3 files changed, 30 insertions, 1 deletions
diff --git a/compiler-rt/lib/hwasan/hwasan.cc b/compiler-rt/lib/hwasan/hwasan.cc
index e2bfea5e422..ff2538316bd 100644
--- a/compiler-rt/lib/hwasan/hwasan.cc
+++ b/compiler-rt/lib/hwasan/hwasan.cc
@@ -88,6 +88,8 @@ static void InitializeFlags() {
     cf.check_printf = false;
     cf.intercept_tls_get_addr = true;
     cf.exitcode = 99;
+    // 8 shadow pages ~512kB, small enough to cover common stack sizes.
+    cf.clear_shadow_mmap_threshold = 4096 * (SANITIZER_ANDROID ? 2 : 8);
     // Sigtrap is used in error reporting.
     cf.handle_sigtrap = kHandleSignalExclusive;
 
diff --git a/compiler-rt/lib/hwasan/hwasan_poisoning.cc b/compiler-rt/lib/hwasan/hwasan_poisoning.cc
index 9c8e16b12ad..6fb7d15db3a 100644
--- a/compiler-rt/lib/hwasan/hwasan_poisoning.cc
+++ b/compiler-rt/lib/hwasan/hwasan_poisoning.cc
@@ -16,6 +16,7 @@
 #include "hwasan_mapping.h"
 #include "interception/interception.h"
 #include "sanitizer_common/sanitizer_common.h"
+#include "sanitizer_common/sanitizer_linux.h"
 
 namespace __hwasan {
 
@@ -24,7 +25,22 @@ uptr TagMemoryAligned(uptr p, uptr size, tag_t tag) {
   CHECK(IsAligned(size, kShadowAlignment));
   uptr shadow_start = MemToShadow(p);
   uptr shadow_size = MemToShadowSize(size);
-  internal_memset((void *)shadow_start, tag, shadow_size);
+
+  uptr page_size = GetPageSizeCached();
+  uptr page_start = RoundUpTo(shadow_start, page_size);
+  uptr page_end = RoundDownTo(shadow_start + shadow_size, page_size);
+  uptr threshold = common_flags()->clear_shadow_mmap_threshold;
+  if (SANITIZER_LINUX &&
+      UNLIKELY(page_end >= page_start + threshold && tag == 0)) {
+    internal_memset((void *)shadow_start, tag, page_start - shadow_start);
+    internal_memset((void *)page_end, tag,
+                    shadow_start + shadow_size - page_end);
+    // For an anonymous private mapping MADV_DONTNEED will return a zero page on
+    // Linux.
+    ReleaseMemoryPagesToOSAndZeroFill(page_start, page_end);
+  } else {
+    internal_memset((void *)shadow_start, tag, shadow_size);
+  }
   return AddTagToPointer(p, tag);
 }
 
diff --git a/compiler-rt/lib/sanitizer_common/sanitizer_linux.h b/compiler-rt/lib/sanitizer_common/sanitizer_linux.h
index c309e33f81b..522eb092a6d 100644
--- a/compiler-rt/lib/sanitizer_common/sanitizer_linux.h
+++ b/compiler-rt/lib/sanitizer_common/sanitizer_linux.h
@@ -106,6 +106,17 @@ bool LibraryNameIs(const char *full_name, const char *base_name);
 // Call cb for each region mapped by map.
 void ForEachMappedRegion(link_map *map, void (*cb)(const void *, uptr));
 
+// Releases memory pages entirely within the [beg, end] address range.
+// The pages no longer count toward RSS; reads are guaranteed to return 0.
+// Requires (but does not verify!) that pages are MAP_PRIVATE.
+INLINE void ReleaseMemoryPagesToOSAndZeroFill(uptr beg, uptr end) {
+  // man madvise on Linux promises zero-fill for anonymous private pages.
+  // Testing shows the same behaviour for private (but not anonymous) mappings
+  // of shm_open() files, as long as the underlying file is untouched.
+  CHECK(SANITIZER_LINUX);
+  ReleaseMemoryPagesToOS(beg, end);
+}
+
 #if SANITIZER_ANDROID
 
 #if defined(__aarch64__)