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authorGravatar misterg <misterg@google.com>2017-09-19 16:54:40 -0400
committerGravatar misterg <misterg@google.com>2017-09-19 16:54:40 -0400
commitc2e754829628d1e9b7a16b3389cfdace76950fdf (patch)
tree5a7f056f44e27c30e10025113b644f0b3b5801fc /absl/debugging/internal/vdso_support.cc
Initial Commit
Diffstat (limited to 'absl/debugging/internal/vdso_support.cc')
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1 files changed, 177 insertions, 0 deletions
diff --git a/absl/debugging/internal/vdso_support.cc b/absl/debugging/internal/vdso_support.cc
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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
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// 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 <fcntl.h>
+#include <sys/syscall.h>
+#include <unistd.h>
+
+#include "absl/base/dynamic_annotations.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/port.h"
+
+#ifndef AT_SYSINFO_EHDR
+#define AT_SYSINFO_EHDR 33 // for crosstoolv10
+#endif
+
+namespace absl {
+namespace debug_internal {
+
+std::atomic<const void *> VDSOSupport::vdso_base_(
+ debug_internal::ElfMemImage::kInvalidBase);
+std::atomic<VDSOSupport::GetCpuFn> VDSOSupport::getcpu_fn_(&InitAndGetCPU);
+VDSOSupport::VDSOSupport()
+ // 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) ==
+ debug_internal::ElfMemImage::kInvalidBase
+ ? 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() {
+ if (vdso_base_.load(std::memory_order_relaxed) ==
+ debug_internal::ElfMemImage::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 (RunningOnValgrind()) {
+ vdso_base_.store(nullptr, std::memory_order_relaxed);
+ getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);
+ return nullptr;
+ }
+ int fd = open("/proc/self/auxv", O_RDONLY);
+ if (fd == -1) {
+ // Kernel too old to have a VDSO.
+ vdso_base_.store(nullptr, std::memory_order_relaxed);
+ getcpu_fn_.store(&GetCPUViaSyscall, 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) {
+ vdso_base_.store(reinterpret_cast<void *>(aux.a_un.a_val),
+ std::memory_order_relaxed);
+ break;
+ }
+ }
+ close(fd);
+ }
+ if (vdso_base_.load(std::memory_order_relaxed) ==
+ debug_internal::ElfMemImage::kInvalidBase) {
+ // Didn't find AT_SYSINFO_EHDR in auxv[].
+ vdso_base_.store(nullptr, 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.
+ getcpu_fn_.store(fn, std::memory_order_relaxed);
+ return vdso_base_.load(std::memory_order_relaxed);
+}
+
+const void *VDSOSupport::SetBase(const void *base) {
+ ABSL_RAW_CHECK(base != debug_internal::ElfMemImage::kInvalidBase,
+ "internal error");
+ const void *old_base = vdso_base_.load(std::memory_order_relaxed);
+ vdso_base_.store(base, std::memory_order_relaxed);
+ image_.Init(base);
+ // Also reset getcpu_fn_, so GetCPU could be tested with simulated VDSO.
+ getcpu_fn_.store(&InitAndGetCPU, 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);
+#else
+ // x86_64 never implemented sys_getcpu(), except as a VDSO call.
+ errno = ENOSYS;
+ return -1;
+#endif
+}
+
+// Use fast __vdso_getcpu if available.
+long VDSOSupport::InitAndGetCPU(unsigned *cpu, // NOLINT(runtime/int)
+ void *x, void *y) {
+ Init();
+ 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.
+ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY
+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 {
+ public:
+ VDSOInitHelper() { VDSOSupport::Init(); }
+} vdso_init_helper;
+
+} // namespace debug_internal
+} // namespace absl
+
+#endif // ABSL_HAVE_VDSO_SUPPORT