avutil/mem: Use max_alloc_size as-is

The size of a single allocation performed by av_malloc() or av_realloc() is supposed to be bounded by max_alloc_size, which defaults to INT_MAX and can be set by the user; yet currently this is not completely honoured: The actual value used is max_alloc_size - 32. How this came to be can only be understood historically: a) 0ecca7a disallowed allocations > INT_MAX. At that time the size parameter of av_malloc() was an unsigned and the commentary added ("lets disallow possible ambiguous cases") indicates that this was done as a precaution against calling the functions with negative int values. Genuinely limiting the size of allocations to INT_MAX doesn't seem to have been the intention given that at this time the memalign hack introduced in commit da9b170 (which when enabled increased the size of allocations slightly so that one can return a correctly aligned pointer that actually does not point to the beginning of the allocated buffer) was already present. b) Said memalign hack allocated 17 bytes more than actually desired, yet allocating 16 bytes more is actually enough and so this was changed in a949360; this commit also replaced INT_MAX by INT_MAX - 16 (and made the limit therefore a limit on the size of the allocated buffer), but kept the comment, although there is nothing ambiguous about allocating (INT_MAX - 16)..INT_MAX. c) 13dfce3 then increased 16 to 32 for AVX, 6b4c0be replaced INT_MAX by MAX_MALLOC_SIZE (which was of course defined to be INT_MAX) and 5a8e994 added max_alloc_size and made it user-selectable. d) 4fb311c then dropped the memalign hack, yet it kept the -32 (probably because the comment about ambiguous cases was still present?), although it is no longer needed at all after this commit. Therefore this commit removes it and uses max_alloc_size directly. Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
FFmpeg · May 26, 2020 · 731c775 · 731c775
1 parent 1fd8528
commit 731c775
Showing 1 changed file with 4 additions and 6 deletions.
diff --git a/libavutil/mem.c b/libavutil/mem.c
@@ -78,8 +78,7 @@ void *av_malloc(size_t size)
 {
     void *ptr = NULL;
 
-    /* let's disallow possibly ambiguous cases */
-    if (size > (max_alloc_size - 32))
+    if (size > max_alloc_size)
         return NULL;
 
 #if HAVE_POSIX_MEMALIGN
@@ -134,8 +133,7 @@ void *av_malloc(size_t size)
 
 void *av_realloc(void *ptr, size_t size)
 {
-    /* let's disallow possibly ambiguous cases */
-    if (size > (max_alloc_size - 32))
+    if (size > max_alloc_size)
         return NULL;
 
 #if HAVE_ALIGNED_MALLOC
@@ -482,12 +480,12 @@ void *av_fast_realloc(void *ptr, unsigned int *size, size_t min_size)
     if (min_size <= *size)
         return ptr;
 
-    if (min_size > max_alloc_size - 32) {
+    if (min_size > max_alloc_size) {
         *size = 0;
         return NULL;
     }
 
-    min_size = FFMIN(max_alloc_size - 32, FFMAX(min_size + min_size / 16 + 32, min_size));
+    min_size = FFMIN(max_alloc_size, FFMAX(min_size + min_size / 16 + 32, min_size));
 
     ptr = av_realloc(ptr, min_size);
     /* we could set this to the unmodified min_size but this is safer