base / third_party / symbolize / symbolize.cc [blame]

// Copyright (c) 2006, Google Inc.
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
//     * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
// Author: Satoru Takabayashi
// Stack-footprint reduction work done by Raksit Ashok
//
// Implementation note:
//
// We don't use heaps but only use stacks.  We want to reduce the
// stack consumption so that the symbolizer can run on small stacks.
//
// Here are some numbers collected with GCC 4.1.0 on x86:
// - sizeof(Elf32_Sym)  = 16
// - sizeof(Elf32_Shdr) = 40
// - sizeof(Elf64_Sym)  = 24
// - sizeof(Elf64_Shdr) = 64
//
// This implementation is intended to be async-signal-safe but uses
// some functions which are not guaranteed to be so, such as memchr()
// and memmove().  We assume they are async-signal-safe.
//
// Additional header can be specified by the GLOG_BUILD_CONFIG_INCLUDE
// macro to add platform specific defines (e.g. GLOG_OS_OPENBSD).

#ifdef GLOG_BUILD_CONFIG_INCLUDE
#include GLOG_BUILD_CONFIG_INCLUDE
#endif  // GLOG_BUILD_CONFIG_INCLUDE

#include "symbolize.h"

#include "utilities.h"

#if defined(HAVE_SYMBOLIZE)

#include <cstring>
#include <cstdlib>

#include <algorithm>
#include <limits>

#include "symbolize.h"
#include "demangle.h"

_START_GOOGLE_NAMESPACE_

// We don't use assert() since it's not guaranteed to be
// async-signal-safe.  Instead we define a minimal assertion
// macro. So far, we don't need pretty printing for __FILE__, etc.

// A wrapper for abort() to make it callable in ? :.
static int AssertFail() {
  abort();
  return 0;  // Should not reach.
}

#define SAFE_ASSERT(expr) ((expr) ? 0 : AssertFail())

static SymbolizeCallback g_symbolize_callback = NULL;
void InstallSymbolizeCallback(SymbolizeCallback callback) {
  g_symbolize_callback = callback;
}

static SymbolizeOpenObjectFileCallback g_symbolize_open_object_file_callback =
    NULL;
void InstallSymbolizeOpenObjectFileCallback(
    SymbolizeOpenObjectFileCallback callback) {
  g_symbolize_open_object_file_callback = callback;
}

// This function wraps the Demangle function to provide an interface
// where the input symbol is demangled in-place.
// To keep stack consumption low, we would like this function to not
// get inlined.
static ATTRIBUTE_NOINLINE void DemangleInplace(char* out, size_t out_size) {
  char demangled[256];  // Big enough for sane demangled symbols.
  if (Demangle(out, demangled, sizeof(demangled))) {
    // Demangling succeeded. Copy to out if the space allows.
    size_t len = strlen(demangled);
    if (len + 1 <= out_size) {  // +1 for '\0'.
      SAFE_ASSERT(len < sizeof(demangled));
      memmove(out, demangled, len + 1);
    }
  }
}

_END_GOOGLE_NAMESPACE_

#if defined(__ELF__)

#if defined(HAVE_DLFCN_H)
#include <dlfcn.h>
#endif
#if defined(GLOG_OS_OPENBSD)
#include <sys/exec_elf.h>
#else
#include <elf.h>
#endif
#include <fcntl.h>
#include <stdint.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <cerrno>
#include <climits>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>

#include "symbolize.h"
#include "config.h"
#include "glog/raw_logging.h"

// Re-runs fn until it doesn't cause EINTR.
#define NO_INTR(fn)   do {} while ((fn) < 0 && errno == EINTR)

_START_GOOGLE_NAMESPACE_

FileDescriptor::~FileDescriptor() {
  if (fd_ >= 0) {
    close(fd_);
  }
}

ssize_t ReadFromOffset(const int fd,
                       void* buf,
                       const size_t count,
                       const size_t offset) {
  SAFE_ASSERT(fd >= 0);
  SAFE_ASSERT(count <=
              static_cast<size_t>(std::numeric_limits<ssize_t>::max()));
  char *buf0 = reinterpret_cast<char *>(buf);
  size_t num_bytes = 0;
  while (num_bytes < count) {
    ssize_t len;
    NO_INTR(len = pread(fd, buf0 + num_bytes, count - num_bytes,
                        static_cast<off_t>(offset + num_bytes)));
    if (len < 0) {  // There was an error other than EINTR.
      return -1;
    }
    if (len == 0) {  // Reached EOF.
      break;
    }
    num_bytes += static_cast<size_t>(len);
  }
  SAFE_ASSERT(num_bytes <= count);
  return static_cast<ssize_t>(num_bytes);
}

