tor-browser

symbolize_elf.inc (61273B)

---

// Copyright 2018 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.

// This library provides Symbolize() function that symbolizes program
// counters to their corresponding symbol names on linux platforms.
// This library has a minimal implementation of an ELF symbol table
// reader (i.e. it doesn't depend on libelf, etc.).
//
// The algorithm used in Symbolize() is as follows.
//
//   1. Go through a list of maps in /proc/self/maps and find the map
//   containing the program counter.
//
//   2. Open the mapped file and find a regular symbol table inside.
//   Iterate over symbols in the symbol table and look for the symbol
//   containing the program counter.  If such a symbol is found,
//   obtain the symbol name, and demangle the symbol if possible.
//   If the symbol isn't found in the regular symbol table (binary is
//   stripped), try the same thing with a dynamic symbol table.
//
// Note that Symbolize() is originally implemented to be used in
// signal handlers, hence it doesn't use malloc() and other unsafe
// operations.  It should be both thread-safe and async-signal-safe.
//
// 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.

#include <dlfcn.h>
#include <elf.h>
#include <fcntl.h>
#include <link.h>  // For ElfW() macro.
#include <sys/resource.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

#include <algorithm>
#include <array>
#include <atomic>
#include <cerrno>
#include <cinttypes>
#include <climits>
#include <cstdint>
#include <cstdio>
#include <cstdlib>
#include <cstring>

#include "absl/base/casts.h"
#include "absl/base/dynamic_annotations.h"
#include "absl/base/internal/low_level_alloc.h"
#include "absl/base/internal/raw_logging.h"
#include "absl/base/internal/spinlock.h"
#include "absl/base/port.h"
#include "absl/debugging/internal/demangle.h"
#include "absl/debugging/internal/vdso_support.h"
#include "absl/strings/string_view.h"

#if defined(__FreeBSD__) && !defined(ElfW)
#define ElfW(x) __ElfN(x)
#endif

namespace absl {
ABSL_NAMESPACE_BEGIN

// Value of argv[0]. Used by MaybeInitializeObjFile().
static char *argv0_value = nullptr;

void InitializeSymbolizer(const char *argv0) {
#ifdef ABSL_HAVE_VDSO_SUPPORT
 // We need to make sure VDSOSupport::Init() is called before any setuid or
 // chroot calls, so InitializeSymbolizer() should be called very early in the
 // life of a program.
 absl::debugging_internal::VDSOSupport::Init();
#endif
 if (argv0_value != nullptr) {
   free(argv0_value);
   argv0_value = nullptr;
 }
 if (argv0 != nullptr && argv0[0] != '\0') {
   argv0_value = strdup(argv0);
 }
}

namespace debugging_internal {
namespace {

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

// On Linux, ELF_ST_* are defined in <linux/elf.h>.  To make this portable
// we define our own ELF_ST_BIND and ELF_ST_TYPE if not available.
#ifndef ELF_ST_BIND
#define ELF_ST_BIND(info) (((unsigned char)(info)) >> 4)
#endif

#ifndef ELF_ST_TYPE
#define ELF_ST_TYPE(info) (((unsigned char)(info)) & 0xF)
#endif

// Some platforms use a special .opd section to store function pointers.
const char kOpdSectionName[] = ".opd";

#if (defined(__powerpc__) && !(_CALL_ELF > 1)) || defined(__ia64)
// Use opd section for function descriptors on these platforms, the function
// address is the first word of the descriptor.
enum { kPlatformUsesOPDSections = 1 };
#else  // not PPC or IA64
enum { kPlatformUsesOPDSections = 0 };
#endif

// This works for PowerPC & IA64 only.  A function descriptor consist of two
// pointers and the first one is the function's entry.
const size_t kFunctionDescriptorSize = sizeof(void *) * 2;

const int kMaxDecorators = 10;  // Seems like a reasonable upper limit.

struct InstalledSymbolDecorator {
 SymbolDecorator fn;
 void *arg;
 int ticket;
};

int g_num_decorators;
InstalledSymbolDecorator g_decorators[kMaxDecorators];

struct FileMappingHint {
 const void *start;
 const void *end;
 uint64_t offset;
 const char *filename;
};

// Protects g_decorators.
// We are using SpinLock and not a Mutex here, because we may be called
// from inside Mutex::Lock itself, and it prohibits recursive calls.
// This happens in e.g. base/stacktrace_syscall_unittest.
// Moreover, we are using only TryLock(), if the decorator list
// is being modified (is busy), we skip all decorators, and possibly
// loose some info. Sorry, that's the best we could do.
ABSL_CONST_INIT absl::base_internal::SpinLock g_decorators_mu(
   absl::kConstInit, absl::base_internal::SCHEDULE_KERNEL_ONLY);

const int kMaxFileMappingHints = 8;
int g_num_file_mapping_hints;
FileMappingHint g_file_mapping_hints[kMaxFileMappingHints];
// Protects g_file_mapping_hints.
ABSL_CONST_INIT absl::base_internal::SpinLock g_file_mapping_mu(
   absl::kConstInit, absl::base_internal::SCHEDULE_KERNEL_ONLY);

// Async-signal-safe function to zero a buffer.
// memset() is not guaranteed to be async-signal-safe.
static void SafeMemZero(void* p, size_t size) {
 unsigned char *c = static_cast<unsigned char *>(p);
 while (size--) {
   *c++ = 0;
 }
}

struct ObjFile {
 ObjFile()
     : filename(nullptr),
       start_addr(nullptr),
       end_addr(nullptr),
       offset(0),
       fd(-1),
       elf_type(-1) {
   SafeMemZero(&elf_header, sizeof(elf_header));
   SafeMemZero(&phdr[0], sizeof(phdr));
 }

 char *filename;
 const void *start_addr;
 const void *end_addr;
 uint64_t offset;

 // The following fields are initialized on the first access to the
 // object file.
 int fd;
 int elf_type;
 ElfW(Ehdr) elf_header;

 // PT_LOAD program header describing executable code.
 // Normally we expect just one, but SWIFT binaries have two.
 // CUDA binaries have 3 (see cr/473913254 description).
 std::array<ElfW(Phdr), 4> phdr;
};

// Build 4-way associative cache for symbols. Within each cache line, symbols
// are replaced in LRU order.
enum {
 ASSOCIATIVITY = 4,
};
struct SymbolCacheLine {
 const void *pc[ASSOCIATIVITY];
 char *name[ASSOCIATIVITY];

 // age[i] is incremented when a line is accessed. it's reset to zero if the
 // i'th entry is read.
 uint32_t age[ASSOCIATIVITY];
};

// ---------------------------------------------------------------
// An async-signal-safe arena for LowLevelAlloc
static std::atomic<base_internal::LowLevelAlloc::Arena *> g_sig_safe_arena;

static base_internal::LowLevelAlloc::Arena *SigSafeArena() {
 return g_sig_safe_arena.load(std::memory_order_acquire);
}

static void InitSigSafeArena() {
 if (SigSafeArena() == nullptr) {
   base_internal::LowLevelAlloc::Arena *new_arena =
       base_internal::LowLevelAlloc::NewArena(
           base_internal::LowLevelAlloc::kAsyncSignalSafe);
   base_internal::LowLevelAlloc::Arena *old_value = nullptr;
   if (!g_sig_safe_arena.compare_exchange_strong(old_value, new_arena,
                                                 std::memory_order_release,
                                                 std::memory_order_relaxed)) {
     // We lost a race to allocate an arena; deallocate.
     base_internal::LowLevelAlloc::DeleteArena(new_arena);
   }
 }
}

