[asan][fuchsia] Implement PlatformUnpoisonStacks

[charco]

Summary

This CL modifies the PlatformUnpoisonStacks so that fuchsia can
implement its own logic for unpoisoning the stacks.

For the general case, the behavior is the same as with regular asan: it
will unpoison everything from the current stack pointer until the base
of the stack (stack top).

In some situations, the current stack might not be the same as the
default stack. In those cases, the code will now unpoison the entire
default stack, and will also unpoison the current page of the stack.

[mcgrathr]

lgtm

compiler-rt/lib/asan/asan_fuchsia.cpp

 
 namespace __asan {
 
 // The system already set up the shadow memory for us.
 // __sanitizer::GetMaxUserVirtualAddress has already been called by
 // AsanInitInternal->InitializeHighMemEnd (asan_rtl.cpp).
 // Just do some additional sanity checks here.
 void InitializeShadowMemory() {
-  if (Verbosity()) PrintAddressSpaceLayout();
+  if (Verbosity())
+    PrintAddressSpaceLayout();
 
   // Make sure SHADOW_OFFSET doesn't use __asan_shadow_memory_dynamic_address.
   __asan_shadow_memory_dynamic_address = kDefaultShadowSentinel;
   DCHECK(kLowShadowBeg != kDefaultShadowSentinel);
   __asan_shadow_memory_dynamic_address = kLowShadowBeg;
 
   CHECK_EQ(kShadowGapEnd, kHighShadowBeg - 1);
   CHECK_EQ(kHighMemEnd, __sanitizer::ShadowBounds.memory_limit - 1);

 
 void *AsanDoesNotSupportStaticLinkage() { return nullptr; }
 
 void InitializePlatformExceptionHandlers() {}
 void AsanOnDeadlySignal(int signo, void *siginfo, void *context) {
   UNIMPLEMENTED();
 }
 
-bool PlatformUnpoisonStacks() { return false; }
+bool PlatformUnpoisonStacks() {
+  // The current sp might not point to the default stack. This
+  // could be because we are in a crash stack from fuzzing for example.
+  // Unpoison the default stack and the current stack page.
+  AsanThread *curr_thread = GetCurrentThread();
+  CHECK(curr_thread != nullptr);
+  uptr top = curr_thread->stack_top();
+  uptr bottom = curr_thread->stack_bottom();
+  // The default stack grows from top to bottom. (bottom < top).
+
+  uptr local_stack = reinterpret_cast<uptr>(__builtin_frame_address(0));
+  if (local_stack >= bottom && local_stack <= top) {
+    // The current stack is the default stack.
+    // We only need to unpoison from where we are using until the end.
+    bottom = RoundDownTo(local_stack, GetPageSize());
+    UnpoisonStack(bottom, top, "default");
+  } else {
+    // The current stack is not the default stack.
+    // Unpoison the entire default stack and the current stack page.
+    UnpoisonStack(bottom, top, "default");
+    bottom = RoundDownTo(local_stack, GetPageSize());
+    top = bottom + GetPageSize();
+    UnpoisonStack(bottom, top, "unknown");
+    return true;
+  }
+
+  return false;
+}
 
 // We can use a plain thread_local variable for TSD.
 static thread_local void *per_thread;
 
 void *AsanTSDGet() { return per_thread; }
 
 void AsanTSDSet(void *tsd) { per_thread = tsd; }
 

 
 // This is called before each thread creation is attempted.  So, in
 // its first call, the calling thread is the initial and sole thread.
 static void *BeforeThreadCreateHook(uptr user_id, bool detached,
                                     const char *name, uptr stack_bottom,
                                     uptr stack_size) {
   EnsureMainThreadIDIsCorrect();
   // Strict init-order checking is thread-hostile.
-  if (flags()->strict_init_order) StopInitOrderChecking();
+  if (flags()->strict_init_order)
+    StopInitOrderChecking();
 
   GET_STACK_TRACE_THREAD;
   u32 parent_tid = GetCurrentTidOrInvalid();
 
   AsanThread *thread =
       CreateAsanThread(&stack, parent_tid, user_id, detached, name);
 
   // On other systems, AsanThread::Init() is called from the new