Elf.cpp

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/*
 * Copyright 2012-present Facebook, Inc.
 *
 * 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.
 */
#include <folly/experimental/symbolizer/Elf.h>

#include <fcntl.h>
#include <folly/portability/SysMman.h>
#include <sys/stat.h>
#include <sys/types.h>

#include <cstring>
#include <string>

#include <glog/logging.h>

#include <folly/Conv.h>
#include <folly/Exception.h>
#include <folly/ScopeGuard.h>

#ifndef STT_GNU_IFUNC
#define STT_GNU_IFUNC 10
#endif

namespace folly {
namespace symbolizer {

ElfFile::ElfFile() noexcept
    : fd_(-1),
      file_(static_cast<char*>(MAP_FAILED)),
      length_(0),
      baseAddress_(0) {}

ElfFile::ElfFile(const char* name, bool readOnly)
    : fd_(-1),
      file_(static_cast<char*>(MAP_FAILED)),
      length_(0),
      baseAddress_(0) {
  open(name, readOnly);
}

void ElfFile::open(const char* name, bool readOnly) {
  const char* msg = "";
  int r = openNoThrow(name, readOnly, &msg);
  if (r == kSystemError) {
    throwSystemError(msg);
  } else {
    CHECK_EQ(r, kSuccess) << msg;
  }
}

int ElfFile::openNoThrow(
    const char* name,
    bool readOnly,
    const char** msg) noexcept {
  FOLLY_SAFE_CHECK(fd_ == -1, "File already open");
  strncat(filepath_, name, kFilepathMaxLen - 1);
  fd_ = ::open(name, readOnly ? O_RDONLY : O_RDWR);
  if (fd_ == -1) {
    if (msg) {
      *msg = "open";
    }
    return kSystemError;
  }
  // Always close fd and unmap in case of failure along the way to avoid
  // check failure above if we leave fd != -1 and the object is recycled
  // like it is inside SignalSafeElfCache
  auto guard = makeGuard([&] { reset(); });
  struct stat st;
  int r = fstat(fd_, &st);
  if (r == -1) {
    if (msg) {
      *msg = "fstat";
    }
    return kSystemError;
  }

  length_ = st.st_size;
  int prot = PROT_READ;
  if (!readOnly) {
    prot |= PROT_WRITE;
  }
  file_ = static_cast<char*>(mmap(nullptr, length_, prot, MAP_SHARED, fd_, 0));
  if (file_ == MAP_FAILED) {
    if (msg) {
      *msg = "mmap";
    }
    return kSystemError;
  }
  if (!init(msg)) {
    reset();
    errno = EINVAL;
    return kInvalidElfFile;
  }
  guard.dismiss();
  return kSuccess;
}

int ElfFile::openAndFollow(
    const char* name,
    bool readOnly,
    const char** msg) noexcept {
  auto result = openNoThrow(name, readOnly, msg);
  if (!readOnly || result != kSuccess) {
    return result;
  }

  /* NOTE .gnu_debuglink specifies only the name of the debugging info file
   * (with no directory components). GDB checks 3 different directories, but
   * ElfFile only supports the first version:
   *     - dirname(name)
   *     - dirname(name) + /.debug/
   *     - X/dirname(name)/ - where X is set in gdb's `debug-file-directory`.
   */
  auto dirend = strrchr(name, '/');
  // include ending '/' if any.
  auto dirlen = dirend != nullptr ? dirend + 1 - name : 0;

  auto debuginfo = getSectionByName(".gnu_debuglink");
  if (!debuginfo) {
    return result;
  }

  // The section starts with the filename, with any leading directory
  // components removed, followed by a zero byte.
  auto debugFileName = getSectionBody(*debuginfo);
  auto debugFileLen = strlen(debugFileName.begin());
  if (dirlen + debugFileLen >= PATH_MAX) {
    return result;
  }

  char linkname[PATH_MAX];
  memcpy(linkname, name, dirlen);
  memcpy(linkname + dirlen, debugFileName.begin(), debugFileLen + 1);
  reset();
  result = openNoThrow(linkname, readOnly, msg);
  if (result == kSuccess) {
    return result;
  }
  return openNoThrow(name, readOnly, msg);
}

ElfFile::~ElfFile() {
  reset();
}

ElfFile::ElfFile(ElfFile&& other) noexcept
    : fd_(other.fd_),
      file_(other.file_),
      length_(other.length_),
      baseAddress_(other.baseAddress_) {
  // copy other.filepath_, leaving filepath_ zero-terminated, always.
  strncat(filepath_, other.filepath_, kFilepathMaxLen - 1);
  other.filepath_[0] = 0;
  other.fd_ = -1;
  other.file_ = static_cast<char*>(MAP_FAILED);
  other.length_ = 0;
  other.baseAddress_ = 0;
}