// ---------------------------------------------------------------
// An AddrMap is a vector of ObjFile, using SigSafeArena() for allocation.

class AddrMap {
public:
 AddrMap() : size_(0), allocated_(0), obj_(nullptr) {}
 ~AddrMap() { base_internal::LowLevelAlloc::Free(obj_); }
 size_t Size() const { return size_; }
 ObjFile *At(size_t i) { return &obj_[i]; }
 ObjFile *Add();
 void Clear();

private:
 size_t size_;       // count of valid elements (<= allocated_)
 size_t allocated_;  // count of allocated elements
 ObjFile *obj_;      // array of allocated_ elements
 AddrMap(const AddrMap &) = delete;
 AddrMap &operator=(const AddrMap &) = delete;
};

void AddrMap::Clear() {
 for (size_t i = 0; i != size_; i++) {
   At(i)->~ObjFile();
 }
 size_ = 0;
}

ObjFile *AddrMap::Add() {
 if (size_ == allocated_) {
   size_t new_allocated = allocated_ * 2 + 50;
   ObjFile *new_obj_ =
       static_cast<ObjFile *>(base_internal::LowLevelAlloc::AllocWithArena(
           new_allocated * sizeof(*new_obj_), SigSafeArena()));
   if (obj_) {
     memcpy(new_obj_, obj_, allocated_ * sizeof(*new_obj_));
     base_internal::LowLevelAlloc::Free(obj_);
   }
   obj_ = new_obj_;
   allocated_ = new_allocated;
 }
 return new (&obj_[size_++]) ObjFile;
}

class CachingFile {
public:
 // Setup reader for fd that uses buf[0, buf_size-1] as a cache.
 CachingFile(int fd, char *buf, size_t buf_size)
     : fd_(fd),
       cache_(buf),
       cache_size_(buf_size),
       cache_start_(0),
       cache_limit_(0) {}

 int fd() const { return fd_; }
 ssize_t ReadFromOffset(void *buf, size_t count, off_t offset);
 bool ReadFromOffsetExact(void *buf, size_t count, off_t offset);

private:
 // Bytes [cache_start_, cache_limit_-1] from fd_ are stored in
 // a prefix of cache_[0, cache_size_-1].
 int fd_;
 char *cache_;
 size_t cache_size_;
 off_t cache_start_;
 off_t cache_limit_;
};

// ---------------------------------------------------------------

enum FindSymbolResult { SYMBOL_NOT_FOUND = 1, SYMBOL_TRUNCATED, SYMBOL_FOUND };

class Symbolizer {
public:
 Symbolizer();
 ~Symbolizer();
 const char *GetSymbol(const void *const pc);

private:
 char *CopyString(const char *s) {
   size_t len = strlen(s);
   char *dst = static_cast<char *>(
       base_internal::LowLevelAlloc::AllocWithArena(len + 1, SigSafeArena()));
   ABSL_RAW_CHECK(dst != nullptr, "out of memory");
   memcpy(dst, s, len + 1);
   return dst;
 }
 ObjFile *FindObjFile(const void *const start,
                      size_t size) ABSL_ATTRIBUTE_NOINLINE;
 static bool RegisterObjFile(const char *filename,
                             const void *const start_addr,
                             const void *const end_addr, uint64_t offset,
                             void *arg);
 SymbolCacheLine *GetCacheLine(const void *const pc);
 const char *FindSymbolInCache(const void *const pc);
 const char *InsertSymbolInCache(const void *const pc, const char *name);
 void AgeSymbols(SymbolCacheLine *line);
 void ClearAddrMap();
 FindSymbolResult GetSymbolFromObjectFile(const ObjFile &obj,
                                          const void *const pc,
                                          const ptrdiff_t relocation,
                                          char *out, size_t out_size,
                                          char *tmp_buf, size_t tmp_buf_size);
 const char *GetUncachedSymbol(const void *pc);

 enum {
   SYMBOL_BUF_SIZE = 3072,
   TMP_BUF_SIZE = 1024,
   SYMBOL_CACHE_LINES = 128,
   FILE_CACHE_SIZE = 8192,
 };

 AddrMap addr_map_;

 bool ok_;
 bool addr_map_read_;

 char symbol_buf_[SYMBOL_BUF_SIZE];
 char file_cache_[FILE_CACHE_SIZE];

 // tmp_buf_ will be used to store arrays of ElfW(Shdr) and ElfW(Sym)
 // so we ensure that tmp_buf_ is properly aligned to store either.
 alignas(16) char tmp_buf_[TMP_BUF_SIZE];
 static_assert(alignof(ElfW(Shdr)) <= 16,
               "alignment of tmp buf too small for Shdr");
 static_assert(alignof(ElfW(Sym)) <= 16,
               "alignment of tmp buf too small for Sym");

 SymbolCacheLine symbol_cache_[SYMBOL_CACHE_LINES];
};

// Protect against client code closing low-valued file descriptors it doesn't
// actually own.
int OpenReadOnlyWithHighFD(const char *fname) {
 static int high_fd = [] {
   struct rlimit rlim{};
   const int rc = getrlimit(RLIMIT_NOFILE, &rlim);
   if (rc == 0 && rlim.rlim_cur >= 2000) {
     const int max_fd = static_cast<int>(rlim.rlim_cur);

     // This will return 2000 on reasonably-configured systems.
     return std::min<int>(2000, max_fd - 1000);
   }
   ABSL_RAW_LOG(WARNING, "Unable to get high fd: rc=%d, limit=%ld",  //
                rc, static_cast<long>(rlim.rlim_cur));
   return -1;
 }();
 constexpr int kOpenFlags = O_RDONLY | O_CLOEXEC;
 if (high_fd >= 1000) {
   const int fd = open(fname, kOpenFlags);
   if (fd != -1 && fd < high_fd) {
     // Try to relocate fd to high range.
     static_assert(kOpenFlags & O_CLOEXEC,
                   "F_DUPFD_CLOEXEC assumes O_CLOEXEC");
     const int fd2 = fcntl(fd, F_DUPFD_CLOEXEC, high_fd);
     if (fd2 != -1) {
       // Successfully obtained high fd. Use it.
       close(fd);
       return fd2;
     } else {
       ABSL_RAW_LOG(WARNING, "Unable to dup fd=%d above %d, errno=%d", fd,
                    high_fd, errno);
     }
   }
   // Either open failed and fd==-1, or fd is already above high_fd, or fcntl
   // failed and fd is valid (but low).
   return fd;
 }
 return open(fname, kOpenFlags);
}

static std::atomic<Symbolizer *> g_cached_symbolizer;

}  // namespace

static size_t SymbolizerSize() {
#if defined(__wasm__) || defined(__asmjs__)
 auto pagesize = static_cast<size_t>(getpagesize());
#else
 auto pagesize = static_cast<size_t>(sysconf(_SC_PAGESIZE));
#endif
 return ((sizeof(Symbolizer) - 1) / pagesize + 1) * pagesize;
}

// Return (and set null) g_cached_symbolized_state if it is not null.
// Otherwise return a new symbolizer.
static Symbolizer *AllocateSymbolizer() {
 InitSigSafeArena();
 Symbolizer *symbolizer =
     g_cached_symbolizer.exchange(nullptr, std::memory_order_acquire);
 if (symbolizer != nullptr) {
   return symbolizer;
 }
 return new (base_internal::LowLevelAlloc::AllocWithArena(
     SymbolizerSize(), SigSafeArena())) Symbolizer();
}

