[compiler-rt][asan] decommit shadow memory for unmaps in fuchsia.

This CL allows asan allocator in fuchsia to decommit shadow memory
for memory allocated using mmap.

Big allocations in asan end up being allocated via `mmap` and freed with
`munmap`. However, when that memory is freed, asan returns the
corresponding shadow memory back to the OS via a call to
`ReleaseMemoryPagesToOs`.

In fuchsia, `ReleaseMemoryPagesToOs` is a no-op: to be able to free
memory back to the OS, you have to hold a handle to the vmo you want to
modify, which is tricky at the ReleaseMemoryPagesToOs level as that
function is not exclusively used for shadow memory.

The function `__sanitizer_fill_shadow` fills a given shadow memory range
with a specific value, and if that value is 0 (unpoison) and the memory
range is bigger than a threshold parameter, it will decommit that memory
if it is all zeroes.

This CL modifies the `FlushUnneededASanShadowMemory` function in
`asan_poisoning.cpp` to add a call to `__sanitizer_fill_shadow` with
value and threshold = 0. This way, all the unneeded shadow memory gets
returned back to the OS.

A test for this behavior can be found in fxrev.dev/391974

Differential Revision: https://reviews.llvm.org/D80355

Change-Id: Id6dd85693e78a222f0329d5b2201e0da753e01c0

compiler-rt/lib/asan/asan_fuchsia.cpp

@@ -198,6 +198,10 @@ bool HandleDlopenInit() {
   return false;
 }
 
+void FlushUnneededASanShadowMemory(uptr p, uptr size) {
+  __sanitizer_fill_shadow(p, size, 0, 0);
+}
+
 }  // namespace __asan
 
 // These are declared (in extern "C") by <zircon/sanitizer.h>.

compiler-rt/lib/asan/asan_linux.cpp

@@ -114,6 +114,12 @@ void AsanApplyToGlobals(globals_op_fptr op, const void *needle) {
   UNIMPLEMENTED();
 }
 
+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));
+}
+
 #if SANITIZER_ANDROID
 // FIXME: should we do anything for Android?
 void AsanCheckDynamicRTPrereqs() {}

compiler-rt/lib/asan/asan_mac.cpp

@@ -89,6 +89,12 @@ void AsanApplyToGlobals(globals_op_fptr op, const void *needle) {
   op(globals, size / sizeof(__asan_global));
 }
 
+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 ReadContextStack(void *context, uptr *stack, uptr *ssize) {
   UNIMPLEMENTED();
 }

compiler-rt/lib/asan/asan_poisoning.cpp

@@ -62,12 +62,6 @@ struct ShadowSegmentEndpoint {
   }
 };
 
-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()) {

compiler-rt/lib/asan/asan_rtems.cpp

@@ -50,6 +50,12 @@ void AsanApplyToGlobals(globals_op_fptr op, const void *needle) {
   UNIMPLEMENTED();
 }
 
+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 AsanCheckDynamicRTPrereqs() {}
 void AsanCheckIncompatibleRT() {}
 void InitializeAsanInterceptors() {}

compiler-rt/lib/asan/asan_win.cpp

@@ -191,6 +191,12 @@ void AsanApplyToGlobals(globals_op_fptr op, const void *needle) {
   UNIMPLEMENTED();
 }
 
+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));
+}
+
 // ---------------------- TSD ---------------- {{{
 static bool tsd_key_inited = false;