[msan] Mark allocator padding as uninitialized, with new origin tag

This is follow-up work per discussion in llvm#155944 (comment).

If the allocator reserves more space than the user requested (e.g.,
malloc(7) actually has 16 bytes reserved), the padding bytes will now be
marked as uninitialized; the origin will be set as a new tag,
ALLOC_PADDING (in the case of ambiguity caused by origin
granularity, ALLOC will take precedence).

Padding poisoning is controlled by the existing flag poison_in_malloc and a new flag,
poison_in_calloc.

Author: thurstond

compiler-rt/lib/msan/msan.h

@@ -303,6 +303,7 @@ u32 ChainOrigin(u32 id, StackTrace *stack);
 const int STACK_TRACE_TAG_POISON = StackTrace::TAG_CUSTOM + 1;
 const int STACK_TRACE_TAG_FIELDS = STACK_TRACE_TAG_POISON + 1;
 const int STACK_TRACE_TAG_VPTR = STACK_TRACE_TAG_FIELDS + 1;
+const int STACK_TRACE_TAG_ALLOC_PADDING = STACK_TRACE_TAG_VPTR + 1;
 
 #define GET_MALLOC_STACK_TRACE                                             \
   UNINITIALIZED BufferedStackTrace stack;                                  \

compiler-rt/lib/msan/msan_allocator.cpp

@@ -217,25 +217,42 @@ static void *MsanAllocate(BufferedStackTrace *stack, uptr size, uptr alignment,
   }
   auto *meta = reinterpret_cast<Metadata *>(allocator.GetMetaData(allocated));
   meta->requested_size = size;
+  uptr actually_allocated_size = allocator.GetActuallyAllocatedSize(allocated);
+  void* padding_start =
+      reinterpret_cast<void*>(reinterpret_cast<uptr>(allocated) + size);
+  uptr padding_size = actually_allocated_size - size;
+
+  // Origins have 4-byte granularity. Set the TAG_ALLOC_PADDING origin first,
+  // so the TAG_ALLOC origin will take precedence if necessary e.g.,
+  // - if we have malloc(7) that actually takes up 16 bytes, bytes 0-7 will
+  //   have TAG_ALLOC and bytes 8-15 will have TAG_ALLOC_PADDING.
+  // - with calloc, bytes 4-15 will have TAG_ALLOC_PADDING (but bytes 4-6 are
+  //   initialized, hence the origin is unused). Note that this means, unlike
+  //   with malloc, byte 7 is TAG_ALLOC_PADDING instead of TAG_ALLOC.
+  if (__msan_get_track_origins() && ((zero && flags()->poison_in_calloc) ||
+                                     (!zero && flags()->poison_in_malloc))) {
+    stack->tag = STACK_TRACE_TAG_ALLOC_PADDING;
+    Origin o2 = Origin::CreateHeapOrigin(stack);
+    __msan_set_origin(padding_start, padding_size, o2.raw_id());
+  }
+
   if (zero) {
     if (allocator.FromPrimary(allocated))
       __msan_clear_and_unpoison(allocated, size);
     else
       __msan_unpoison(allocated, size);  // Mem is already zeroed.
+
+    if (flags()->poison_in_calloc)
+      __msan_poison(padding_start, padding_size);
   } else if (flags()->poison_in_malloc) {
-    __msan_poison(allocated, size);
+    __msan_poison(allocated, actually_allocated_size);
     if (__msan_get_track_origins()) {
       stack->tag = StackTrace::TAG_ALLOC;
       Origin o = Origin::CreateHeapOrigin(stack);
       __msan_set_origin(allocated, size, o.raw_id());
     }
   }
 
-  uptr actually_allocated_size = allocator.GetActuallyAllocatedSize(allocated);
-  // For compatibility, the allocator converted 0-sized allocations into 1 byte
-  if (size == 0 && actually_allocated_size > 0 && flags()->poison_in_malloc)
-    __msan_poison(allocated, 1);
-
   UnpoisonParam(2);
   RunMallocHooks(allocated, size);
   return allocated;
@@ -247,7 +264,7 @@ void __msan::MsanDeallocate(BufferedStackTrace *stack, void *p) {
   RunFreeHooks(p);
 