// Try reading exactly "count" bytes from "offset" bytes in a file
// pointed by "fd" into the buffer starting at "buf" while handling
// short reads and EINTR.  On success, return true. Otherwise, return
// false.
static bool ReadFromOffsetExact(const int fd,
                                void* buf,
                                const size_t count,
                                const size_t offset) {
  ssize_t len = ReadFromOffset(fd, buf, count, offset);
  return static_cast<size_t>(len) == count;
}

// Returns elf_header.e_type if the file pointed by fd is an ELF binary.
static int FileGetElfType(const int fd) {
  ElfW(Ehdr) elf_header;
  if (!ReadFromOffsetExact(fd, &elf_header, sizeof(elf_header), 0)) {
    return -1;
  }
  if (memcmp(elf_header.e_ident, ELFMAG, SELFMAG) != 0) {
    return -1;
  }
  return elf_header.e_type;
}

// Read the section headers in the given ELF binary, and if a section
// of the specified type is found, set the output to this section header
// and return true.  Otherwise, return false.
// To keep stack consumption low, we would like this function to not get
// inlined.
static ATTRIBUTE_NOINLINE bool GetSectionHeaderByType(const int fd,
                                                      ElfW(Half) sh_num,
                                                      const size_t sh_offset,
                                                      ElfW(Word) type,
                                                      ElfW(Shdr) * out) {
  // Read at most 16 section headers at a time to save read calls.
  ElfW(Shdr) buf[16];
  for (size_t i = 0; i < sh_num;) {
    const size_t num_bytes_left = (sh_num - i) * sizeof(buf[0]);
    const size_t num_bytes_to_read =
        (sizeof(buf) > num_bytes_left) ? num_bytes_left : sizeof(buf);
    const ssize_t len = ReadFromOffset(fd, buf, num_bytes_to_read,
                                       sh_offset + i * sizeof(buf[0]));
    if (len == -1) {
      return false;
    }
    SAFE_ASSERT(static_cast<size_t>(len) % sizeof(buf[0]) == 0);
    const size_t num_headers_in_buf = static_cast<size_t>(len) / sizeof(buf[0]);
    SAFE_ASSERT(num_headers_in_buf <= sizeof(buf) / sizeof(buf[0]));
    for (size_t j = 0; j < num_headers_in_buf; ++j) {
      if (buf[j].sh_type == type) {
        *out = buf[j];
        return true;
      }
    }
    i += num_headers_in_buf;
  }
  return false;
}

// There is no particular reason to limit section name to 63 characters,
// but there has (as yet) been no need for anything longer either.
const int kMaxSectionNameLen = 64;

// name_len should include terminating '\0'.
bool GetSectionHeaderByName(int fd, const char *name, size_t name_len,
                            ElfW(Shdr) *out) {
  ElfW(Ehdr) elf_header;
  if (!ReadFromOffsetExact(fd, &elf_header, sizeof(elf_header), 0)) {
    return false;
  }

  ElfW(Shdr) shstrtab;
  size_t shstrtab_offset =
      (elf_header.e_shoff + static_cast<size_t>(elf_header.e_shentsize) *
                                static_cast<size_t>(elf_header.e_shstrndx));
  if (!ReadFromOffsetExact(fd, &shstrtab, sizeof(shstrtab), shstrtab_offset)) {
    return false;
  }