// Set g_cached_symbolize_state to s if it is null, otherwise
// delete s.
static void FreeSymbolizer(Symbolizer *s) {
 Symbolizer *old_cached_symbolizer = nullptr;
 if (!g_cached_symbolizer.compare_exchange_strong(old_cached_symbolizer, s,
                                                  std::memory_order_release,
                                                  std::memory_order_relaxed)) {
   s->~Symbolizer();
   base_internal::LowLevelAlloc::Free(s);
 }
}

Symbolizer::Symbolizer() : ok_(true), addr_map_read_(false) {
 for (SymbolCacheLine &symbol_cache_line : symbol_cache_) {
   for (size_t j = 0; j < ABSL_ARRAYSIZE(symbol_cache_line.name); ++j) {
     symbol_cache_line.pc[j] = nullptr;
     symbol_cache_line.name[j] = nullptr;
     symbol_cache_line.age[j] = 0;
   }
 }
}

Symbolizer::~Symbolizer() {
 for (SymbolCacheLine &symbol_cache_line : symbol_cache_) {
   for (char *s : symbol_cache_line.name) {
     base_internal::LowLevelAlloc::Free(s);
   }
 }
 ClearAddrMap();
}

// 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.
#define SAFE_ASSERT(expr) ((expr) ? static_cast<void>(0) : abort())

// Read up to "count" bytes from file descriptor "fd" into the buffer
// starting at "buf" while handling short reads and EINTR.  On
// success, return the number of bytes read.  Otherwise, return -1.
static ssize_t ReadPersistent(int fd, void *buf, size_t count) {
 SAFE_ASSERT(fd >= 0);
 SAFE_ASSERT(count <= SSIZE_MAX);
 char *buf0 = reinterpret_cast<char *>(buf);
 size_t num_bytes = 0;
 while (num_bytes < count) {
   ssize_t len;
   NO_INTR(len = read(fd, buf0 + num_bytes, count - num_bytes));
   if (len < 0) {  // There was an error other than EINTR.
     ABSL_RAW_LOG(WARNING, "read failed: errno=%d", errno);
     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);
}

// Read up to "count" bytes from "offset" into the buffer starting at "buf",
// while handling short reads and EINTR.  On success, return the number of bytes
// read.  Otherwise, return -1.
ssize_t CachingFile::ReadFromOffset(void *buf, size_t count, off_t offset) {
 char *dst = static_cast<char *>(buf);
 size_t read = 0;
 while (read < count) {
   // Look in cache first.
   if (offset >= cache_start_ && offset < cache_limit_) {
     const char *hit_start = &cache_[offset - cache_start_];
     const size_t n =
         std::min(count - read, static_cast<size_t>(cache_limit_ - offset));
     memcpy(dst, hit_start, n);
     dst += n;
     read += static_cast<size_t>(n);
     offset += static_cast<off_t>(n);
     continue;
   }

   cache_start_ = 0;
   cache_limit_ = 0;
   ssize_t n = pread(fd_, cache_, cache_size_, offset);
   if (n < 0) {
     if (errno == EINTR) {
       continue;
     }
     ABSL_RAW_LOG(WARNING, "read failed: errno=%d", errno);
     return -1;
   }
   if (n == 0) {  // Reached EOF.
     break;
   }

   cache_start_ = offset;
   cache_limit_ = offset + static_cast<off_t>(n);
   // Next iteration will copy from cache into dst.
 }
 return static_cast<ssize_t>(read);
}

// Try reading exactly "count" bytes from "offset" bytes into the buffer
// starting at "buf" while handling short reads and EINTR.  On success, return
// true. Otherwise, return false.
bool CachingFile::ReadFromOffsetExact(void *buf, size_t count, off_t offset) {
 ssize_t len = ReadFromOffset(buf, count, offset);
 return len >= 0 && static_cast<size_t>(len) == count;
}

// Returns elf_header.e_type if the file pointed by fd is an ELF binary.
static int FileGetElfType(CachingFile *file) {
 ElfW(Ehdr) elf_header;
 if (!file->ReadFromOffsetExact(&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 ABSL_ATTRIBUTE_NOINLINE bool GetSectionHeaderByType(
   CachingFile *file, ElfW(Half) sh_num, const off_t sh_offset,
   ElfW(Word) type, ElfW(Shdr) * out, char *tmp_buf, size_t tmp_buf_size) {
 ElfW(Shdr) *buf = reinterpret_cast<ElfW(Shdr) *>(tmp_buf);
 const size_t buf_entries = tmp_buf_size / sizeof(buf[0]);
 const size_t buf_bytes = buf_entries * sizeof(buf[0]);

 for (size_t i = 0; static_cast<int>(i) < sh_num;) {
   const size_t num_bytes_left =
       (static_cast<size_t>(sh_num) - i) * sizeof(buf[0]);
   const size_t num_bytes_to_read =
       (buf_bytes > num_bytes_left) ? num_bytes_left : buf_bytes;
   const off_t offset = sh_offset + static_cast<off_t>(i * sizeof(buf[0]));
   const ssize_t len = file->ReadFromOffset(buf, num_bytes_to_read, offset);
   if (len <= 0) {
     ABSL_RAW_LOG(WARNING, "Reading %zu bytes from offset %ju returned %zd.",
                  num_bytes_to_read, static_cast<intmax_t>(offset), len);
     return false;
   }
   if (static_cast<size_t>(len) % sizeof(buf[0]) != 0) {
     ABSL_RAW_LOG(
         WARNING,
         "Reading %zu bytes from offset %jd returned %zd which is not a "
         "multiple of %zu.",
         num_bytes_to_read, static_cast<intmax_t>(offset), len,
         sizeof(buf[0]));
     return false;
   }
   const size_t num_headers_in_buf = static_cast<size_t>(len) / sizeof(buf[0]);
   SAFE_ASSERT(num_headers_in_buf <= buf_entries);
   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;

// Small cache to use for miscellaneous file reads.
const int kSmallFileCacheSize = 100;

bool ForEachSection(int fd,
                   const std::function<bool(absl::string_view name,
                                            const ElfW(Shdr) &)> &callback) {
 char buf[kSmallFileCacheSize];
 CachingFile file(fd, buf, sizeof(buf));

 ElfW(Ehdr) elf_header;
 if (!file.ReadFromOffsetExact(&elf_header, sizeof(elf_header), 0)) {
   return false;
 }

 // Technically it can be larger, but in practice this never happens.
 if (elf_header.e_shentsize != sizeof(ElfW(Shdr))) {
   return false;
 }

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

 for (int i = 0; i < elf_header.e_shnum; ++i) {
   ElfW(Shdr) out;
   off_t section_header_offset =
       static_cast<off_t>(elf_header.e_shoff) + elf_header.e_shentsize * i;
   if (!file.ReadFromOffsetExact(&out, sizeof(out), section_header_offset)) {
     return false;
   }
   off_t name_offset = static_cast<off_t>(shstrtab.sh_offset) + out.sh_name;
   char header_name[kMaxSectionNameLen];
   ssize_t n_read =
       file.ReadFromOffset(&header_name, kMaxSectionNameLen, name_offset);
   if (n_read < 0) {
     return false;
   } else if (n_read > kMaxSectionNameLen) {
     // Long read?
     return false;
   }

   absl::string_view name(header_name,
                          strnlen(header_name, static_cast<size_t>(n_read)));
   if (!callback(name, out)) {
     break;
   }
 }
 return true;
}