ElfFile& ElfFile::operator=(ElfFile&& other) {
  assert(this != &other);
  reset();

  // copy other.filepath_, leaving filepath_ zero-terminated, always.
  strncat(filepath_, other.filepath_, kFilepathMaxLen - 1);
  fd_ = other.fd_;
  file_ = other.file_;
  length_ = other.length_;
  baseAddress_ = other.baseAddress_;

  other.filepath_[0] = 0;
  other.fd_ = -1;
  other.file_ = static_cast<char*>(MAP_FAILED);
  other.length_ = 0;
  other.baseAddress_ = 0;

  return *this;
}

void ElfFile::reset() {
  filepath_[0] = 0;

  if (file_ != MAP_FAILED) {
    munmap(file_, length_);
    file_ = static_cast<char*>(MAP_FAILED);
  }

  if (fd_ != -1) {
    close(fd_);
    fd_ = -1;
  }
}

bool ElfFile::init(const char** msg) {
  if (length_ < 4) {
    if (msg) {
      *msg = "not an ELF file (too short)";
    }
    return false;
  }

  std::array<char, 5> elfMagBuf = {{0, 0, 0, 0, 0}};
  if (::lseek(fd_, 0, SEEK_SET) != 0 || ::read(fd_, elfMagBuf.data(), 4) != 4) {
    if (msg) {
      *msg = "unable to read ELF file for magic number";
    }
    return false;
  }
  if (std::strncmp(elfMagBuf.data(), ELFMAG, sizeof(ELFMAG)) != 0) {
    if (msg) {
      *msg = "invalid ELF magic";
    }
    return false;
  }
  if (::lseek(fd_, 0, SEEK_SET) != 0) {
    if (msg) {
      *msg = "unable to reset file descriptor after reading ELF magic number";
    }
    return false;
  }

  auto& elfHeader = this->elfHeader();

#define EXPECTED_CLASS P1(ELFCLASS, __ELF_NATIVE_CLASS)
#define P1(a, b) P2(a, b)
#define P2(a, b) a##b
  // Validate ELF class (32/64 bits)
  if (elfHeader.e_ident[EI_CLASS] != EXPECTED_CLASS) {
    if (msg) {
      *msg = "invalid ELF class";
    }
    return false;
  }
#undef P1
#undef P2
#undef EXPECTED_CLASS

  // Validate ELF data encoding (LSB/MSB)
  static constexpr auto kExpectedEncoding =
      kIsLittleEndian ? ELFDATA2LSB : ELFDATA2MSB;
  if (elfHeader.e_ident[EI_DATA] != kExpectedEncoding) {
    if (msg) {
      *msg = "invalid ELF encoding";
    }
    return false;
  }

  // Validate ELF version (1)
  if (elfHeader.e_ident[EI_VERSION] != EV_CURRENT ||
      elfHeader.e_version != EV_CURRENT) {
    if (msg) {
      *msg = "invalid ELF version";
    }
    return false;
  }

  // We only support executable and shared object files
  if (elfHeader.e_type != ET_EXEC && elfHeader.e_type != ET_DYN) {
    if (msg) {
      *msg = "invalid ELF file type";
    }
    return false;
  }

  if (elfHeader.e_phnum == 0) {
    if (msg) {
      *msg = "no program header!";
    }
    return false;
  }

  if (elfHeader.e_phentsize != sizeof(ElfPhdr)) {
    if (msg) {
      *msg = "invalid program header entry size";
    }
    return false;
  }

  if (elfHeader.e_shentsize != sizeof(ElfShdr)) {
    if (msg) {
      *msg = "invalid section header entry size";
    }
  }

  // Program headers are sorted by load address, so the first PT_LOAD
  // header gives us the base address.
  const ElfPhdr* programHeader =
      iterateProgramHeaders([](auto& h) { return h.p_type == PT_LOAD; });

  if (!programHeader) {
    if (msg) {
      *msg = "could not find base address";
    }
    return false;
  }
  baseAddress_ = programHeader->p_vaddr;

  return true;
}

const ElfShdr* ElfFile::getSectionByIndex(size_t idx) const {
  FOLLY_SAFE_CHECK(idx < elfHeader().e_shnum, "invalid section index");
  return &at<ElfShdr>(elfHeader().e_shoff + idx * sizeof(ElfShdr));
}

folly::StringPiece ElfFile::getSectionBody(const ElfShdr& section) const {
  return folly::StringPiece(file_ + section.sh_offset, section.sh_size);
}

