[sanitizer][Fuchsia] Add callback at end of __sanitizer_startup_hook

compiler-rt/lib/asan/asan_fuchsia.cpp

@@ -25,6 +25,11 @@
 #  include "asan_thread.h"
 #  include "lsan/lsan_common.h"
 
+namespace __sanitizer {
+// ASan doesn't need to do anything else special in the startup hook.
+void EarlySanitizerInit() {}
+}  // namespace __sanitizer
+
 namespace __asan {
 
 // The system already set up the shadow memory for us.

compiler-rt/lib/hwasan/hwasan_fuchsia.cpp

@@ -31,6 +31,15 @@
 SANITIZER_INTERFACE_ATTRIBUTE
 THREADLOCAL uptr __hwasan_tls;
 
+namespace __sanitizer {
+void EarlySanitizerInit() {
+  // Setup the hwasan runtime before any `__libc_extensions_init`s are called.
+  // This is needed because libraries which define this function (like fdio)
+  // may be instrumented and either access `__hwasan_tls` or make runtime calls.
+  __hwasan_init();
+}
+}  // namespace __sanitizer
+
 namespace __hwasan {
 
 bool InitShadow() {

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cpp

@@ -547,6 +547,8 @@ void __sanitizer_startup_hook(int argc, char **argv, char **envp,
   __sanitizer::StoredEnviron = envp;
   __sanitizer::MainThreadStackBase = reinterpret_cast<uintptr_t>(stack_base);
   __sanitizer::MainThreadStackSize = stack_size;
+
+  EarlySanitizerInit();
 }
 
 void __sanitizer_set_report_path(const char *path) {

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.h

@@ -32,6 +32,13 @@ struct MemoryMappingLayoutData {
 
 void InitShadowBounds();
 
+// Individual sanitizers can define this to explicitly run something at the end
+// of `__sanitizer_startup_hook`. This can be useful if a sanitizer needs to do
+// extra work after the common startup hook code is called and before module
+// ctors are invoked. For example, hwasan can explicitly call its initializing
+// function here so it can be set up before libc extensions are initialized.
+void EarlySanitizerInit();
+
 }  // namespace __sanitizer
 
 #endif  // SANITIZER_FUCHSIA

compiler-rt/lib/ubsan/ubsan_init_standalone.cpp

@@ -19,6 +19,13 @@
 #include "ubsan_init.h"
 #include "ubsan_signals_standalone.h"
 
+#if SANITIZER_FUCHSIA
+namespace __sanitizer {
+// UBSan doesn't need to do anything else special in the startup hook.
+void EarlySanitizerInit() {}
+} // namespace __sanitizer
+#endif // SANITIZER_FUCHSIA
+
 namespace __ubsan {
 
 class UbsanStandaloneInitializer {