   Metadata *meta = reinterpret_cast<Metadata *>(allocator.GetMetaData(p));
-  uptr size = meta->requested_size;
+  uptr size = allocator.GetActuallyAllocatedSize(p);
   meta->requested_size = 0;
   // This memory will not be reused by anyone else, so we are free to keep it
   // poisoned. The secondary allocator will unmap and unpoison by

compiler-rt/lib/msan/msan_flags.inc

@@ -22,6 +22,7 @@ MSAN_FLAG(int, origin_history_size, Origin::kMaxDepth, "")
 MSAN_FLAG(int, origin_history_per_stack_limit, 20000, "")
 MSAN_FLAG(bool, poison_heap_with_zeroes, false, "")
 MSAN_FLAG(bool, poison_stack_with_zeroes, false, "")
+MSAN_FLAG(bool, poison_in_calloc, true, "")
 MSAN_FLAG(bool, poison_in_malloc, true, "")
 MSAN_FLAG(bool, poison_in_free, true, "")
 MSAN_FLAG(bool, poison_in_dtor, true, "")

compiler-rt/lib/msan/msan_report.cpp

@@ -90,6 +90,11 @@ static void DescribeOrigin(u32 id) {
         Printf("  %sVirtual table ptr was destroyed%s\n", d.Origin(),
                d.Default());
         break;
+      case STACK_TRACE_TAG_ALLOC_PADDING:
+        Printf(
+            "  %sUninitialized value was created by heap allocator padding%s\n",
+            d.Origin(), d.Default());
+        break;
       default:
         Printf("  %sUninitialized value was created%s\n", d.Origin(),
                d.Default());

compiler-rt/test/msan/allocator_padding.cpp

@@ -0,0 +1,59 @@
+// malloc: all bytes are uninitialized
+// RUN: %clang_msan -fsanitize-memory-track-origins=2 %s -o %t && not %run %t 0 2>&1 \
+// RUN:     | FileCheck %s --check-prefixes=CHECK,ORIGIN-ALLOC
+// RUN: %clang_msan -fsanitize-memory-track-origins=2 %s -o %t && not %run %t 6 2>&1 \
+// RUN:     | FileCheck %s --check-prefixes=CHECK,ORIGIN-ALLOC
+//
+// This test assumes the allocator allocates 16 bytes for malloc(7). Bytes
+// 7-15 are padding.
+// Edge case: when the origin granularity spans both ALLOC and ALLOC_PADDING,
+//            ALLOC takes precedence
+// RUN: %clang_msan -fsanitize-memory-track-origins=2 %s -o %t && not %run %t 7 2>&1 \
+// RUN:     | FileCheck %s --check-prefixes=CHECK,ORIGIN-ALLOC
+//
+// Bytes 8-15 are tagged as ALLOC_PADDING.
+// RUN: %clang_msan -fsanitize-memory-track-origins=2 %s -o %t && not %run %t 8 2>&1 \
+// RUN:     | FileCheck %s --check-prefixes=CHECK,ORIGIN-ALLOC-PADDING
+// RUN: %clang_msan -fsanitize-memory-track-origins=2 %s -o %t && not %run %t 15 2>&1 \
+// RUN:     | FileCheck %s --check-prefixes=CHECK,ORIGIN-ALLOC-PADDING
+
+// calloc
+// Bytes 0-6 are fully initialized, so no MSan report should happen.
+// RUN: %clang_msan -fsanitize-memory-track-origins=2 -DUSE_CALLOC %s -o %t && %run %t 0 2>&1
+// RUN: %clang_msan -fsanitize-memory-track-origins=2 -DUSE_CALLOC %s -o %t && %run %t 6 2>&1
+//
+// Byte 7 is uninitialized. Unlike malloc, this is tagged as ALLOC_PADDING
+// (since the origin does not need to track bytes 4-6).
+// RUN: %clang_msan -fsanitize-memory-track-origins=2 -DUSE_CALLOC %s -o %t && not %run %t 7 2>&1 \
+// RUN:     | FileCheck %s --check-prefixes=CHECK,ORIGIN-ALLOC-PADDING
+//
+// As with malloc, Bytes 8-15 are tagged as ALLOC_PADDING.
+// RUN: %clang_msan -fsanitize-memory-track-origins=2 -DUSE_CALLOC %s -o %t && not %run %t 8 2>&1 \
+// RUN:     | FileCheck %s --check-prefixes=CHECK,ORIGIN-ALLOC-PADDING
+// RUN: %clang_msan -fsanitize-memory-track-origins=2 -DUSE_CALLOC %s -o %t && not %run %t 15 2>&1 \
+// RUN:     | FileCheck %s --check-prefixes=CHECK,ORIGIN-ALLOC-PADDING
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+int main(int argc, char **argv) {
+#ifdef USE_CALLOC
+  char *p = (char *)calloc(7, 1);
+#else
+  char *p = (char *)malloc(7);
+#endif
+
+  if (argc == 2) {
+    int index = atoi(argv[1]);
+
+    printf("p[%d] = %d\n", index, p[index]);
+    // CHECK: WARNING: MemorySanitizer: use-of-uninitialized-value
+    // CHECK: {{#0 0x.* in main .*allocator_padding.cpp:}}[[@LINE-2]]
+    // ORIGIN-ALLOC: Uninitialized value was created by a heap allocation
+    // ORIGIN-ALLOC-PADDING: Uninitialized value was created by heap allocator padding
+    free(p);
+  }
+
+  return 0;
+}