  for (size_t i = 0; i < elf_header.e_shnum; ++i) {
    size_t section_header_offset =
        (elf_header.e_shoff + elf_header.e_shentsize * i);
    if (!ReadFromOffsetExact(fd, out, sizeof(*out), section_header_offset)) {
      return false;
    }
    char header_name[kMaxSectionNameLen];
    if (sizeof(header_name) < name_len) {
      RAW_LOG(WARNING, "Section name '%s' is too long (%" PRIuS "); "
              "section will not be found (even if present).", name, name_len);
      // No point in even trying.
      return false;
    }
    size_t name_offset = shstrtab.sh_offset + out->sh_name;
    ssize_t n_read = ReadFromOffset(fd, &header_name, name_len, name_offset);
    if (n_read == -1) {
      return false;
    } else if (static_cast<size_t>(n_read) != name_len) {
      // Short read -- name could be at end of file.
      continue;
    }
    if (memcmp(header_name, name, name_len) == 0) {
      return true;
    }
  }
  return false;
}

// Read a symbol table and look for the symbol containing the
// pc. Iterate over symbols in a symbol table and look for the symbol
// containing "pc".  On success, return true and write the symbol name
// to out.  Otherwise, return false.
// To keep stack consumption low, we would like this function to not get
// inlined.
static ATTRIBUTE_NOINLINE bool FindSymbol(uint64_t pc,
                                          const int fd,
                                          char* out,
                                          size_t out_size,
                                          uint64_t symbol_offset,
                                          const ElfW(Shdr) * strtab,
                                          const ElfW(Shdr) * symtab) {
  if (symtab == NULL) {
    return false;
  }
  const size_t num_symbols = symtab->sh_size / symtab->sh_entsize;
  for (unsigned i = 0; i < num_symbols;) {
    size_t offset = symtab->sh_offset + i * symtab->sh_entsize;

    // If we are reading Elf64_Sym's, we want to limit this array to
    // 32 elements (to keep stack consumption low), otherwise we can
    // have a 64 element Elf32_Sym array.
#if defined(__WORDSIZE) && __WORDSIZE == 64
    const size_t NUM_SYMBOLS = 32U;
#else
    const size_t NUM_SYMBOLS = 64U;
#endif

    // Read at most NUM_SYMBOLS symbols at once to save read() calls.
    ElfW(Sym) buf[NUM_SYMBOLS];
    size_t num_symbols_to_read = std::min(NUM_SYMBOLS, num_symbols - i);
    const ssize_t len =
        ReadFromOffset(fd, &buf, sizeof(buf[0]) * num_symbols_to_read, offset);
    SAFE_ASSERT(static_cast<size_t>(len) % sizeof(buf[0]) == 0);
    const size_t num_symbols_in_buf = static_cast<size_t>(len) / sizeof(buf[0]);
    SAFE_ASSERT(num_symbols_in_buf <= num_symbols_to_read);
    for (unsigned j = 0; j < num_symbols_in_buf; ++j) {
      const ElfW(Sym)& symbol = buf[j];
      uint64_t start_address = symbol.st_value;
      start_address += symbol_offset;
      uint64_t end_address = start_address + symbol.st_size;
      if (symbol.st_value != 0 &&  // Skip null value symbols.
          symbol.st_shndx != 0 &&  // Skip undefined symbols.
          start_address <= pc && pc < end_address) {
        ssize_t len1 = ReadFromOffset(fd, out, out_size,
                                      strtab->sh_offset + symbol.st_name);
        if (len1 <= 0 || memchr(out, '\0', out_size) == NULL) {
          memset(out, 0, out_size);
          return false;
        }
        return true;  // Obtained the symbol name.
      }
    }
    i += num_symbols_in_buf;
  }
  return false;
}

// Get the symbol name of "pc" from the file pointed by "fd".  Process
// both regular and dynamic symbol tables if necessary.  On success,
// write the symbol name to "out" and return true.  Otherwise, return
// false.
static bool GetSymbolFromObjectFile(const int fd,
                                    uint64_t pc,
                                    char* out,
                                    size_t out_size,
                                    uint64_t base_address) {
  // Read the ELF header.
  ElfW(Ehdr) elf_header;
  if (!ReadFromOffsetExact(fd, &elf_header, sizeof(elf_header), 0)) {
    return false;
  }

  ElfW(Shdr) symtab, strtab;

