elf_mem_image.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
 *
 *      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 for in-memory Elf images.

#ifndef ABSL_DEBUGGING_INTERNAL_ELF_MEM_IMAGE_H_
#define ABSL_DEBUGGING_INTERNAL_ELF_MEM_IMAGE_H_

// Including this will define the __GLIBC__ macro if glibc is being
// used.
#include <climits>

// Maybe one day we can rewrite this file not to require the elf
// symbol extensions in glibc, but for right now we need them.
#ifdef ABSL_HAVE_ELF_MEM_IMAGE
#error ABSL_HAVE_ELF_MEM_IMAGE cannot be directly set
#endif

#if defined(__ELF__) && defined(__GLIBC__) && !defined(__native_client__) && \
    !defined(__asmjs__) && !defined(__wasm__)
#define ABSL_HAVE_ELF_MEM_IMAGE 1
#endif

#if ABSL_HAVE_ELF_MEM_IMAGE

#include <link.h>  // for ElfW

namespace absl {
namespace debugging_internal {

// An in-memory ELF image (may not exist on disk).
class ElfMemImage {
 private:
  // Sentinel: there could never be an elf image at &kInvalidBaseSentinel.
  static const int kInvalidBaseSentinel;

 public:
  // Sentinel: there could never be an elf image at this address.
  static constexpr const void *const kInvalidBase =
    static_cast<const void*>(&kInvalidBaseSentinel);

  // Information about a single vdso symbol.
  // All pointers are into .dynsym, .dynstr, or .text of the VDSO.
  // Do not free() them or modify through them.
  struct SymbolInfo {
    const char      *name;      // E.g. "__vdso_getcpu"
    const char      *version;   // E.g. "LINUX_2.6", could be ""
                                // for unversioned symbol.
    const void      *address;   // Relocated symbol address.
    const ElfW(Sym) *symbol;    // Symbol in the dynamic symbol table.
  };

  // Supports iteration over all dynamic symbols.
  class SymbolIterator {
   public:
    friend class ElfMemImage;
    const SymbolInfo *operator->() const;
    const SymbolInfo &operator*() const;
    SymbolIterator& operator++();
    bool operator!=(const SymbolIterator &rhs) const;
    bool operator==(const SymbolIterator &rhs) const;
   private:
    SymbolIterator(const void *const image, int index);
    void Update(int incr);
    SymbolInfo info_;
    int index_;
    const void *const image_;
  };


  explicit ElfMemImage(const void *base);
  void                 Init(const void *base);
  bool                 IsPresent() const { return ehdr_ != nullptr; }
  const ElfW(Phdr)*    GetPhdr(int index) const;
  const ElfW(Sym)*     GetDynsym(int index) const;
  const ElfW(Versym)*  GetVersym(int index) const;
  const ElfW(Verdef)*  GetVerdef(int index) const;
  const ElfW(Verdaux)* GetVerdefAux(const ElfW(Verdef) *verdef) const;
  const char*          GetDynstr(ElfW(Word) offset) const;
  const void*          GetSymAddr(const ElfW(Sym) *sym) const;
  const char*          GetVerstr(ElfW(Word) offset) const;
  int                  GetNumSymbols() const;

  SymbolIterator begin() const;
  SymbolIterator end() const;

  // Look up versioned dynamic symbol in the image.
  // Returns false if image is not present, or doesn't contain given
  // symbol/version/type combination.
  // If info_out is non-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 image isn't present
  // or doesn't have a symbol overlapping given address.
  // If info_out is non-null, additional details are filled in.
  bool LookupSymbolByAddress(const void *address, SymbolInfo *info_out) const;

 private:
  const ElfW(Ehdr) *ehdr_;
  const ElfW(Sym) *dynsym_;
  const ElfW(Versym) *versym_;
  const ElfW(Verdef) *verdef_;
  const ElfW(Word) *hash_;
  const char *dynstr_;
  size_t strsize_;
  size_t verdefnum_;
  ElfW(Addr) link_base_;     // Link-time base (p_vaddr of first PT_LOAD).
};

}  // namespace debugging_internal
}  // namespace absl

#endif  // ABSL_HAVE_ELF_MEM_IMAGE

#endif  // ABSL_DEBUGGING_INTERNAL_ELF_MEM_IMAGE_H_