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// Copyright 2017 The Abseil Authors.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// Allow dynamic symbol lookup in the kernel VDSO page.
// VDSOSupport -- a class representing kernel VDSO (if present).
#include "absl/debugging/internal/vdso_support.h"
#ifdef ABSL_HAVE_VDSO_SUPPORT // defined in vdso_support.h
#include <errno.h>
#include <fcntl.h>
#include <sys/syscall.h>
#include <unistd.h>
#if __GLIBC_PREREQ(2, 16) // GLIBC-2.16 implements getauxval.
#include <sys/auxv.h>
#include "absl/base/dynamic_annotations.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/base/port.h"
#define AT_SYSINFO_EHDR 33 // for crosstoolv10
namespace absl {
namespace debugging_internal {
std::atomic<const void *> VDSOSupport::vdso_base_(
std::atomic<VDSOSupport::GetCpuFn> VDSOSupport::getcpu_fn_(&InitAndGetCPU);
// If vdso_base_ is still set to kInvalidBase, we got here
// before VDSOSupport::Init has been called. Call it now.
: image_(vdso_base_.load(std::memory_order_relaxed) ==
? Init()
: vdso_base_.load(std::memory_order_relaxed)) {}
// NOTE: we can't use GoogleOnceInit() below, because we can be
// called by tcmalloc, and none of the *once* stuff may be functional yet.
// In addition, we hope that the VDSOSupportHelper constructor
// causes this code to run before there are any threads, and before
// InitGoogle() has executed any chroot or setuid calls.
// Finally, even if there is a race here, it is harmless, because
// the operation should be idempotent.
const void *VDSOSupport::Init() {
const auto kInvalidBase = debugging_internal::ElfMemImage::kInvalidBase;
#if __GLIBC_PREREQ(2, 16)
if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
errno = 0;
const void *const sysinfo_ehdr =
reinterpret_cast<const void *>(getauxval(AT_SYSINFO_EHDR));
if (errno == 0) {, std::memory_order_relaxed);
#endif // __GLIBC_PREREQ(2, 16)
if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
// Valgrind zaps AT_SYSINFO_EHDR and friends from the auxv[]
// on stack, and so glibc works as if VDSO was not present.
// But going directly to kernel via /proc/self/auxv below bypasses
// Valgrind zapping. So we check for Valgrind separately.
if (AbslRunningOnValgrind()) {, std::memory_order_relaxed);, std::memory_order_relaxed);
return nullptr;
int fd = open("/proc/self/auxv", O_RDONLY);
if (fd == -1) {
// Kernel too old to have a VDSO., std::memory_order_relaxed);, std::memory_order_relaxed);
return nullptr;
ElfW(auxv_t) aux;
while (read(fd, &aux, sizeof(aux)) == sizeof(aux)) {
if (aux.a_type == AT_SYSINFO_EHDR) {<void *>(aux.a_un.a_val),
if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
// Didn't find AT_SYSINFO_EHDR in auxv[]., std::memory_order_relaxed);
GetCpuFn fn = &GetCPUViaSyscall; // default if VDSO not present.
if (vdso_base_.load(std::memory_order_relaxed)) {
VDSOSupport vdso;
SymbolInfo info;
if (vdso.LookupSymbol("__vdso_getcpu", "LINUX_2.6", STT_FUNC, &info)) {
fn = reinterpret_cast<GetCpuFn>(const_cast<void *>(info.address));
// Subtle: this code runs outside of any locks; prevent compiler
// from assigning to getcpu_fn_ more than once., std::memory_order_relaxed);
return vdso_base_.load(std::memory_order_relaxed);
const void *VDSOSupport::SetBase(const void *base) {
ABSL_RAW_CHECK(base != debugging_internal::ElfMemImage::kInvalidBase,
"internal error");
const void *old_base = vdso_base_.load(std::memory_order_relaxed);, std::memory_order_relaxed);
// Also reset getcpu_fn_, so GetCPU could be tested with simulated VDSO., std::memory_order_relaxed);
return old_base;
bool VDSOSupport::LookupSymbol(const char *name,
const char *version,
int type,
SymbolInfo *info) const {
return image_.LookupSymbol(name, version, type, info);
bool VDSOSupport::LookupSymbolByAddress(const void *address,
SymbolInfo *info_out) const {
return image_.LookupSymbolByAddress(address, info_out);
// NOLINT on 'long' because this routine mimics kernel api.
long VDSOSupport::GetCPUViaSyscall(unsigned *cpu, // NOLINT(runtime/int)
void *, void *) {
#ifdef SYS_getcpu
return syscall(SYS_getcpu, cpu, nullptr, nullptr);
// x86_64 never implemented sys_getcpu(), except as a VDSO call.
static_cast<void>(cpu); // Avoid an unused argument compiler warning.
errno = ENOSYS;
return -1;
// Use fast __vdso_getcpu if available.
long VDSOSupport::InitAndGetCPU(unsigned *cpu, // NOLINT(runtime/int)
void *x, void *y) {
GetCpuFn fn = getcpu_fn_.load(std::memory_order_relaxed);
ABSL_RAW_CHECK(fn != &InitAndGetCPU, "Init() did not set getcpu_fn_");
return (*fn)(cpu, x, y);
// This function must be very fast, and may be called from very
// low level (e.g. tcmalloc). Hence I avoid things like
// GoogleOnceInit() and ::operator new.
int GetCPU() {
unsigned cpu;
int ret_code = (*VDSOSupport::getcpu_fn_)(&cpu, nullptr, nullptr);
return ret_code == 0 ? cpu : ret_code;
// We need to make sure VDSOSupport::Init() is called before
// InitGoogle() does any setuid or chroot calls. If VDSOSupport
// is used in any global constructor, this will happen, since
// VDSOSupport's constructor calls Init. But if not, we need to
// ensure it here, with a global constructor of our own. This
// is an allowed exception to the normal rule against non-trivial
// global constructors.
static class VDSOInitHelper {
VDSOInitHelper() { VDSOSupport::Init(); }
} vdso_init_helper;
} // namespace debugging_internal
} // namespace absl