// name_len should include terminating '\0'.
bool GetSectionHeaderByName(int fd, const char *name, size_t name_len,
                           ElfW(Shdr) * out) {
 char header_name[kMaxSectionNameLen];
 if (sizeof(header_name) < name_len) {
   ABSL_RAW_LOG(WARNING,
                "Section name '%s' is too long (%zu); "
                "section will not be found (even if present).",
                name, name_len);
   // No point in even trying.
   return false;
 }

 char buf[kSmallFileCacheSize];
 CachingFile file(fd, buf, sizeof(buf));
 ElfW(Ehdr) elf_header;
 if (!file.ReadFromOffsetExact(&elf_header, sizeof(elf_header), 0)) {
   return false;
 }

 // Technically it can be larger, but in practice this never happens.
 if (elf_header.e_shentsize != sizeof(ElfW(Shdr))) {
   return false;
 }

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

 for (int i = 0; i < elf_header.e_shnum; ++i) {
   off_t section_header_offset =
       static_cast<off_t>(elf_header.e_shoff) + elf_header.e_shentsize * i;
   if (!file.ReadFromOffsetExact(out, sizeof(*out), section_header_offset)) {
     return false;
   }
   off_t name_offset = static_cast<off_t>(shstrtab.sh_offset) + out->sh_name;
   ssize_t n_read = file.ReadFromOffset(&header_name, name_len, name_offset);
   if (n_read < 0) {
     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;
}

// Compare symbols at in the same address.
// Return true if we should pick symbol1.
static bool ShouldPickFirstSymbol(const ElfW(Sym) & symbol1,
                                 const ElfW(Sym) & symbol2) {
 // If one of the symbols is weak and the other is not, pick the one
 // this is not a weak symbol.
 char bind1 = ELF_ST_BIND(symbol1.st_info);
 char bind2 = ELF_ST_BIND(symbol1.st_info);
 if (bind1 == STB_WEAK && bind2 != STB_WEAK) return false;
 if (bind2 == STB_WEAK && bind1 != STB_WEAK) return true;

 // If one of the symbols has zero size and the other is not, pick the
 // one that has non-zero size.
 if (symbol1.st_size != 0 && symbol2.st_size == 0) {
   return true;
 }
 if (symbol1.st_size == 0 && symbol2.st_size != 0) {
   return false;
 }

 // If one of the symbols has no type and the other is not, pick the
 // one that has a type.
 char type1 = ELF_ST_TYPE(symbol1.st_info);
 char type2 = ELF_ST_TYPE(symbol1.st_info);
 if (type1 != STT_NOTYPE && type2 == STT_NOTYPE) {
   return true;
 }
 if (type1 == STT_NOTYPE && type2 != STT_NOTYPE) {
   return false;
 }

 // Pick the first one, if we still cannot decide.
 return true;
}

// Return true if an address is inside a section.
static bool InSection(const void *address, ptrdiff_t relocation,
                     const ElfW(Shdr) * section) {
 const char *start = reinterpret_cast<const char *>(
     section->sh_addr + static_cast<ElfW(Addr)>(relocation));
 size_t size = static_cast<size_t>(section->sh_size);
 return start <= address && address < (start + size);
}

static const char *ComputeOffset(const char *base, ptrdiff_t offset) {
 // Note: cast to intptr_t to avoid undefined behavior when base evaluates to
 // zero and offset is non-zero.
 return reinterpret_cast<const char *>(reinterpret_cast<intptr_t>(base) +
                                       offset);
}

// 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".  If the symbol is found, and its name fits in
// out_size, the name is written into out and SYMBOL_FOUND is returned.
// If the name does not fit, truncated name is written into out,
// and SYMBOL_TRUNCATED is returned. Out is NUL-terminated.
// If the symbol is not found, SYMBOL_NOT_FOUND is returned;
// To keep stack consumption low, we would like this function to not get
// inlined.
static ABSL_ATTRIBUTE_NOINLINE FindSymbolResult FindSymbol(
   const void *const pc, CachingFile *file, char *out, size_t out_size,
   ptrdiff_t relocation, const ElfW(Shdr) * strtab, const ElfW(Shdr) * symtab,
   const ElfW(Shdr) * opd, char *tmp_buf, size_t tmp_buf_size) {
 if (symtab == nullptr) {
   return SYMBOL_NOT_FOUND;
 }

 // Read multiple symbols at once to save read() calls.
 ElfW(Sym) *buf = reinterpret_cast<ElfW(Sym) *>(tmp_buf);
 const size_t buf_entries = tmp_buf_size / sizeof(buf[0]);

 const size_t num_symbols = symtab->sh_size / symtab->sh_entsize;

 // On platforms using an .opd section (PowerPC & IA64), a function symbol
 // has the address of a function descriptor, which contains the real
 // starting address.  However, we do not always want to use the real
 // starting address because we sometimes want to symbolize a function
 // pointer into the .opd section, e.g. FindSymbol(&foo,...).
 const bool pc_in_opd = kPlatformUsesOPDSections && opd != nullptr &&
                        InSection(pc, relocation, opd);
 const bool deref_function_descriptor_pointer =
     kPlatformUsesOPDSections && opd != nullptr && !pc_in_opd;

 ElfW(Sym) best_match;
 SafeMemZero(&best_match, sizeof(best_match));
 bool found_match = false;
 for (size_t i = 0; i < num_symbols;) {
   off_t offset =
       static_cast<off_t>(symtab->sh_offset + i * symtab->sh_entsize);
   const size_t num_remaining_symbols = num_symbols - i;
   const size_t entries_in_chunk =
       std::min(num_remaining_symbols, buf_entries);
   const size_t bytes_in_chunk = entries_in_chunk * sizeof(buf[0]);
   const ssize_t len = file->ReadFromOffset(buf, bytes_in_chunk, offset);
   SAFE_ASSERT(len >= 0);
   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 <= entries_in_chunk);
   for (size_t j = 0; j < num_symbols_in_buf; ++j) {
     const ElfW(Sym) &symbol = buf[j];

     // For a DSO, a symbol address is relocated by the loading address.
     // We keep the original address for opd redirection below.
     const char *const original_start_address =
         reinterpret_cast<const char *>(symbol.st_value);
     const char *start_address =
         ComputeOffset(original_start_address, relocation);

#ifdef __arm__
     // ARM functions are always aligned to multiples of two bytes; the
     // lowest-order bit in start_address is ignored by the CPU and indicates
     // whether the function contains ARM (0) or Thumb (1) code. We don't care
     // about what encoding is being used; we just want the real start address
     // of the function.
     start_address = reinterpret_cast<const char *>(
         reinterpret_cast<uintptr_t>(start_address) & ~1u);
#endif

     if (deref_function_descriptor_pointer &&
         InSection(original_start_address, /*relocation=*/0, opd)) {
       // The opd section is mapped into memory.  Just dereference
       // start_address to get the first double word, which points to the
       // function entry.
       start_address = *reinterpret_cast<const char *const *>(start_address);
     }

