */
- Elf64_Verdaux *aux = (Elf64_Verdaux*)((char *)def + def->vd_aux);
+ ELF(Verdaux) *aux = (ELF(Verdaux)*)((char *)def + def->vd_aux);
return def->vd_hash == hash
&& !strcmp(name, vdso_info.symstrings + aux->vda_name);
}
@@ -213,10 +218,10 @@ void *vdso_sym(const char *version, const char *name)
return 0;

ver_hash = elf_hash(version);
- Elf64_Word chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];
+ ELF(Word) chain = vdso_info.bucket[elf_hash(name) % vdso_info.nbucket];

for (; chain != STN_UNDEF; chain = vdso_info.chain[chain]) {
- Elf64_Sym *sym = &vdso_info.symtab[chain];
+ ELF(Sym) *sym = &vdso_info.symtab[chain];

/* Check for a defined global or weak function w/ right name. */
if (ELF64_ST_TYPE(sym->st_info) != STT_FUNC)
@@ -243,7 +248,7 @@ void *vdso_sym(const char *version, const char *name)

void vdso_init_from_auxv(void *auxv)
{
- Elf64_auxv_t *elf_auxv = auxv;
+ ELF(auxv_t) *elf_auxv = auxv;
for (int i = 0; elf_auxv[i].a_type != AT_NULL; i++)
{
if (elf_auxv[i].a_type == AT_SYSINFO_EHDR) {
diff --git a/Documentation/vDSO/vdso_standalone_test_x86.c b/Documentation/vDSO/vdso_standalone_test_x86.c
index 1523f5d..d462402 100644
--- a/Documentation/vDSO/vdso_standalone_test_x86.c
+++ b/Documentation/vDSO/vdso_standalone_test_x86.c
@@ -1,13 +1,14 @@
/*
- * vdso_test.c: Sample code to test parse_vdso.c on x86_64
- * Copyright (c) 2011 Andy Lutomirski
+ * vdso_test.c: Sample code to test parse_vdso.c on x86
+ * Copyright (c) 2011-2014 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
+ * -Os -fno-asynchronous-unwind-tables -flto -lgcc_s
* vdso_standalone_test_x86.c parse_vdso.c
- * to generate a small binary with no dependencies at all.
+ * to generate a small binary. On x86_64, you can omit -lgcc_s
+ * if you want the binary to be completely standalone.
*/

#include <sys/syscall.h>
@@ -35,21 +36,31 @@ int strcmp(const char *a, const char *b)
return 0;
}

-/* ...and two syscalls. This is x86_64-specific. */
-static inline long linux_write(int fd, const void *data, size_t len)
+/* ...and two syscalls. This is x86-specific. */
+static inline long x86_syscall3(long nr, long a0, long a1, long a2)
{
-
long ret;
- asm volatile ("syscall" : "=a" (ret) : "a" (__NR_write),
- "D" (fd), "S" (data), "d" (len) :
+#ifdef __x86_64__
+ asm volatile ("syscall" : "=a" (ret) : "a" (nr),
+ "D" (a0), "S" (a1), "d" (a2) :
"cc", "memory", "rcx",
"r8", "r9", "r10", "r11" );
+#else
+ asm volatile ("int $0x80" : "=a" (ret) : "a" (nr),
+ "b" (a0), "c" (a1), "d" (a2) :
+ "cc", "memory" );
+#endif
return ret;
}

+static inline long linux_write(int fd, const void *data, size_t len)
+{
+ return x86_syscall3(__NR_write, fd, (long)data, (long)len);
+}
+
static inline void linux_exit(int code)
{
- asm volatile ("syscall" : : "a" (__NR_exit), "D" (code));
+ x86_syscall3(__NR_exit, code, 0, 0);
}

void to_base10(char *lastdig, uint64_t n)
@@ -104,8 +115,14 @@ __attribute__((externally_visible)) void c_main(void **stack)
asm (
".text\n"
".global _start\n"
- ".type _start,@function\n"
- "_start:\n\t"
- "mov %rsp,%rdi\n\t"
- "jmp c_main"
+ ".type _start,@function\n"
+ "_start:\n\t"
+#ifdef __x86_64__
+ "mov %rsp,%rdi\n\t"
+ "jmp c_main"
+#else
+ "push %esp\n\t"
+ "call c_main\n\t"
+ "int $3"
+#endif
);
diff --git a/Documentation/vDSO/vdso_test.c b/Documentation/vDSO/vdso_test.c
new file mode 100644
index 0000000..75b1da2
--- /dev/null
+++ b/Documentation/vDSO/vdso_test.c
@@ -0,0 +1,50 @@
+/*
+ * vdso_test.c: Sample code to test parse_vdso.c
+ * Copyright (c) 2014 Andy Lutomirski
+ * Subject to the GNU General Public License, version 2
+ *
+ * Compile with:
+ * gcc -std=gnu99 vdso_test.c parse_vdso.c
+ *
+ * Tested on x86, 32-bit and 64-bit. It may work on other architectures, too.
+ */
+
+#include <stdint.h>
+#include <elf.h>
+#include <stdio.h>
+#include <sys/auxv.h>
+#include <sys/time.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);
+
+int main(int argc, char **argv)
+{
+ unsigned long sysinfo_ehdr = getauxval(AT_SYSINFO_EHDR);
+ if (!sysinfo_ehdr) {
+ printf("AT_SYSINFO_EHDR is not present!\n");
+ return 0;
+ }
+
+ vdso_init_from_sysinfo_ehdr(getauxval(AT_SYSINFO_EHDR));
+
+ /* 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)
+ printf("Could not find __vdso_gettimeofday\n");
+
+ struct timeval tv;
+ long ret = gtod(&tv, 0);
+
+ if (ret == 0) {
+ printf("The time is %lld.%06lld\n",
+ (long long)tv.tv_sec, (long long)tv.tv_usec);
+ } else {
+ printf("__vdso_gettimeofday failed\n");
+ }
+
+ return 0;
+}
--
1.9.3