  // Consult a regular symbol table first.
  if (GetSectionHeaderByType(fd, elf_header.e_shnum, elf_header.e_shoff,
                             SHT_SYMTAB, &symtab)) {
    if (!ReadFromOffsetExact(fd, &strtab, sizeof(strtab), elf_header.e_shoff +
                             symtab.sh_link * sizeof(symtab))) {
      return false;
    }
    if (FindSymbol(pc, fd, out, out_size, base_address, &strtab, &symtab)) {
      return true;  // Found the symbol in a regular symbol table.
    }
  }

  // If the symbol is not found, then consult a dynamic symbol table.
  if (GetSectionHeaderByType(fd, elf_header.e_shnum, elf_header.e_shoff,
                             SHT_DYNSYM, &symtab)) {
    if (!ReadFromOffsetExact(fd, &strtab, sizeof(strtab), elf_header.e_shoff +
                             symtab.sh_link * sizeof(symtab))) {
      return false;
    }
    if (FindSymbol(pc, fd, out, out_size, base_address, &strtab, &symtab)) {
      return true;  // Found the symbol in a dynamic symbol table.
    }
  }

  return false;
}

namespace {

// Helper class for reading lines from file.
//
// Note: we don't use ProcMapsIterator since the object is big (it has
// a 5k array member) and uses async-unsafe functions such as sscanf()
// and snprintf().
class LineReader {
 public:
  explicit LineReader(int fd, char* buf, size_t buf_len, size_t offset)
      : fd_(fd),
        buf_(buf),
        buf_len_(buf_len),
        offset_(offset),
        bol_(buf),
        eol_(buf),
        eod_(buf) {}

  // Read '\n'-terminated line from file.  On success, modify "bol"
  // and "eol", then return true.  Otherwise, return false.
  //
  // Note: if the last line doesn't end with '\n', the line will be
  // dropped.  It's an intentional behavior to make the code simple.
  bool ReadLine(const char **bol, const char **eol) {
    if (BufferIsEmpty()) {  // First time.
      const ssize_t num_bytes = ReadFromOffset(fd_, buf_, buf_len_, offset_);
      if (num_bytes <= 0) {  // EOF or error.
        return false;
      }
      offset_ += static_cast<size_t>(num_bytes);
      eod_ = buf_ + num_bytes;
      bol_ = buf_;
    } else {
      bol_ = eol_ + 1;  // Advance to the next line in the buffer.
      SAFE_ASSERT(bol_ <= eod_);  // "bol_" can point to "eod_".
      if (!HasCompleteLine()) {
        const size_t incomplete_line_length = static_cast<size_t>(eod_ - bol_);
        // Move the trailing incomplete line to the beginning.
        memmove(buf_, bol_, incomplete_line_length);
        // Read text from file and append it.
        char * const append_pos = buf_ + incomplete_line_length;
        const size_t capacity_left = buf_len_ - incomplete_line_length;
        const ssize_t num_bytes =
            ReadFromOffset(fd_, append_pos, capacity_left, offset_);
        if (num_bytes <= 0) {  // EOF or error.
          return false;
        }
        offset_ += static_cast<size_t>(num_bytes);
        eod_ = append_pos + num_bytes;
        bol_ = buf_;
      }
    }
    eol_ = FindLineFeed();
    if (eol_ == NULL) {  // '\n' not found.  Malformed line.
      return false;
    }
    *eol_ = '\0';  // Replace '\n' with '\0'.

    *bol = bol_;
    *eol = eol_;
    return true;
  }

  // Beginning of line.
  const char *bol() {
    return bol_;
  }

  // End of line.
  const char *eol() {
    return eol_;
  }

 private:
  LineReader(const LineReader&);
  void operator=(const LineReader&);

  char *FindLineFeed() {
    return reinterpret_cast<char*>(
        memchr(bol_, '\n', static_cast<size_t>(eod_ - bol_)));
  }

  bool BufferIsEmpty() {
    return buf_ == eod_;
  }

  bool HasCompleteLine() {
    return !BufferIsEmpty() && FindLineFeed() != NULL;
  }

  const int fd_;
  char * const buf_;
  const size_t buf_len_;
  size_t offset_;
  char *bol_;
  char *eol_;
  const char *eod_;  // End of data in "buf_".
};
}  // namespace