compiler-rt/test/msan/zero_alloc.cpp

@@ -1,4 +1,7 @@
-// RUN: %clang_msan -Wno-alloc-size -fsanitize-recover=memory %s -o %t && not %run %t 2>&1 | FileCheck %s
+// RUN: %clang_msan -Wno-alloc-size -fsanitize-recover=memory %s -o %t && not %run %t 2>&1 \
+// RUN:     | FileCheck %s --check-prefix=CHECK
+// RUN: %clang_msan -Wno-alloc-size -fsanitize-recover=memory -fsanitize-memory-track-origins=2 %s -o %t && not %run %t 2>&1 \
+// RUN:     | FileCheck %s --check-prefixes=CHECK,ORIGINS
 
 #include <stdio.h>
 #include <stdlib.h>
@@ -10,6 +13,7 @@ int main(int argc, char **argv) {
     printf("Content of p1 is: %d\n", *p1);
     // CHECK: WARNING: MemorySanitizer: use-of-uninitialized-value
     // CHECK: {{#0 0x.* in main .*zero_alloc.cpp:}}[[@LINE-2]]
+    // ORIGINS: Uninitialized value was created by heap allocator padding
     free(p1);
   }
 
@@ -19,6 +23,7 @@ int main(int argc, char **argv) {
     printf("Content of p2 is: %d\n", *p2);
     // CHECK: WARNING: MemorySanitizer: use-of-uninitialized-value
     // CHECK: {{#0 0x.* in main .*zero_alloc.cpp:}}[[@LINE-2]]
+    // ORIGINS: Uninitialized value was created by heap allocator padding
     free(p2);
   }
 
@@ -28,6 +33,7 @@ int main(int argc, char **argv) {
     printf("Content of p2 is: %d\n", *p3);
     // CHECK: WARNING: MemorySanitizer: use-of-uninitialized-value
     // CHECK: {{#0 0x.* in main .*zero_alloc.cpp:}}[[@LINE-2]]
+    // ORIGINS: Uninitialized value was created by heap allocator padding
     free(p3);
   }