[Concurrency] Clear Slab's fake metadata pointer before freeing it.

This helps ensure that memory analysis tools don't get confused by leftover metadata pointers in reallocated memory that hasn't been initialized yet.

rdar://88494373

Author: mikeash

stdlib/public/Concurrency/CMakeLists.txt

@@ -61,6 +61,9 @@ else()
   list(APPEND SWIFT_RUNTIME_CONCURRENCY_C_FLAGS "-fswift-async-fp=never")
 endif()
 
+list(APPEND SWIFT_RUNTIME_CONCURRENCY_C_FLAGS
+  "-D__STDC_WANT_LIB_EXT1__=1")
+
 add_swift_target_library(swift_Concurrency ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} IS_STDLIB
   ../CompatibilityOverride/CompatibilityOverride.cpp
   Actor.cpp

stdlib/public/runtime/StackAllocator.h

@@ -14,6 +14,13 @@
 //
 //===----------------------------------------------------------------------===//
 
+// Define __STDC_WANT_LIB_EXT1__ to get memset_s on platforms that have it.
+// Other files may have included string.h without it already, so we also set
+// this with a -D flag when building, but this allows tests to build without
+// additional trouble.
+#define __STDC_WANT_LIB_EXT1__ 1
+#include <string.h>
+
 #include "swift/ABI/MetadataValues.h"
 #include "swift/Runtime/Debug.h"
 #include "llvm/Support/Alignment.h"
@@ -118,6 +125,22 @@ class StackAllocator {
       return headerSize() + size;
     }
 
+    /// Clear the fake metadata pointer. Call before freeing so that leftover
+    /// heap garbage doesn't have slab metadata pointers in it.
+    void clearMetadata() {
+      // Use memset_s or explicit_bzero where available. Fall back to a plain
+      // assignment on unknown platforms. This is not necessary for correctness,
+      // just as an aid to analysis tools, so it's OK if the fallback gets
+      // optimized out.
+#if defined(__APPLE__)
+      memset_s(&metadata, sizeof(metadata), 0, sizeof(metadata));
+#elif defined(__linux__)
+      explicit_bzero(&metadata, sizeof(metadata));
+#else
+      metadata = 0;
+#endif
+    }
+
     /// Return the payload buffer address at \p atOffset.
     ///
     /// Note: it's valid to call this function on a not-yet-constructed slab.
@@ -247,6 +270,7 @@ class StackAllocator {
     while (slab) {
       Slab *next = slab->next;
       freedCapacity += slab->capacity;
+      slab->clearMetadata();
       free(slab);
       numAllocatedSlabs--;
       slab = next;
@@ -272,6 +296,8 @@ class StackAllocator {
   ~StackAllocator() {
     if (lastAllocation)
       SWIFT_FATAL_ERROR(0, "not all allocations are deallocated");
+    if (firstSlabIsPreallocated)
+      firstSlab->clearMetadata();
     (void)freeAllSlabs(firstSlabIsPreallocated ? firstSlab->next : firstSlab);
     assert(getNumAllocatedSlabs() == 0);
   }