void ElfFile::validateStringTable(const ElfShdr& stringTable) const {
  FOLLY_SAFE_CHECK(
      stringTable.sh_type == SHT_STRTAB, "invalid type for string table");

  const char* start = file_ + stringTable.sh_offset;
  // First and last bytes must be 0
  FOLLY_SAFE_CHECK(
      stringTable.sh_size == 0 ||
          (start[0] == '\0' && start[stringTable.sh_size - 1] == '\0'),
      "invalid string table");
}

const char* ElfFile::getString(const ElfShdr& stringTable, size_t offset)
    const {
  validateStringTable(stringTable);
  FOLLY_SAFE_CHECK(
      offset < stringTable.sh_size, "invalid offset in string table");

  return file_ + stringTable.sh_offset + offset;
}

const char* ElfFile::getSectionName(const ElfShdr& section) const {
  if (elfHeader().e_shstrndx == SHN_UNDEF) {
    return nullptr; // no section name string table
  }

  const ElfShdr& sectionNames = *getSectionByIndex(elfHeader().e_shstrndx);
  return getString(sectionNames, section.sh_name);
}

const ElfShdr* ElfFile::getSectionByName(const char* name) const {
  if (elfHeader().e_shstrndx == SHN_UNDEF) {
    return nullptr; // no section name string table
  }

  const ElfShdr& sectionNames = *getSectionByIndex(elfHeader().e_shstrndx);
  const char* start = file_ + sectionNames.sh_offset;

  // Find section with the appropriate sh_name offset
  const ElfShdr* foundSection = iterateSections([&](const ElfShdr& sh) {
    if (sh.sh_name >= sectionNames.sh_size) {
      return false;
    }
    return !strcmp(start + sh.sh_name, name);
  });
  return foundSection;
}

ElfFile::Symbol ElfFile::getDefinitionByAddress(uintptr_t address) const {
  Symbol foundSymbol{nullptr, nullptr};

  auto findSection = [&](const ElfShdr& section) {
    auto findSymbols = [&](const ElfSym& sym) {
      if (sym.st_shndx == SHN_UNDEF) {
        return false; // not a definition
      }
      if (address >= sym.st_value && address < sym.st_value + sym.st_size) {
        foundSymbol.first = &section;
        foundSymbol.second = &sym;
        return true;
      }

      return false;
    };

    return iterateSymbolsWithTypes(
        section, {STT_OBJECT, STT_FUNC, STT_GNU_IFUNC}, findSymbols);
  };

  // Try the .dynsym section first if it exists, it's smaller.
  (iterateSectionsWithType(SHT_DYNSYM, findSection) ||
   iterateSectionsWithType(SHT_SYMTAB, findSection));

  return foundSymbol;
}

ElfFile::Symbol ElfFile::getSymbolByName(const char* name) const {
  Symbol foundSymbol{nullptr, nullptr};

  auto findSection = [&](const ElfShdr& section) -> bool {
    // This section has no string table associated w/ its symbols; hence we
    // can't get names for them
    if (section.sh_link == SHN_UNDEF) {
      return false;
    }

    auto findSymbols = [&](const ElfSym& sym) -> bool {
      if (sym.st_shndx == SHN_UNDEF) {
        return false; // not a definition
      }
      if (sym.st_name == 0) {
        return false; // no name for this symbol
      }
      const char* sym_name =
          getString(*getSectionByIndex(section.sh_link), sym.st_name);
      if (strcmp(sym_name, name) == 0) {
        foundSymbol.first = &section;
        foundSymbol.second = &sym;
        return true;
      }

      return false;
    };

    return iterateSymbolsWithTypes(
        section, {STT_OBJECT, STT_FUNC, STT_GNU_IFUNC}, findSymbols);
  };

  // Try the .dynsym section first if it exists, it's smaller.
  iterateSectionsWithType(SHT_DYNSYM, findSection) ||
      iterateSectionsWithType(SHT_SYMTAB, findSection);

  return foundSymbol;
}

const ElfShdr* ElfFile::getSectionContainingAddress(ElfAddr addr) const {
  return iterateSections([&](const ElfShdr& sh) -> bool {
    return (addr >= sh.sh_addr) && (addr < (sh.sh_addr + sh.sh_size));
  });
}

const char* ElfFile::getSymbolName(Symbol symbol) const {
  if (!symbol.first || !symbol.second) {
    return nullptr;
  }

  if (symbol.second->st_name == 0) {
    return nullptr; // symbol has no name
  }

  if (symbol.first->sh_link == SHN_UNDEF) {
    return nullptr; // symbol table has no strings
  }

  return getString(
      *getSectionByIndex(symbol.first->sh_link), symbol.second->st_name);
}

} // namespace symbolizer
} // namespace folly