// Place the hex number read from "start" into "*hex".  The pointer to
// the first non-hex character or "end" is returned.
static char *GetHex(const char *start, const char *end, uint64_t *hex) {
  *hex = 0;
  const char *p;
  for (p = start; p < end; ++p) {
    int ch = *p;
    if ((ch >= '0' && ch <= '9') ||
        (ch >= 'A' && ch <= 'F') || (ch >= 'a' && ch <= 'f')) {
      *hex = (*hex << 4U) |
             (ch < 'A' ? static_cast<uint64_t>(ch - '0') : (ch & 0xF) + 9U);
    } else {  // Encountered the first non-hex character.
      break;
    }
  }
  SAFE_ASSERT(p <= end);
  return const_cast<char *>(p);
}

static int OpenObjectFileContainingPcAndGetStartAddressNoHook(
    uint64_t pc,
    uint64_t& start_address,
    uint64_t& base_address,
    char* out_file_name,
    size_t out_file_name_size) {
  int object_fd;

  int maps_fd;
  NO_INTR(maps_fd = open("/proc/self/maps", O_RDONLY));
  FileDescriptor wrapped_maps_fd(maps_fd);
  if (wrapped_maps_fd.get() < 0) {
    return -1;
  }

  int mem_fd;
  NO_INTR(mem_fd = open("/proc/self/mem", O_RDONLY));
  FileDescriptor wrapped_mem_fd(mem_fd);
  if (wrapped_mem_fd.get() < 0) {
    return -1;
  }

  // Iterate over maps and look for the map containing the pc.  Then
  // look into the symbol tables inside.
  char buf[1024];  // Big enough for line of sane /proc/self/maps
  unsigned num_maps = 0;
  LineReader reader(wrapped_maps_fd.get(), buf, sizeof(buf), 0);
  while (true) {
    num_maps++;
    const char *cursor;
    const char *eol;
    if (!reader.ReadLine(&cursor, &eol)) {  // EOF or malformed line.
      return -1;
    }

    // Start parsing line in /proc/self/maps.  Here is an example:
    //
    // 08048000-0804c000 r-xp 00000000 08:01 2142121    /bin/cat
    //
    // We want start address (08048000), end address (0804c000), flags
    // (r-xp) and file name (/bin/cat).

    // Read start address.
    cursor = GetHex(cursor, eol, &start_address);
    if (cursor == eol || *cursor != '-') {
      return -1;  // Malformed line.
    }
    ++cursor;  // Skip '-'.

    // Read end address.
    uint64_t end_address;
    cursor = GetHex(cursor, eol, &end_address);
    if (cursor == eol || *cursor != ' ') {
      return -1;  // Malformed line.
    }
    ++cursor;  // Skip ' '.

    // Read flags.  Skip flags until we encounter a space or eol.
    const char * const flags_start = cursor;
    while (cursor < eol && *cursor != ' ') {
      ++cursor;
    }
    // We expect at least four letters for flags (ex. "r-xp").
    if (cursor == eol || cursor < flags_start + 4) {
      return -1;  // Malformed line.
    }

    // Determine the base address by reading ELF headers in process memory.
    ElfW(Ehdr) ehdr;
    // Skip non-readable maps.
    if (flags_start[0] == 'r' &&
        ReadFromOffsetExact(mem_fd, &ehdr, sizeof(ElfW(Ehdr)), start_address) &&
        memcmp(ehdr.e_ident, ELFMAG, SELFMAG) == 0) {
      switch (ehdr.e_type) {
        case ET_EXEC:
          base_address = 0;
          break;
        case ET_DYN:
          // Find the segment containing file offset 0. This will correspond
          // to the ELF header that we just read. Normally this will have
          // virtual address 0, but this is not guaranteed. We must subtract
          // the virtual address from the address where the ELF header was
          // mapped to get the base address.
          //
          // If we fail to find a segment for file offset 0, use the address
          // of the ELF header as the base address.
          base_address = start_address;
          for (unsigned i = 0; i != ehdr.e_phnum; ++i) {
            ElfW(Phdr) phdr;
            if (ReadFromOffsetExact(
                    mem_fd, &phdr, sizeof(phdr),
                    start_address + ehdr.e_phoff + i * sizeof(phdr)) &&
                phdr.p_type == PT_LOAD && phdr.p_offset == 0) {
              base_address = start_address - phdr.p_vaddr;
              break;
            }
          }
          break;
        default:
          // ET_REL or ET_CORE. These aren't directly executable, so they don't
          // affect the base address.
          break;
      }
    }