     // If pc is inside the .opd section, it points to a function descriptor.
     const size_t size = pc_in_opd ? kFunctionDescriptorSize : symbol.st_size;
     const void *const end_address =
         ComputeOffset(start_address, static_cast<ptrdiff_t>(size));
     if (symbol.st_value != 0 &&  // Skip null value symbols.
         symbol.st_shndx != 0 &&  // Skip undefined symbols.
#ifdef STT_TLS
         ELF_ST_TYPE(symbol.st_info) != STT_TLS &&  // Skip thread-local data.
#endif                                               // STT_TLS
         ((start_address <= pc && pc < end_address) ||
          (start_address == pc && pc == end_address))) {
       if (!found_match || ShouldPickFirstSymbol(symbol, best_match)) {
         found_match = true;
         best_match = symbol;
       }
     }
   }
   i += num_symbols_in_buf;
 }

 if (found_match) {
   const off_t off =
       static_cast<off_t>(strtab->sh_offset) + best_match.st_name;
   const ssize_t n_read = file->ReadFromOffset(out, out_size, off);
   if (n_read <= 0) {
     // This should never happen.
     ABSL_RAW_LOG(WARNING,
                  "Unable to read from fd %d at offset %lld: n_read = %zd",
                  file->fd(), static_cast<long long>(off), n_read);
     return SYMBOL_NOT_FOUND;
   }
   ABSL_RAW_CHECK(static_cast<size_t>(n_read) <= out_size,
                  "ReadFromOffset read too much data.");

   // strtab->sh_offset points into .strtab-like section that contains
   // NUL-terminated strings: '\0foo\0barbaz\0...".
   //
   // sh_offset+st_name points to the start of symbol name, but we don't know
   // how long the symbol is, so we try to read as much as we have space for,
   // and usually over-read (i.e. there is a NUL somewhere before n_read).
   if (memchr(out, '\0', static_cast<size_t>(n_read)) == nullptr) {
     // Either out_size was too small (n_read == out_size and no NUL), or
     // we tried to read past the EOF (n_read < out_size) and .strtab is
     // corrupt (missing terminating NUL; should never happen for valid ELF).
     out[n_read - 1] = '\0';
     return SYMBOL_TRUNCATED;
   }
   return SYMBOL_FOUND;
 }

 return SYMBOL_NOT_FOUND;
}

// Get the symbol name of "pc" from the file pointed by "fd".  Process
// both regular and dynamic symbol tables if necessary.
// See FindSymbol() comment for description of return value.
FindSymbolResult Symbolizer::GetSymbolFromObjectFile(
   const ObjFile &obj, const void *const pc, const ptrdiff_t relocation,
   char *out, size_t out_size, char *tmp_buf, size_t tmp_buf_size) {
 ElfW(Shdr) symtab;
 ElfW(Shdr) strtab;
 ElfW(Shdr) opd;
 ElfW(Shdr) *opd_ptr = nullptr;

 // On platforms using an .opd sections for function descriptor, read
 // the section header.  The .opd section is in data segment and should be
 // loaded but we check that it is mapped just to be extra careful.
 if (kPlatformUsesOPDSections) {
   if (GetSectionHeaderByName(obj.fd, kOpdSectionName,
                              sizeof(kOpdSectionName) - 1, &opd) &&
       FindObjFile(reinterpret_cast<const char *>(opd.sh_addr) + relocation,
                   opd.sh_size) != nullptr) {
     opd_ptr = &opd;
   } else {
     return SYMBOL_NOT_FOUND;
   }
 }

 CachingFile file(obj.fd, file_cache_, sizeof(file_cache_));

 // Consult a regular symbol table, then fall back to the dynamic symbol table.
 for (const auto symbol_table_type : {SHT_SYMTAB, SHT_DYNSYM}) {
   if (!GetSectionHeaderByType(&file, obj.elf_header.e_shnum,
                               static_cast<off_t>(obj.elf_header.e_shoff),
                               static_cast<ElfW(Word)>(symbol_table_type),
                               &symtab, tmp_buf, tmp_buf_size)) {
     continue;
   }
   if (!file.ReadFromOffsetExact(
           &strtab, sizeof(strtab),
           static_cast<off_t>(obj.elf_header.e_shoff +
                              symtab.sh_link * sizeof(symtab)))) {
     continue;
   }
   const FindSymbolResult rc =
       FindSymbol(pc, &file, out, out_size, relocation, &strtab, &symtab,
                  opd_ptr, tmp_buf, tmp_buf_size);
   if (rc != SYMBOL_NOT_FOUND) {
     return rc;
   }
 }

 return SYMBOL_NOT_FOUND;
}

namespace {
// Thin wrapper around a file descriptor so that the file descriptor
// gets closed for sure.
class FileDescriptor {
public:
 explicit FileDescriptor(int fd) : fd_(fd) {}
 FileDescriptor(const FileDescriptor &) = delete;
 FileDescriptor &operator=(const FileDescriptor &) = delete;

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

 int get() const { return fd_; }

private:
 const int fd_;
};

// 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)
     : fd_(fd),
       buf_len_(buf_len),
       buf_(buf),
       bol_(buf),
       eol_(buf),
       eod_(buf) {}

 LineReader(const LineReader &) = delete;
 LineReader &operator=(const LineReader &) = delete;

 // 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 = ReadPersistent(fd_, buf_, buf_len_);
     if (num_bytes <= 0) {  // EOF or error.
       return false;
     }
     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 auto 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 =
           ReadPersistent(fd_, append_pos, capacity_left);
       if (num_bytes <= 0) {  // EOF or error.
         return false;
       }
       eod_ = append_pos + num_bytes;
       bol_ = buf_;
     }
   }
   eol_ = FindLineFeed();
   if (eol_ == nullptr) {  // '\n' not found.  Malformed line.
     return false;
   }
   *eol_ = '\0';  // Replace '\n' with '\0'.

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

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

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

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

 const int fd_;
 const size_t buf_len_;
 char *const buf_;
 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 const char *GetHex(const char *start, const char *end,
                         uint64_t *const value) {
 uint64_t 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 << 4) |
           static_cast<uint64_t>(ch < 'A' ? ch - '0' : (ch & 0xF) + 9);
   } else {  // Encountered the first non-hex character.
     break;
   }
 }
 SAFE_ASSERT(p <= end);
 *value = hex;
 return p;
}

static const char *GetHex(const char *start, const char *end,
                         const void **const addr) {
 uint64_t hex = 0;
 const char *p = GetHex(start, end, &hex);
 *addr = reinterpret_cast<void *>(hex);
 return p;
}

// Normally we are only interested in "r?x" maps.
// On the PowerPC, function pointers point to descriptors in the .opd
// section.  The descriptors themselves are not executable code, so
// we need to relax the check below to "r??".
static bool ShouldUseMapping(const char *const flags) {
 return flags[0] == 'r' && (kPlatformUsesOPDSections || flags[2] == 'x');
}

// Read /proc/self/maps and run "callback" for each mmapped file found.  If
// "callback" returns false, stop scanning and return true. Else continue
// scanning /proc/self/maps. Return true if no parse error is found.
static ABSL_ATTRIBUTE_NOINLINE bool ReadAddrMap(
   bool (*callback)(const char *filename, const void *const start_addr,
                    const void *const end_addr, uint64_t offset, void *arg),
   void *arg, void *tmp_buf, size_t tmp_buf_size) {
 // Use /proc/self/task/<pid>/maps instead of /proc/self/maps. The latter
 // requires kernel to stop all threads, and is significantly slower when there
 // are 1000s of threads.
 char maps_path[80];
 snprintf(maps_path, sizeof(maps_path), "/proc/self/task/%d/maps", getpid());

 int maps_fd;
 NO_INTR(maps_fd = OpenReadOnlyWithHighFD(maps_path));
 FileDescriptor wrapped_maps_fd(maps_fd);
 if (wrapped_maps_fd.get() < 0) {
   ABSL_RAW_LOG(WARNING, "%s: errno=%d", maps_path, errno);
   return false;
 }

 // Iterate over maps and look for the map containing the pc.  Then
 // look into the symbol tables inside.
 LineReader reader(wrapped_maps_fd.get(), static_cast<char *>(tmp_buf),
                   tmp_buf_size);
 while (true) {
   const char *cursor;
   const char *eol;
   if (!reader.ReadLine(&cursor, &eol)) {  // EOF or malformed line.
     break;
   }

   const char *line = cursor;
   const void *start_address;
   // 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 != '-') {
     ABSL_RAW_LOG(WARNING, "Corrupt /proc/self/maps line: %s", line);
     return false;
   }
   ++cursor;  // Skip '-'.

