[ASan] Make FlushUnneededASanShadowMemory a per-platform function

[phosek]

Summary

Currently, FlushUnneededASanShadowMemory invokes ReleaseMemoryPagesToOS
on all platforms. The latter function is complex and used by other parts
and implementing it on systems like Fuchsia is tricky. However, the
former is fairly straightforward to implement.

This change decouples the two functions and makes
FlushUnneededASanShadowMemory implementation per-platform which allows
implementing this function separately from ReleaseMemoryPagesToOS. On
all systems except for Fuchsia, FlushUnneededASanShadowMemory is still
implemented in terms of ReleaseMemoryPagesToOS so this change should be
NFC.

[eugenis]

LGTM

[phosek]

This is no longer needed now that D64166 landed.

compiler-rt/lib/asan/asan_fuchsia.cc

 
 bool HandleDlopenInit() {
   // Not supported on this platform.
   static_assert(!SANITIZER_SUPPORTS_INIT_FOR_DLOPEN,
                 "Expected SANITIZER_SUPPORTS_INIT_FOR_DLOPEN to be false");
   return false;
 }
 
+void FlushUnneededASanShadowMemory(uptr p, uptr size) {
+}
+
 }  // namespace __asan
 
 // These are declared (in extern "C") by <zircon/sanitizer.h>.
 // The system runtime will call our definitions directly.
 
 void *__sanitizer_before_thread_create_hook(thrd_t thread, bool detached,
                                             const char *name, void *stack_base,
                                             size_t stack_size) {

compiler-rt/lib/asan/asan_linux.cc

 
 bool HandleDlopenInit() {
   // Not supported on this platform.
   static_assert(!SANITIZER_SUPPORTS_INIT_FOR_DLOPEN,
                 "Expected SANITIZER_SUPPORTS_INIT_FOR_DLOPEN to be false");
   return false;
 }
 
+void FlushUnneededASanShadowMemory(uptr p, uptr size) {
+  // Since asan's mapping is compacting, the shadow chunk may be
+  // not page-aligned, so we only flush the page-aligned portion.
+  ReleaseMemoryPagesToOS(MemToShadow(p), MemToShadow(p + size));
+}
+
 } // namespace __asan
 
 #endif  // SANITIZER_FREEBSD || SANITIZER_LINUX || SANITIZER_NETBSD ||
         // SANITIZER_SOLARIS

compiler-rt/lib/asan/asan_mac.cc

VReport(2,
           "context: %p, pthread_self: %p\n",
           block, pthread_self());
   asan_register_worker_thread(context->parent_tid, &stack);
   // Call the original dispatcher for the block.
   context->func(context->block);
   asan_free(context, &stack, FROM_MALLOC);
 }
 
+void FlushUnneededASanShadowMemory(uptr p, uptr size) {
+  // Since asan's mapping is compacting, the shadow chunk may be
+  // not page-aligned, so we only flush the page-aligned portion.
+  ReleaseMemoryPagesToOS(MemToShadow(p), MemToShadow(p + size));
+}
+
 }  // namespace __asan
 
 using namespace __asan;  // NOLINT
 
 // Wrap |ctxt| and |func| into an asan_block_context_t.
 // The caller retains control of the allocated context.
 extern "C"
 asan_block_context_t *alloc_asan_context(void *ctxt, dispatch_function_t func,

compiler-rt/lib/asan/asan_poisoning.cc

struct ShadowSegmentEndpoint {
 
   explicit ShadowSegmentEndpoint(uptr address) {
     chunk = (u8*)MemToShadow(address);
     offset = address & (SHADOW_GRANULARITY - 1);
     value = *chunk;
   }
 };
 
-void FlushUnneededASanShadowMemory(uptr p, uptr size) {
-  // Since asan's mapping is compacting, the shadow chunk may be
-  // not page-aligned, so we only flush the page-aligned portion.
-  ReleaseMemoryPagesToOS(MemToShadow(p), MemToShadow(p + size));
-}
-
 void AsanPoisonOrUnpoisonIntraObjectRedzone(uptr ptr, uptr size, bool poison) {
   uptr end = ptr + size;
   if (Verbosity()) {
     Printf("__asan_%spoison_intra_object_redzone [%p,%p) %zd\n",
            poison ? "" : "un", ptr, end, size);
     if (Verbosity() >= 2)
       PRINT_CURRENT_STACK();
   }

compiler-rt/lib/asan/asan_win.cc

 }
 
 #pragma section(".CRT$XLY", long, read)  // NOLINT
 __declspec(allocate(".CRT$XLY")) void (NTAPI *__asan_tls_exit)(void *,
     unsigned long, void *) = asan_thread_exit;
 
 WIN_FORCE_LINK(__asan_dso_reg_hook)
 
+void FlushUnneededASanShadowMemory(uptr p, uptr size) {
+  // Since asan's mapping is compacting, the shadow chunk may be
+  // not page-aligned, so we only flush the page-aligned portion.
+  ReleaseMemoryPagesToOS(MemToShadow(p), MemToShadow(p + size));
+}
+
 // }}}
 }  // namespace __asan
 
 #endif  // SANITIZER_WINDOWS