    // Check start and end addresses.
    if (!(start_address <= pc && pc < end_address)) {
      continue;  // We skip this map.  PC isn't in this map.
    }

   // Check flags.  We are only interested in "r*x" maps.
    if (flags_start[0] != 'r' || flags_start[2] != 'x') {
      continue;  // We skip this map.
    }
    ++cursor;  // Skip ' '.

    // Read file offset.
    uint64_t file_offset;
    cursor = GetHex(cursor, eol, &file_offset);
    if (cursor == eol || *cursor != ' ') {
      return -1;  // Malformed line.
    }
    ++cursor;  // Skip ' '.

    // Skip to file name.  "cursor" now points to dev.  We need to
    // skip at least two spaces for dev and inode.
    int num_spaces = 0;
    while (cursor < eol) {
      if (*cursor == ' ') {
        ++num_spaces;
      } else if (num_spaces >= 2) {
        // The first non-space character after skipping two spaces
        // is the beginning of the file name.
        break;
      }
      ++cursor;
    }
    if (cursor == eol) {
      return -1;  // Malformed line.
    }

    // Finally, "cursor" now points to file name of our interest.
    NO_INTR(object_fd = open(cursor, O_RDONLY));
    if (object_fd < 0) {
      // Failed to open object file.  Copy the object file name to
      // |out_file_name|.
      strncpy(out_file_name, cursor, out_file_name_size);
      // Making sure |out_file_name| is always null-terminated.
      out_file_name[out_file_name_size - 1] = '\0';
      return -1;
    }
    return object_fd;
  }
}

int OpenObjectFileContainingPcAndGetStartAddress(uint64_t pc,
                                                 uint64_t& start_address,
                                                 uint64_t& base_address,
                                                 char* out_file_name,
                                                 size_t out_file_name_size) {
  if (g_symbolize_open_object_file_callback) {
    return g_symbolize_open_object_file_callback(
        pc, start_address, base_address, out_file_name, out_file_name_size);
  }
  return OpenObjectFileContainingPcAndGetStartAddressNoHook(
      pc, start_address, base_address, out_file_name, out_file_name_size);
}

// POSIX doesn't define any async-signal safe function for converting
// an integer to ASCII. We'll have to define our own version.
// itoa_r() converts an (unsigned) integer to ASCII. It returns "buf", if the
// conversion was successful or NULL otherwise. It never writes more than "sz"
// bytes. Output will be truncated as needed, and a NUL character is always
// appended.
// NOTE: code from sandbox/linux/seccomp-bpf/demo.cc.
static char* itoa_r(uintptr_t i,
                    char* buf,
                    size_t sz,
                    unsigned base,
                    size_t padding) {
  // Make sure we can write at least one NUL byte.
  size_t n = 1;
  if (n > sz) {
    return NULL;
  }

  if (base < 2 || base > 16) {
    buf[0] = '\000';
    return NULL;
  }

  char *start = buf;

  // Loop until we have converted the entire number. Output at least one
  // character (i.e. '0').
  char *ptr = start;
  do {
    // Make sure there is still enough space left in our output buffer.
    if (++n > sz) {
      buf[0] = '\000';
      return NULL;
    }

    // Output the next digit.
    *ptr++ = "0123456789abcdef"[i % base];
    i /= base;

    if (padding > 0) {
      padding--;
    }
  } while (i > 0 || padding > 0);

  // Terminate the output with a NUL character.
  *ptr = '\000';

  // Conversion to ASCII actually resulted in the digits being in reverse
  // order. We can't easily generate them in forward order, as we can't tell
  // the number of characters needed until we are done converting.
  // So, now, we reverse the string (except for the possible "-" sign).
  while (--ptr > start) {
    char ch = *ptr;
    *ptr = *start;
    *start++ = ch;
  }
  return buf;
}

