//

// 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

//

//      https://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/base/config.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 {

ABSL_NAMESPACE_BEGIN

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

ABSL_NAMESPACE_END

}  // namespace absl

#endif  // ABSL_HAVE_ELF_MEM_IMAGE

#endif  // ABSL_DEBUGGING_INTERNAL_VDSO_SUPPORT_H_