Initial Commit

absl/debugging/internal/vdso_support.h

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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.
+//
+// VDSO stands for "Virtual Dynamic Shared Object" -- a page of
+// executable code, which looks like a shared library, but doesn't
+// necessarily exist anywhere on disk, and which gets mmap()ed into
+// every process by kernels which support VDSO, such as 2.6.x for 32-bit
+// executables, and 2.6.24 and above for 64-bit executables.
+//
+// More details could be found here:
+// http://www.trilithium.com/johan/2005/08/linux-gate/
+//
+// VDSOSupport -- a class representing kernel VDSO (if present).
+//
+// Example usage:
+//  VDSOSupport vdso;
+//  VDSOSupport::SymbolInfo info;
+//  typedef (*FN)(unsigned *, void *, void *);
+//  FN fn = nullptr;
+//  if (vdso.LookupSymbol("__vdso_getcpu", "LINUX_2.6", STT_FUNC, &info)) {
+//     fn = reinterpret_cast<FN>(info.address);
+//  }
+
+#ifndef ABSL_DEBUGGING_INTERNAL_VDSO_SUPPORT_H_
+#define ABSL_DEBUGGING_INTERNAL_VDSO_SUPPORT_H_
+
+#include <atomic>
+
+#include "absl/debugging/internal/elf_mem_image.h"
+
+#ifdef ABSL_HAVE_ELF_MEM_IMAGE
+
+#ifdef ABSL_HAVE_VDSO_SUPPORT
+#error ABSL_HAVE_VDSO_SUPPORT cannot be directly set
+#else
+#define ABSL_HAVE_VDSO_SUPPORT 1
+#endif
+
+namespace absl {
+namespace debug_internal {
+
+// NOTE: this class may be used from within tcmalloc, and can not
+// use any memory allocation routines.
+class VDSOSupport {
+ public:
+  VDSOSupport();
+
+  typedef ElfMemImage::SymbolInfo SymbolInfo;
+  typedef ElfMemImage::SymbolIterator SymbolIterator;
+
+  // On PowerPC64 VDSO symbols can either be of type STT_FUNC or STT_NOTYPE
+  // depending on how the kernel is built.  The kernel is normally built with
+  // STT_NOTYPE type VDSO symbols.  Let's make things simpler first by using a
+  // compile-time constant.
+#ifdef __powerpc64__
+  enum { kVDSOSymbolType = STT_NOTYPE };
+#else
+  enum { kVDSOSymbolType = STT_FUNC };
+#endif
+
+  // Answers whether we have a vdso at all.
+  bool IsPresent() const { return image_.IsPresent(); }
+
+  // Allow to iterate over all VDSO symbols.
+  SymbolIterator begin() const { return image_.begin(); }
+  SymbolIterator end() const { return image_.end(); }
+
+  // Look up versioned dynamic symbol in the kernel VDSO.
+  // Returns false if VDSO is not present, or doesn't contain given
+  // symbol/version/type combination.
+  // If info_out != nullptr, additional details are filled in.
+  bool LookupSymbol(const char *name, const char *version,
+                    int symbol_type, SymbolInfo *info_out) const;
+
+  // Find info about symbol (if any) which overlaps given address.
+  // Returns true if symbol was found; false if VDSO isn't present
+  // or doesn't have a symbol overlapping given address.
+  // If info_out != nullptr, additional details are filled in.
+  bool LookupSymbolByAddress(const void *address, SymbolInfo *info_out) const;
+
+  // Used only for testing. Replace real VDSO base with a mock.
+  // Returns previous value of vdso_base_. After you are done testing,
+  // you are expected to call SetBase() with previous value, in order to
+  // reset state to the way it was.
+  const void *SetBase(const void *s);
+
+  // Computes vdso_base_ and returns it. Should be called as early as
+  // possible; before any thread creation, chroot or setuid.
+  static const void *Init();
+
+ private:
+  // image_ represents VDSO ELF image in memory.
+  // image_.ehdr_ == nullptr implies there is no VDSO.
+  ElfMemImage image_;
+
+  // Cached value of auxv AT_SYSINFO_EHDR, computed once.
+  // This is a tri-state:
+  //   kInvalidBase   => value hasn't been determined yet.
+  //              0   => there is no VDSO.
+  //           else   => vma of VDSO Elf{32,64}_Ehdr.
+  //
+  // When testing with mock VDSO, low bit is set.
+  // The low bit is always available because vdso_base_ is
+  // page-aligned.
+  static std::atomic<const void *> vdso_base_;
+
+  // NOLINT on 'long' because these routines mimic kernel api.
+  // The 'cache' parameter may be used by some versions of the kernel,
+  // and should be nullptr or point to a static buffer containing at
+  // least two 'long's.
+  static long InitAndGetCPU(unsigned *cpu, void *cache,     // NOLINT 'long'.
+                            void *unused);
+  static long GetCPUViaSyscall(unsigned *cpu, void *cache,  // NOLINT 'long'.
+                               void *unused);
+  typedef long (*GetCpuFn)(unsigned *cpu, void *cache,      // NOLINT 'long'.
+                           void *unused);
+
+  // This function pointer may point to InitAndGetCPU,
+  // GetCPUViaSyscall, or __vdso_getcpu at different stages of initialization.
+  static std::atomic<GetCpuFn> getcpu_fn_;
+
+  friend int GetCPU(void);  // Needs access to getcpu_fn_.
+
+  VDSOSupport(const VDSOSupport&) = delete;
+  VDSOSupport& operator=(const VDSOSupport&) = delete;
+};
+
+// Same as sched_getcpu() on later glibc versions.
+// Return current CPU, using (fast) __vdso_getcpu@LINUX_2.6 if present,
+// otherwise use syscall(SYS_getcpu,...).
+// May return -1 with errno == ENOSYS if the kernel doesn't
+// support SYS_getcpu.
+int GetCPU();
+
+}  // namespace debug_internal
+}  // namespace absl
+
+#endif  // ABSL_HAVE_ELF_MEM_IMAGE
+
+#endif  // ABSL_DEBUGGING_INTERNAL_VDSO_SUPPORT_H_