// Safely appends string |source| to string |dest|.  Never writes past the
// buffer size |dest_size| and guarantees that |dest| is null-terminated.
static void SafeAppendString(const char* source, char* dest, size_t dest_size) {
  size_t dest_string_length = strlen(dest);
  SAFE_ASSERT(dest_string_length < dest_size);
  dest += dest_string_length;
  dest_size -= dest_string_length;
  strncpy(dest, source, dest_size);
  // Making sure |dest| is always null-terminated.
  dest[dest_size - 1] = '\0';
}

// Converts a 64-bit value into a hex string, and safely appends it to |dest|.
// Never writes past the buffer size |dest_size| and guarantees that |dest| is
// null-terminated.
static void SafeAppendHexNumber(uint64_t value, char* dest, size_t dest_size) {
  // 64-bit numbers in hex can have up to 16 digits.
  char buf[17] = {'\0'};
  SafeAppendString(itoa_r(value, buf, sizeof(buf), 16, 0), dest, dest_size);
}

// The implementation of our symbolization routine.  If it
// successfully finds the symbol containing "pc" and obtains the
// symbol name, returns true and write the symbol name to "out".
// Otherwise, returns false. If Callback function is installed via
// InstallSymbolizeCallback(), the function is also called in this function,
// and "out" is used as its output.
// To keep stack consumption low, we would like this function to not
// get inlined.
static ATTRIBUTE_NOINLINE bool SymbolizeAndDemangle(void* pc,
                                                    char* out,
                                                    size_t out_size) {
  uint64_t pc0 = reinterpret_cast<uintptr_t>(pc);
  uint64_t start_address = 0;
  uint64_t base_address = 0;

  if (out_size < 1) {
    return false;
  }
  out[0] = '\0';
  SafeAppendString("(", out, out_size);

  int object_fd = OpenObjectFileContainingPcAndGetStartAddress(
      pc0, start_address, base_address, out + 1, out_size - 1);

  FileDescriptor wrapped_object_fd(object_fd);

#if defined(PRINT_UNSYMBOLIZED_STACK_TRACES)
  {
#else
  // Check whether a file name was returned.
  if (object_fd < 0) {
#endif
    if (out[1]) {
      // The object file containing PC was determined successfully however the
      // object file was not opened successfully.  This is still considered
      // success because the object file name and offset are known and tools
      // like asan_symbolize.py can be used for the symbolization.
      out[out_size - 1] = '\0';  // Making sure |out| is always null-terminated.
      SafeAppendString("+0x", out, out_size);
      SafeAppendHexNumber(pc0 - base_address, out, out_size);
      SafeAppendString(")", out, out_size);
      return true;
    }
    // Failed to determine the object file containing PC.  Bail out.
    return false;
  }
  int elf_type = FileGetElfType(wrapped_object_fd.get());
  if (elf_type == -1) {
    return false;
  }
  if (g_symbolize_callback) {
    // Run the call back if it's installed.
    // Note: relocation (and much of the rest of this code) will be
    // wrong for prelinked shared libraries and PIE executables.
    uint64_t relocation = (elf_type == ET_DYN) ? start_address : 0;
    int num_bytes_written = g_symbolize_callback(wrapped_object_fd.get(),
                                                 pc, out, out_size,
                                                 relocation);
    if (num_bytes_written > 0) {
      out += static_cast<size_t>(num_bytes_written);
      out_size -= static_cast<size_t>(num_bytes_written);
    }
  }
  if (!GetSymbolFromObjectFile(wrapped_object_fd.get(), pc0,
                               out, out_size, base_address)) {
    if (out[1] && !g_symbolize_callback) {
      // The object file containing PC was opened successfully however the
      // symbol was not found. The object may have been stripped. This is still
      // considered success because the object file name and offset are known
      // and tools like asan_symbolize.py can be used for the symbolization.
      out[out_size - 1] = '\0';  // Making sure |out| is always null-terminated.
      SafeAppendString("+0x", out, out_size);
      SafeAppendHexNumber(pc0 - base_address, out, out_size);
      SafeAppendString(")", out, out_size);
      return true;
    }
    return false;
  }

  // Symbolization succeeded.  Now we try to demangle the symbol.
  DemangleInplace(out, out_size);
  return true;
}

