src/base/vdso_support.h

// Copyright (c) 2008, Google Inc.
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// ---
// Author: Paul Pluzhnikov
//
// 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 = NULL;
//  if (vdso.LookupSymbol("__vdso_getcpu", "LINUX_2.6", STT_FUNC, &info)) {
//     fn = reinterpret_cast<FN>(info.address);
//  }

#ifndef BASE_VDSO_SUPPORT_H_
#define BASE_VDSO_SUPPORT_H_

#include <config.h>
#include "base/basictypes.h"
#include "base/elf_mem_image.h"

#ifdef HAVE_ELF_MEM_IMAGE

// Enable VDSO support only for the architectures/operating systems that
// support it.
#if defined(__linux__) && (defined(__i386__) || defined(__PPC__))
#define HAVE_VDSO_SUPPORT 1
#endif

#include <stdlib.h>     // for NULL

namespace base {

// 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;

  // 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 != NULL, 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 != NULL, 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_ == NULL 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 const void *vdso_base_;

  DISALLOW_COPY_AND_ASSIGN(VDSOSupport);
};

}  // namespace base

#endif  // HAVE_ELF_MEM_IMAGE

#endif  // BASE_VDSO_SUPPORT_H_