   // Read end address.
   const void *end_address;
   cursor = GetHex(cursor, eol, &end_address);
   if (cursor == eol || *cursor != ' ') {
     ABSL_RAW_LOG(WARNING, "Corrupt /proc/self/maps line: %s", line);
     return false;
   }
   ++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) {
     ABSL_RAW_LOG(WARNING, "Corrupt /proc/self/maps: %s", line);
     return false;
   }

   // Check flags.
   if (!ShouldUseMapping(flags_start)) {
     continue;  // We skip this map.
   }
   ++cursor;  // Skip ' '.

   // Read file offset.
   uint64_t offset;
   cursor = GetHex(cursor, eol, &offset);
   ++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;
   }

   // Check whether this entry corresponds to our hint table for the true
   // filename.
   bool hinted =
       GetFileMappingHint(&start_address, &end_address, &offset, &cursor);
   if (!hinted && (cursor == eol || cursor[0] == '[')) {
     // not an object file, typically [vdso] or [vsyscall]
     continue;
   }
   if (!callback(cursor, start_address, end_address, offset, arg)) break;
 }
 return true;
}

// Find the objfile mapped in address region containing [addr, addr + len).
ObjFile *Symbolizer::FindObjFile(const void *const addr, size_t len) {
 for (int i = 0; i < 2; ++i) {
   if (!ok_) return nullptr;

   // Read /proc/self/maps if necessary
   if (!addr_map_read_) {
     addr_map_read_ = true;
     if (!ReadAddrMap(RegisterObjFile, this, tmp_buf_, TMP_BUF_SIZE)) {
       ok_ = false;
       return nullptr;
     }
   }

   size_t lo = 0;
   size_t hi = addr_map_.Size();
   while (lo < hi) {
     size_t mid = (lo + hi) / 2;
     if (addr < addr_map_.At(mid)->end_addr) {
       hi = mid;
     } else {
       lo = mid + 1;
     }
   }
   if (lo != addr_map_.Size()) {
     ObjFile *obj = addr_map_.At(lo);
     SAFE_ASSERT(obj->end_addr > addr);
     if (addr >= obj->start_addr &&
         reinterpret_cast<const char *>(addr) + len <= obj->end_addr)
       return obj;
   }

   // The address mapping may have changed since it was last read.  Retry.
   ClearAddrMap();
 }
 return nullptr;
}

void Symbolizer::ClearAddrMap() {
 for (size_t i = 0; i != addr_map_.Size(); i++) {
   ObjFile *o = addr_map_.At(i);
   base_internal::LowLevelAlloc::Free(o->filename);
   if (o->fd >= 0) {
     close(o->fd);
   }
 }
 addr_map_.Clear();
 addr_map_read_ = false;
}

// Callback for ReadAddrMap to register objfiles in an in-memory table.
bool Symbolizer::RegisterObjFile(const char *filename,
                                const void *const start_addr,
                                const void *const end_addr, uint64_t offset,
                                void *arg) {
 Symbolizer *impl = static_cast<Symbolizer *>(arg);

 // Files are supposed to be added in the increasing address order.  Make
 // sure that's the case.
 size_t addr_map_size = impl->addr_map_.Size();
 if (addr_map_size != 0) {
   ObjFile *old = impl->addr_map_.At(addr_map_size - 1);
   if (old->end_addr > end_addr) {
     ABSL_RAW_LOG(ERROR,
                  "Unsorted addr map entry: 0x%" PRIxPTR ": %s <-> 0x%" PRIxPTR
                  ": %s",
                  reinterpret_cast<uintptr_t>(end_addr), filename,
                  reinterpret_cast<uintptr_t>(old->end_addr), old->filename);
     return true;
   } else if (old->end_addr == end_addr) {
     // The same entry appears twice. This sometimes happens for [vdso].
     if (old->start_addr != start_addr ||
         strcmp(old->filename, filename) != 0) {
       ABSL_RAW_LOG(ERROR,
                    "Duplicate addr 0x%" PRIxPTR ": %s <-> 0x%" PRIxPTR ": %s",
                    reinterpret_cast<uintptr_t>(end_addr), filename,
                    reinterpret_cast<uintptr_t>(old->end_addr), old->filename);
     }
     return true;
   } else if (old->end_addr == start_addr &&
              reinterpret_cast<uintptr_t>(old->start_addr) - old->offset ==
                  reinterpret_cast<uintptr_t>(start_addr) - offset &&
              strcmp(old->filename, filename) == 0) {
     // Two contiguous map entries that span a contiguous region of the file,
     // perhaps because some part of the file was mlock()ed. Combine them.
     old->end_addr = end_addr;
     return true;
   }
 }
 ObjFile *obj = impl->addr_map_.Add();
 obj->filename = impl->CopyString(filename);
 obj->start_addr = start_addr;
 obj->end_addr = end_addr;
 obj->offset = offset;
 obj->elf_type = -1;  // filled on demand
 obj->fd = -1;        // opened on demand
 return true;
}

// 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 ABSL_ATTRIBUTE_NOINLINE void DemangleInplace(char *out, size_t out_size,
                                                   char *tmp_buf,
                                                   size_t tmp_buf_size) {
 if (Demangle(out, tmp_buf, tmp_buf_size)) {
   // Demangling succeeded. Copy to out if the space allows.
   size_t len = strlen(tmp_buf);
   if (len + 1 <= out_size) {  // +1 for '\0'.
     SAFE_ASSERT(len < tmp_buf_size);
     memmove(out, tmp_buf, len + 1);
   }
 }
}

SymbolCacheLine *Symbolizer::GetCacheLine(const void *const pc) {
 uintptr_t pc0 = reinterpret_cast<uintptr_t>(pc);
 pc0 >>= 3;  // drop the low 3 bits

 // Shuffle bits.
 pc0 ^= (pc0 >> 6) ^ (pc0 >> 12) ^ (pc0 >> 18);
 return &symbol_cache_[pc0 % SYMBOL_CACHE_LINES];
}

void Symbolizer::AgeSymbols(SymbolCacheLine *line) {
 for (uint32_t &age : line->age) {
   ++age;
 }
}

const char *Symbolizer::FindSymbolInCache(const void *const pc) {
 if (pc == nullptr) return nullptr;

 SymbolCacheLine *line = GetCacheLine(pc);
 for (size_t i = 0; i < ABSL_ARRAYSIZE(line->pc); ++i) {
   if (line->pc[i] == pc) {
     AgeSymbols(line);
     line->age[i] = 0;
     return line->name[i];
   }
 }
 return nullptr;
}

const char *Symbolizer::InsertSymbolInCache(const void *const pc,
                                           const char *name) {
 SAFE_ASSERT(pc != nullptr);

 SymbolCacheLine *line = GetCacheLine(pc);
 uint32_t max_age = 0;
 size_t oldest_index = 0;
 bool found_oldest_index = false;
 for (size_t i = 0; i < ABSL_ARRAYSIZE(line->pc); ++i) {
   if (line->pc[i] == nullptr) {
     AgeSymbols(line);
     line->pc[i] = pc;
     line->name[i] = CopyString(name);
     line->age[i] = 0;
     return line->name[i];
   }
   if (line->age[i] >= max_age) {
     max_age = line->age[i];
     oldest_index = i;
     found_oldest_index = true;
   }
 }

