[scudo] Enable Scudo memory hooks for Fuchsia.

[flowerhack]

Summary

It would be useful for Flutter apps, especially, to be able to use
malloc hooks to debug memory leaks on Fuchsia. They're not able to do
this right now, so it'd be a nice bonus to throw in with the Scudo
switchover.

[cryptoad]

Or maybe wrap it all with # define SCUDO_CAN_USE_HOOKS (SANITIZER_FUCHSIA) or equivalent if it looks cleaner. Up to you.

[flowerhack]

I like that this matches the style of the SCUDO_SHARED_TSD_POOL_SIZE enables, so I may leave it as-is, if that's alright with you.

[cryptoad]

In D48618#1149974, @flowerhack wrote:

I like that this matches the style of the SCUDO_SHARED_TSD_POOL_SIZE enables, so I may leave it as-is, if that's alright with you.

Yup!

lib/scudo/scudo_platform.h

 // The following allows the public interface functions to be disabled.
 #ifndef SCUDO_CAN_USE_PUBLIC_INTERFACE
 # define SCUDO_CAN_USE_PUBLIC_INTERFACE 1
 #endif
 
 // Hooks in the allocation & deallocation paths can become a security concern if
 // implemented improperly, or if overwritten by an attacker. Use with caution.
 #ifndef SCUDO_CAN_USE_HOOKS
-# define SCUDO_CAN_USE_HOOKS 0
-#endif
+# if SANITIZER_FUCHSIA
+#  define SCUDO_CAN_USE_HOOKS 1
+# else
+#  define SCUDO_CAN_USE_HOOKS 0
+# endif  // SANITIZER_FUCHSIA
+#endif  // SCUDO_CAN_USE_HOOKS
 
 namespace __scudo {
 
 #if SANITIZER_CAN_USE_ALLOCATOR64
 # if defined(__aarch64__) && SANITIZER_ANDROID
 const uptr AllocatorSize = 0x4000000000ULL;  // 256G.
 # elif defined(__aarch64__)
 const uptr AllocatorSize = 0x10000000000ULL;  // 1T.