_END_GOOGLE_NAMESPACE_

#elif (defined(GLOG_OS_MACOSX) || defined(GLOG_OS_EMSCRIPTEN)) && \
    defined(HAVE_DLADDR)

#include <dlfcn.h>
#include <cstring>

_START_GOOGLE_NAMESPACE_

static ATTRIBUTE_NOINLINE bool SymbolizeAndDemangle(void* pc,
                                                    char* out,
                                                    size_t out_size) {
  Dl_info info;
  if (dladdr(pc, &info)) {
    if (info.dli_sname) {
      if (strlen(info.dli_sname) < out_size) {
        strcpy(out, info.dli_sname);
        // Symbolization succeeded.  Now we try to demangle the symbol.
        DemangleInplace(out, out_size);
        return true;
      }
    }
  }
  return false;
}

_END_GOOGLE_NAMESPACE_

#elif defined(GLOG_OS_WINDOWS) || defined(GLOG_OS_CYGWIN)

#include <windows.h>
#include <dbghelp.h>

#ifdef _MSC_VER
#pragma comment(lib, "dbghelp")
#endif

_START_GOOGLE_NAMESPACE_

class SymInitializer {
public:
  HANDLE process;
  bool ready;
  SymInitializer() : process(NULL), ready(false) {
    // Initialize the symbol handler.
    // https://msdn.microsoft.com/en-us/library/windows/desktop/ms680344(v=vs.85).aspx
    process = GetCurrentProcess();
    // Defer symbol loading.
    // We do not request undecorated symbols with SYMOPT_UNDNAME
    // because the mangling library calls UnDecorateSymbolName.
    SymSetOptions(SYMOPT_DEFERRED_LOADS);
    if (SymInitialize(process, NULL, true)) {
      ready = true;
    }
  }
  ~SymInitializer() {
    SymCleanup(process);
    // We do not need to close `HANDLE process` because it's a "pseudo handle."
  }
private:
  SymInitializer(const SymInitializer&);
  SymInitializer& operator=(const SymInitializer&);
};

static ATTRIBUTE_NOINLINE bool SymbolizeAndDemangle(void* pc,
                                                    char* out,
                                                    size_t out_size) {
  const static SymInitializer symInitializer;
  if (!symInitializer.ready) {
    return false;
  }
  // Resolve symbol information from address.
  // https://msdn.microsoft.com/en-us/library/windows/desktop/ms680578(v=vs.85).aspx
  char buf[sizeof(SYMBOL_INFO) + MAX_SYM_NAME];
  SYMBOL_INFO *symbol = reinterpret_cast<SYMBOL_INFO *>(buf);
  symbol->SizeOfStruct = sizeof(SYMBOL_INFO);
  symbol->MaxNameLen = MAX_SYM_NAME;
  // We use the ANSI version to ensure the string type is always `char *`.
  // This could break if a symbol has Unicode in it.
  BOOL ret = SymFromAddr(symInitializer.process,
                         reinterpret_cast<DWORD64>(pc), 0, symbol);
  if (ret == 1 && static_cast<ssize_t>(symbol->NameLen) < out_size) {
    // `NameLen` does not include the null terminating character.
    strncpy(out, symbol->Name, static_cast<size_t>(symbol->NameLen) + 1);
    out[static_cast<size_t>(symbol->NameLen)] = '\0';
    // Symbolization succeeded.  Now we try to demangle the symbol.
    DemangleInplace(out, out_size);
    return true;
  }
  return false;
}

_END_GOOGLE_NAMESPACE_

#else
# error BUG: HAVE_SYMBOLIZE was wrongly set
#endif

_START_GOOGLE_NAMESPACE_

bool Symbolize(void* pc, char* out, size_t out_size) {
  return SymbolizeAndDemangle(pc, out, out_size);
}

_END_GOOGLE_NAMESPACE_

#else  /* HAVE_SYMBOLIZE */

#include <cassert>

#include "config.h"

_START_GOOGLE_NAMESPACE_

// TODO: Support other environments.
bool Symbolize(void* /*pc*/, char* /*out*/, size_t /*out_size*/) {
  assert(0);
  return false;
}

_END_GOOGLE_NAMESPACE_

#endif