[compiler-rt] r173776 - [ASan] Do allocate memory even for zero-size allocation requests. Explain why we have to do this in comments.

[Alexander Potapenko]

How about having a reference to the C++ standard here?

On Tue, Jan 29, 2013 at 11:51 AM, Alexey Samsonov <samsonov at google.com> wrote:
> Author: samsonov
> Date: Tue Jan 29 01:51:34 2013
> New Revision: 173776
>
> URL: http://llvm.org/viewvc/llvm-project?rev=173776&view=rev
> Log:
> [ASan] Do allocate memory even for zero-size allocation requests. Explain why we have to do this in comments.
>
> Modified:
>     compiler-rt/trunk/lib/asan/asan_allocator2.cc
>     compiler-rt/trunk/lib/asan/tests/asan_test.cc
>
> Modified: compiler-rt/trunk/lib/asan/asan_allocator2.cc
> URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/asan/asan_allocator2.cc?rev=173776&r1=173775&r2=173776&view=diff
> ==============================================================================
> --- compiler-rt/trunk/lib/asan/asan_allocator2.cc (original)
> +++ compiler-rt/trunk/lib/asan/asan_allocator2.cc Tue Jan 29 01:51:34 2013
> @@ -95,8 +95,6 @@ static const uptr kMaxAllowedMallocSize
>  static const uptr kMaxThreadLocalQuarantine =
>    FIRST_32_SECOND_64(1 << 18, 1 << 20);
>
> -static const uptr kReturnOnZeroMalloc = 2048;  // Zero page is protected.
> -
>  // Every chunk of memory allocated by this allocator can be in one of 3 states:
>  // CHUNK_AVAILABLE: the chunk is in the free list and ready to be allocated.
>  // CHUNK_ALLOCATED: the chunk is allocated and not yet freed.
> @@ -309,10 +307,12 @@ static void *Allocate(uptr size, uptr al
>    if (alignment < min_alignment)
>      alignment = min_alignment;
>    if (size == 0) {
> -    if (alignment <= kReturnOnZeroMalloc)
> -      return reinterpret_cast<void *>(kReturnOnZeroMalloc);
> -    else
> -      return 0;  // 0 bytes with large alignment requested. Just return 0.
> +    // We'd be happy to avoid allocating memory for zero-size requests, but
> +    // some programs/tests depend on this behavior and assume that malloc would
> +    // not return NULL even for zero-size allocations. Moreover, it looks like
> +    // operator new should never return NULL, and results of consecutive "new"
> +    // calls must be different even if the allocated size is zero.
> +    size = 1;
>    }
>    CHECK(IsPowerOfTwo(alignment));
>    uptr rz_log = ComputeRZLog(size);
> @@ -418,7 +418,7 @@ static void *Allocate(uptr size, uptr al
>
>  static void Deallocate(void *ptr, StackTrace *stack, AllocType alloc_type) {
>    uptr p = reinterpret_cast<uptr>(ptr);
> -  if (p == 0 || p == kReturnOnZeroMalloc) return;
> +  if (p == 0) return;
>    uptr chunk_beg = p - kChunkHeaderSize;
>    AsanChunk *m = reinterpret_cast<AsanChunk *>(chunk_beg);
>
> @@ -612,7 +612,7 @@ void *asan_calloc(uptr nmemb, uptr size,
>  }
>
>  void *asan_realloc(void *p, uptr size, StackTrace *stack) {
> -  if (p == 0 || reinterpret_cast<uptr>(p) == kReturnOnZeroMalloc)
> +  if (p == 0)
>      return Allocate(size, 8, stack, FROM_MALLOC);
>    if (size == 0) {
>      Deallocate(p, stack, FROM_MALLOC);
> @@ -678,7 +678,7 @@ uptr __asan_get_estimated_allocated_size
>
>  bool __asan_get_ownership(const void *p) {
>    uptr ptr = reinterpret_cast<uptr>(p);
> -  return (ptr == kReturnOnZeroMalloc) || (AllocationSize(ptr) > 0);
> +  return (AllocationSize(ptr) > 0);
>  }
>
>  uptr __asan_get_allocated_size(const void *p) {
> @@ -686,7 +686,7 @@ uptr __asan_get_allocated_size(const voi
>    uptr ptr = reinterpret_cast<uptr>(p);
>    uptr allocated_size = AllocationSize(ptr);
>    // Die if p is not malloced or if it is already freed.
> -  if (allocated_size == 0 && ptr != kReturnOnZeroMalloc) {
> +  if (allocated_size == 0) {
>      GET_STACK_TRACE_FATAL_HERE;
>      ReportAsanGetAllocatedSizeNotOwned(ptr, &stack);
>    }
>
> Modified: compiler-rt/trunk/lib/asan/tests/asan_test.cc
> URL: http://llvm.org/viewvc/llvm-project/compiler-rt/trunk/lib/asan/tests/asan_test.cc?rev=173776&r1=173775&r2=173776&view=diff
> ==============================================================================
> --- compiler-rt/trunk/lib/asan/tests/asan_test.cc (original)
> +++ compiler-rt/trunk/lib/asan/tests/asan_test.cc Tue Jan 29 01:51:34 2013
> @@ -391,6 +391,26 @@ TEST(AddressSanitizer, ReallocTest) {
>    free(ptr2);
>  }
>
> +TEST(AddressSanitizer, ZeroSizeMallocTest) {
> +  // Test that malloc(0) and similar functions don't return NULL.
> +  void *ptr = Ident(malloc(0));
> +  EXPECT_FALSE(0 == ptr);
> +  free(ptr);
> +#if !defined(__APPLE__) && !defined(ANDROID) && !defined(__ANDROID__)
> +  int pm_res = posix_memalign(&ptr, 1<<20, 0);
> +  EXPECT_EQ(0, pm_res);
> +  EXPECT_FALSE(0 == ptr);
> +  free(ptr);
> +#endif
> +  int *int_ptr = new int [0];
> +  int *int_ptr2 = new int[0];
> +  EXPECT_FALSE(0 == int_ptr);
> +  EXPECT_FALSE(0 == int_ptr2);
> +  EXPECT_FALSE(int_ptr == int_ptr2);
> +  delete[] int_ptr;
> +  delete[] int_ptr2;
> +}
> +
>  #ifndef __APPLE__
>  static const char *kMallocUsableSizeErrorMsg =
>    "AddressSanitizer: attempting to call malloc_usable_size()";
>
>
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-- 
Alexander Potapenko
Software Engineer
Google Moscow