text: //
  // 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/base/attributes.h"
  #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 debugging_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.
    ABSL_CONST_INIT 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 debugging_internal
  }  // namespace absl
  
  #endif  // ABSL_HAVE_ELF_MEM_IMAGE
  
  #endif  // ABSL_DEBUGGING_INTERNAL_VDSO_SUPPORT_H_
  