 AgeSymbols(line);
 ABSL_RAW_CHECK(found_oldest_index, "Corrupt cache");
 base_internal::LowLevelAlloc::Free(line->name[oldest_index]);
 line->pc[oldest_index] = pc;
 line->name[oldest_index] = CopyString(name);
 line->age[oldest_index] = 0;
 return line->name[oldest_index];
}

static void MaybeOpenFdFromSelfExe(ObjFile *obj) {
 if (memcmp(obj->start_addr, ELFMAG, SELFMAG) != 0) {
   return;
 }
 int fd = OpenReadOnlyWithHighFD("/proc/self/exe");
 if (fd == -1) {
   return;
 }
 // Verify that contents of /proc/self/exe matches in-memory image of
 // the binary. This can fail if the "deleted" binary is in fact not
 // the main executable, or for binaries that have the first PT_LOAD
 // segment smaller than 4K. We do it in four steps so that the
 // buffer is smaller and we don't consume too much stack space.
 const char *mem = reinterpret_cast<const char *>(obj->start_addr);
 for (int i = 0; i < 4; ++i) {
   char buf[1024];
   ssize_t n = read(fd, buf, sizeof(buf));
   if (n != sizeof(buf) || memcmp(buf, mem, sizeof(buf)) != 0) {
     close(fd);
     return;
   }
   mem += sizeof(buf);
 }
 obj->fd = fd;
}

static bool MaybeInitializeObjFile(ObjFile *obj) {
 if (obj->fd < 0) {
   obj->fd = OpenReadOnlyWithHighFD(obj->filename);

   if (obj->fd < 0) {
     // Getting /proc/self/exe here means that we were hinted.
     if (strcmp(obj->filename, "/proc/self/exe") == 0) {
       // /proc/self/exe may be inaccessible (due to setuid, etc.), so try
       // accessing the binary via argv0.
       if (argv0_value != nullptr) {
         obj->fd = OpenReadOnlyWithHighFD(argv0_value);
       }
     } else {
       MaybeOpenFdFromSelfExe(obj);
     }
   }

   if (obj->fd < 0) {
     ABSL_RAW_LOG(WARNING, "%s: open failed: errno=%d", obj->filename, errno);
     return false;
   }

   char buf[kSmallFileCacheSize];
   CachingFile file(obj->fd, buf, sizeof(buf));

   obj->elf_type = FileGetElfType(&file);
   if (obj->elf_type < 0) {
     ABSL_RAW_LOG(WARNING, "%s: wrong elf type: %d", obj->filename,
                  obj->elf_type);
     return false;
   }

   if (!file.ReadFromOffsetExact(&obj->elf_header, sizeof(obj->elf_header),
                                 0)) {
     ABSL_RAW_LOG(WARNING, "%s: failed to read elf header", obj->filename);
     return false;
   }
   const int phnum = obj->elf_header.e_phnum;
   const int phentsize = obj->elf_header.e_phentsize;
   auto phoff = static_cast<off_t>(obj->elf_header.e_phoff);
   size_t num_interesting_load_segments = 0;
   for (int j = 0; j < phnum; j++) {
     ElfW(Phdr) phdr;
     if (!file.ReadFromOffsetExact(&phdr, sizeof(phdr), phoff)) {
       ABSL_RAW_LOG(WARNING, "%s: failed to read program header %d",
                    obj->filename, j);
       return false;
     }
     phoff += phentsize;

#if defined(__powerpc__) && !(_CALL_ELF > 1)
     // On the PowerPC ELF v1 ABI, function pointers actually point to function
     // descriptors. These descriptors are stored in an .opd section, which is
     // mapped read-only. We thus need to look at all readable segments, not
     // just the executable ones.
     constexpr int interesting = PF_R;
#else
     constexpr int interesting = PF_X | PF_R;
#endif

     if (phdr.p_type != PT_LOAD
         || (phdr.p_flags & interesting) != interesting) {
       // Not a LOAD segment, not executable code, and not a function
       // descriptor.
       continue;
     }
     if (num_interesting_load_segments < obj->phdr.size()) {
       memcpy(&obj->phdr[num_interesting_load_segments++], &phdr, sizeof(phdr));
     } else {
       ABSL_RAW_LOG(
           WARNING, "%s: too many interesting LOAD segments: %zu >= %zu",
           obj->filename, num_interesting_load_segments, obj->phdr.size());
       break;
     }
   }
   if (num_interesting_load_segments == 0) {
     // This object has no interesting LOAD segments. That's unexpected.
     ABSL_RAW_LOG(WARNING, "%s: no interesting LOAD segments", obj->filename);
     return false;
   }
 }
 return true;
}

// The implementation of our symbolization routine.  If it
// successfully finds the symbol containing "pc" and obtains the
// symbol name, returns pointer to that symbol. Otherwise, returns nullptr.
// If any symbol decorators have been installed via InstallSymbolDecorator(),
// they are called here as well.
// To keep stack consumption low, we would like this function to not
// get inlined.
const char *Symbolizer::GetUncachedSymbol(const void *pc) {
 ObjFile *const obj = FindObjFile(pc, 1);
 ptrdiff_t relocation = 0;
 int fd = -1;
 if (obj != nullptr) {
   if (MaybeInitializeObjFile(obj)) {
     const size_t start_addr = reinterpret_cast<size_t>(obj->start_addr);
     if (obj->elf_type == ET_DYN && start_addr >= obj->offset) {
       // This object was relocated.
       //
       // For obj->offset > 0, adjust the relocation since a mapping at offset
       // X in the file will have a start address of [true relocation]+X.
       relocation = static_cast<ptrdiff_t>(start_addr - obj->offset);

       // Note: some binaries have multiple LOAD segments that can contain
       // function pointers. We must find the right one.
       ElfW(Phdr) *phdr = nullptr;
       for (size_t j = 0; j < obj->phdr.size(); j++) {
         ElfW(Phdr) &p = obj->phdr[j];
         if (p.p_type != PT_LOAD) {
           // We only expect PT_LOADs. This must be PT_NULL that we didn't
           // write over (i.e. we exhausted all interesting PT_LOADs).
           ABSL_RAW_CHECK(p.p_type == PT_NULL, "unexpected p_type");
           break;
         }
         if (pc < reinterpret_cast<void *>(start_addr + p.p_vaddr + p.p_memsz)) {
           phdr = &p;
           break;
         }
       }
       if (phdr == nullptr) {
         // That's unexpected. Hope for the best.
         ABSL_RAW_LOG(
             WARNING,
             "%s: unable to find LOAD segment for pc: %p, start_addr: %zx",
             obj->filename, pc, start_addr);
       } else {
         // Adjust relocation in case phdr.p_vaddr != 0.
         // This happens for binaries linked with `lld --rosegment`, and for
         // binaries linked with BFD `ld -z separate-code`.
         relocation -= phdr->p_vaddr - phdr->p_offset;
       }
     }

