[FFmpeg-devel] [PATCH] avutil/mem: always align by at least 32 bytes

Timo Rothenpieler timo at rothenpieler.org
Sun Dec 3 22:10:27 EET 2023

FFmpeg has instances of DECLARE_ALIGNED(32, ...) in a lot of structs,
which then end up heap-allocated.
By declaring any variable in a struct, or tree of structs, to be 32 byte
aligned, it allows the compiler to safely assume the entire struct
itself is also 32 byte aligned.

This might make the compiler emit code which straight up crashes or
misbehaves in other ways, and at least in one instances is now
documented to actually do (see ticket 10549 on trac).
The issue there is that an unrelated variable in SingleChannelElement is
declared to have an alignment of 32 bytes. So if the compiler does a copy
in decode_cpe() with avx instructions, but ffmpeg is built with
--disable-avx, this results in a crash, since the memory is only 16 byte
Mind you, even if the compiler does not emit avx instructions, the code
is still invalid and could misbehave. It just happens not to. Declaring
any variable in a struct with a 32 byte alignment promises 32 byte
alignment of the whole struct to the compiler.

Instead of now going through all instances of variables in structs
being declared as 32 byte aligned, this patch bumps the minimum alignment
to 32 bytes.
 libavutil/mem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libavutil/mem.c b/libavutil/mem.c
index 36b8940a0c..26a9b9753b 100644
--- a/libavutil/mem.c
+++ b/libavutil/mem.c
@@ -62,7 +62,7 @@ void  free(void *ptr);
 #endif /* MALLOC_PREFIX */
-#define ALIGN (HAVE_AVX512 ? 64 : (HAVE_AVX ? 32 : 16))
+#define ALIGN (HAVE_AVX512 ? 64 : 32)
 /* NOTE: if you want to override these functions with your own
  * implementations (not recommended) you have to link libav* as

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