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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#include "MmapFaultHandler.h"
#if defined(XP_UNIX) && !defined(XP_DARWIN) && !defined(__wasi__)
# include "PlatformMutex.h"
# include "mozilla/Atomics.h"
# include "mozilla/MemoryChecking.h"
# include "mozilla/ThreadLocal.h"
# include <signal.h>
# include <cstring>
static MOZ_THREAD_LOCAL(MmapAccessScope*) sMmapAccessScope;
static struct sigaction sPrevSIGBUSHandler;
static void MmapSIGBUSHandler(int signum, siginfo_t* info, void* context) {
MmapAccessScope* mas = sMmapAccessScope.get();
if (mas && mas->IsInsideBuffer(info->si_addr)) {
// Temporarily instead of handling the signal, we crash intentionally and
// send some diagnostic information to find out why the signal is received.
// The address is inside the buffer, handle the failure.
siglongjmp(mas->mJmpBuf, signum);
// This signal is not caused by accessing region protected by MmapAccessScope.
// Forward the signal to the next handler.
if (sPrevSIGBUSHandler.sa_flags & SA_SIGINFO) {
sPrevSIGBUSHandler.sa_sigaction(signum, info, context);
} else if (sPrevSIGBUSHandler.sa_handler == SIG_DFL ||
sPrevSIGBUSHandler.sa_handler == SIG_IGN) {
// There is no next handler. Uninstalling our handler and returning will
// cause a crash.
sigaction(signum, &sPrevSIGBUSHandler, nullptr);
} else {
mozilla::Atomic<bool> gSIGBUSHandlerInstalled(false);
mozilla::Atomic<bool> gSIGBUSHandlerInstalling(false);
void InstallMmapFaultHandler() {
// This function is called from MmapAccessScope's constructor because there is
// no single point where we could install the handler during startup. This
// means that it's called quite often, so to minimize using of the mutex we
// first check the atomic variable outside the lock.
if (gSIGBUSHandlerInstalled) {
if (gSIGBUSHandlerInstalling.compareExchange(false, true)) {
struct sigaction busHandler;
busHandler.sa_flags = SA_SIGINFO | SA_NODEFER | SA_ONSTACK;
busHandler.sa_sigaction = MmapSIGBUSHandler;
if (sigaction(SIGBUS, &busHandler, &sPrevSIGBUSHandler)) {
MOZ_CRASH("Unable to install SIGBUS handler");
gSIGBUSHandlerInstalled = true;
} else {
// Just spin lock here. It should not take a substantial amount
// of time, so a mutex would likely be a spin lock anyway, and
// this avoids the need to new up a static mutex from within
// mozglue/misc, which complicates things with
while (!gSIGBUSHandlerInstalled) {
MmapAccessScope::MmapAccessScope(void* aBuf, uint32_t aBufLen,
const char* aFilename) {
// Install signal handler if it wasn't installed yet.
// We'll handle the signal only if the crashing address is inside this buffer.
mBuf = aBuf;
mBufLen = aBufLen;
mFilename = aFilename;
MmapAccessScope::~MmapAccessScope() {
MOZ_RELEASE_ASSERT(sMmapAccessScope.get() == this);
void MmapAccessScope::SetThreadLocalScope() {
// mJmpBuf is set outside of this classs for reasons mentioned in the header
// file, but we need to initialize the member here too to make Coverity happy.
memset(mJmpBuf, 0, sizeof(sigjmp_buf));
// If MmapAccessScopes are nested, save the previous one and restore it in
// the destructor.
mPreviousScope = sMmapAccessScope.get();
// MmapAccessScope is now set up (except mJmpBuf for reasons mentioned in the
// header file). Store the pointer in a thread-local variable sMmapAccessScope
// so we can use it in the handler if the signal is triggered.
bool MmapAccessScope::IsInsideBuffer(void* aPtr) {
return aPtr >= mBuf && aPtr < (void*)((char*)mBuf + mBufLen);
void MmapAccessScope::CrashWithInfo(void* aPtr) {
// All we have is the buffer and the crashing address.
"SIGBUS received when accessing mmaped file [buffer=%p, "
"buflen=%u, address=%p, filename=%s]",
mBuf, mBufLen, aPtr, mFilename);