     fd = obj->fd;
     if (GetSymbolFromObjectFile(*obj, pc, relocation, symbol_buf_,
                                 sizeof(symbol_buf_), tmp_buf_,
                                 sizeof(tmp_buf_)) == SYMBOL_FOUND) {
       // Only try to demangle the symbol name if it fit into symbol_buf_.
       DemangleInplace(symbol_buf_, sizeof(symbol_buf_), tmp_buf_,
                       sizeof(tmp_buf_));
     }
   }
 } else {
#if ABSL_HAVE_VDSO_SUPPORT
   VDSOSupport vdso;
   if (vdso.IsPresent()) {
     VDSOSupport::SymbolInfo symbol_info;
     if (vdso.LookupSymbolByAddress(pc, &symbol_info)) {
       // All VDSO symbols are known to be short.
       size_t len = strlen(symbol_info.name);
       ABSL_RAW_CHECK(len + 1 < sizeof(symbol_buf_),
                      "VDSO symbol unexpectedly long");
       memcpy(symbol_buf_, symbol_info.name, len + 1);
     }
   }
#endif
 }

 if (g_decorators_mu.TryLock()) {
   if (g_num_decorators > 0) {
     SymbolDecoratorArgs decorator_args = {
         pc,       relocation,       fd,     symbol_buf_, sizeof(symbol_buf_),
         tmp_buf_, sizeof(tmp_buf_), nullptr};
     for (int i = 0; i < g_num_decorators; ++i) {
       decorator_args.arg = g_decorators[i].arg;
       g_decorators[i].fn(&decorator_args);
     }
   }
   g_decorators_mu.Unlock();
 }
 if (symbol_buf_[0] == '\0') {
   return nullptr;
 }
 symbol_buf_[sizeof(symbol_buf_) - 1] = '\0';  // Paranoia.
 return InsertSymbolInCache(pc, symbol_buf_);
}

const char *Symbolizer::GetSymbol(const void *pc) {
 const char *entry = FindSymbolInCache(pc);
 if (entry != nullptr) {
   return entry;
 }
 symbol_buf_[0] = '\0';

#ifdef __hppa__
 {
   // In some contexts (e.g., return addresses), PA-RISC uses the lowest two
   // bits of the address to indicate the privilege level. Clear those bits
   // before trying to symbolize.
   const auto pc_bits = reinterpret_cast<uintptr_t>(pc);
   const auto address = pc_bits & ~0x3;
   entry = GetUncachedSymbol(reinterpret_cast<const void *>(address));
   if (entry != nullptr) {
     return entry;
   }

   // In some contexts, PA-RISC also uses bit 1 of the address to indicate that
   // this is a cross-DSO function pointer. Such function pointers actually
   // point to a procedure label, a struct whose first 32-bit (pointer) element
   // actually points to the function text. With no symbol found for this
   // address so far, try interpreting it as a cross-DSO function pointer and
   // see how that goes.
   if (pc_bits & 0x2) {
     return GetUncachedSymbol(*reinterpret_cast<const void *const *>(address));
   }

   return nullptr;
 }
#else
 return GetUncachedSymbol(pc);
#endif
}

bool RemoveAllSymbolDecorators(void) {
 if (!g_decorators_mu.TryLock()) {
   // Someone else is using decorators. Get out.
   return false;
 }
 g_num_decorators = 0;
 g_decorators_mu.Unlock();
 return true;
}

bool RemoveSymbolDecorator(int ticket) {
 if (!g_decorators_mu.TryLock()) {
   // Someone else is using decorators. Get out.
   return false;
 }
 for (int i = 0; i < g_num_decorators; ++i) {
   if (g_decorators[i].ticket == ticket) {
     while (i < g_num_decorators - 1) {
       g_decorators[i] = g_decorators[i + 1];
       ++i;
     }
     g_num_decorators = i;
     break;
   }
 }
 g_decorators_mu.Unlock();
 return true;  // Decorator is known to be removed.
}

int InstallSymbolDecorator(SymbolDecorator decorator, void *arg) {
 static int ticket = 0;

 if (!g_decorators_mu.TryLock()) {
   // Someone else is using decorators. Get out.
   return -2;
 }
 int ret = ticket;
 if (g_num_decorators >= kMaxDecorators) {
   ret = -1;
 } else {
   g_decorators[g_num_decorators] = {decorator, arg, ticket++};
   ++g_num_decorators;
 }
 g_decorators_mu.Unlock();
 return ret;
}

bool RegisterFileMappingHint(const void *start, const void *end, uint64_t offset,
                            const char *filename) {
 SAFE_ASSERT(start <= end);
 SAFE_ASSERT(filename != nullptr);

 InitSigSafeArena();

 if (!g_file_mapping_mu.TryLock()) {
   return false;
 }

 bool ret = true;
 if (g_num_file_mapping_hints >= kMaxFileMappingHints) {
   ret = false;
 } else {
   // TODO(ckennelly): Move this into a string copy routine.
   size_t len = strlen(filename);
   char *dst = static_cast<char *>(
       base_internal::LowLevelAlloc::AllocWithArena(len + 1, SigSafeArena()));
   ABSL_RAW_CHECK(dst != nullptr, "out of memory");
   memcpy(dst, filename, len + 1);

   auto &hint = g_file_mapping_hints[g_num_file_mapping_hints++];
   hint.start = start;
   hint.end = end;
   hint.offset = offset;
   hint.filename = dst;
 }

 g_file_mapping_mu.Unlock();
 return ret;
}

bool GetFileMappingHint(const void **start, const void **end, uint64_t *offset,
                       const char **filename) {
 if (!g_file_mapping_mu.TryLock()) {
   return false;
 }
 bool found = false;
 for (int i = 0; i < g_num_file_mapping_hints; i++) {
   if (g_file_mapping_hints[i].start <= *start &&
       *end <= g_file_mapping_hints[i].end) {
     // We assume that the start_address for the mapping is the base
     // address of the ELF section, but when [start_address,end_address) is
     // not strictly equal to [hint.start, hint.end), that assumption is
     // invalid.
     //
     // This uses the hint's start address (even though hint.start is not
     // necessarily equal to start_address) to ensure the correct
     // relocation is computed later.
     *start = g_file_mapping_hints[i].start;
     *end = g_file_mapping_hints[i].end;
     *offset = g_file_mapping_hints[i].offset;
     *filename = g_file_mapping_hints[i].filename;
     found = true;
     break;
   }
 }
 g_file_mapping_mu.Unlock();
 return found;
}

}  // namespace debugging_internal

bool Symbolize(const void *pc, char *out, int out_size) {
 // Symbolization is very slow under tsan.
 ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_BEGIN();
 SAFE_ASSERT(out_size >= 0);
 debugging_internal::Symbolizer *s = debugging_internal::AllocateSymbolizer();
 const char *name = s->GetSymbol(pc);
 bool ok = false;
 if (name != nullptr && out_size > 0) {
   strncpy(out, name, static_cast<size_t>(out_size));
   ok = true;
   if (out[static_cast<size_t>(out_size) - 1] != '\0') {
     // strncpy() does not '\0' terminate when it truncates.  Do so, with
     // trailing ellipsis.
     static constexpr char kEllipsis[] = "...";
     size_t ellipsis_size =
         std::min(strlen(kEllipsis), static_cast<size_t>(out_size) - 1);
     memcpy(out + static_cast<size_t>(out_size) - ellipsis_size - 1, kEllipsis,
            ellipsis_size);
     out[static_cast<size_t>(out_size) - 1] = '\0';
   }
 }
 debugging_internal::FreeSymbolizer(s);
 ABSL_ANNOTATE_IGNORE_READS_AND_WRITES_END();
 return ok;
}

ABSL_NAMESPACE_END
}  // namespace absl

extern "C" bool AbslInternalGetFileMappingHint(const void **start,
                                              const void **end, uint64_t *offset,
                                              const char **filename) {
 return absl::debugging_internal::GetFileMappingHint(start, end, offset,
                                                     filename);
}