Linux ExceptionHandler: don't allocate the CrashContext on the stack

On Android the size of the alternate stack can be very small (8k).
Even if breakpad uses sigaltstack to increase the size of the alternate
stack during initialization, that call affects only the main thread.
On Android, the libc's pthread initializer reset the sigaltstack to 8k.
When entering a signal handler, the kernel typically pushes the context
on the alternate stack. On arm64, sizeof(CrashContext) is ~5k, which
leaves 3k of usable stack for breakpad.
On top of that, breakpad allocates another struct CrashContext on the
stack. In the case of Android arm64, then, breakpad ends up using
5k + 5k > 8k of stack, which causes a stack overflow.
This got unnoticed in Android L, as the alternate stack didn't have
red-zones between them, so breakpad was often happily overflowing onto
the next thread's stack. This is not the case anymore [1].
This CL moves the CrashContext into a global variable. It should be
safe as the ExceptionHandlers are serialized on a mutex.

[1] 595752f623

BUG=374
R=mark@chromium.org

Review URL: https://codereview.chromium.org/1354923002 .

src/client/linux/handler/exception_handler.cc

@@ -212,6 +212,12 @@ void InstallDefaultHandler(int sig) {
 std::vector<ExceptionHandler*>* g_handler_stack_ = NULL;
 pthread_mutex_t g_handler_stack_mutex_ = PTHREAD_MUTEX_INITIALIZER;
 
+// sizeof(CrashContext) can be too big w.r.t the size of alternatate stack
+// for SignalHandler(). Keep the crash context as a .bss field. Exception
+// handlers are serialized by the |g_handler_stack_mutex_| and at most one at a
+// time can use |g_crash_context_|.
+ExceptionHandler::CrashContext g_crash_context_;
+
 }  // namespace
 
 // Runs before crashing: normal context.
@@ -239,6 +245,11 @@ ExceptionHandler::ExceptionHandler(const MinidumpDescriptor& descriptor,
 #endif
 
   pthread_mutex_lock(&g_handler_stack_mutex_);
+
+  // Pre-fault the crash context struct. This is to avoid failing due to OOM
+  // if handling an exception when the process ran out of virtual memory.
+  memset(&g_crash_context_, 0, sizeof(g_crash_context_));
+
   if (!g_handler_stack_)
     g_handler_stack_ = new std::vector<ExceptionHandler*>;
   if (install_handler) {
@@ -424,17 +435,18 @@ bool ExceptionHandler::HandleSignal(int sig, siginfo_t* info, void* uc) {
   if (signal_trusted || (signal_pid_trusted && info->si_pid == getpid())) {
     sys_prctl(PR_SET_DUMPABLE, 1, 0, 0, 0);
   }
-  CrashContext context;
+
   // Fill in all the holes in the struct to make Valgrind happy.
-  memset(&context, 0, sizeof(context));
+  memset(&g_crash_context_, 0, sizeof(g_crash_context_));
-  memcpy(&context.siginfo, info, sizeof(siginfo_t));
+  memcpy(&g_crash_context_.siginfo, info, sizeof(siginfo_t));
-  memcpy(&context.context, uc, sizeof(struct ucontext));
+  memcpy(&g_crash_context_.context, uc, sizeof(struct ucontext));
 #if defined(__aarch64__)
   struct ucontext* uc_ptr = (struct ucontext*)uc;
   struct fpsimd_context* fp_ptr =
       (struct fpsimd_context*)&uc_ptr->uc_mcontext.__reserved;
   if (fp_ptr->head.magic == FPSIMD_MAGIC) {
-    memcpy(&context.float_state, fp_ptr, sizeof(context.float_state));
+    memcpy(&g_crash_context_.float_state, fp_ptr,
+           sizeof(g_crash_context_.float_state));
   }
 #elif !defined(__ARM_EABI__) && !defined(__mips__)
   // FP state is not part of user ABI on ARM Linux.
@@ -442,18 +454,18 @@ bool ExceptionHandler::HandleSignal(int sig, siginfo_t* info, void* uc) {
   // and 'float_state' is not a member of CrashContext.
   struct ucontext* uc_ptr = (struct ucontext*)uc;
   if (uc_ptr->uc_mcontext.fpregs) {
-    memcpy(&context.float_state,
+    memcpy(&g_crash_context_.float_state, uc_ptr->uc_mcontext.fpregs,
-           uc_ptr->uc_mcontext.fpregs,
+           sizeof(g_crash_context_.float_state));
-           sizeof(context.float_state));
   }
 #endif
-  context.tid = syscall(__NR_gettid);
+  g_crash_context_.tid = syscall(__NR_gettid);
   if (crash_handler_ != NULL) {
-    if (crash_handler_(&context, sizeof(context), callback_context_)) {
+    if (crash_handler_(&g_crash_context_, sizeof(g_crash_context_),
+                       callback_context_)) {
       return true;
     }
   }
-  return GenerateDump(&context);
+  return GenerateDump(&g_crash_context_);
 }
 
 // This is a public interface to HandleSignal that allows the client to