Re: [FFmpeg-devel] [PATCH v2] avutil/mem: limit alignment to maximum simd align

[Timo Rothenpieler <timo@rothenpieler.org>]

On 13.01.2024 16:46, Timo Rothenpieler wrote:
> 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
> aligned.
> 
> 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.
> 
> This patch limits the maximum alignment to the maximum possible simd
> alignment according to configure.
> While not perfect, it at the very least gets rid of a lot of UB, by
> matching up the maximum DECLARE_ALIGNED value with the alignment of heap
> allocations done by lavu.
> ---
>   libavutil/mem.c          |  8 +++++++-
>   libavutil/mem_internal.h | 14 ++++++++------
>   2 files changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/libavutil/mem.c b/libavutil/mem.c
> index 36b8940a0c..b5bcaab164 100644
> --- a/libavutil/mem.c
> +++ b/libavutil/mem.c
> @@ -62,7 +62,13 @@ void  free(void *ptr);
>   
>   #endif /* MALLOC_PREFIX */
>   
> -#define ALIGN (HAVE_AVX512 ? 64 : (HAVE_AVX ? 32 : 16))
> +#if defined(_MSC_VER)
> +/* MSVC does not support conditionally limiting alignment.
> +   Set minimum value here to maximum used throughout the codebase. */
> +#define ALIGN (HAVE_SIMD_ALIGN_64 ? 64 : 32)
> +#else
> +#define ALIGN (HAVE_SIMD_ALIGN_64 ? 64 : (HAVE_SIMD_ALIGN_32 ? 32 : 16))
> +#endif
>   
>   /* NOTE: if you want to override these functions with your own
>    * implementations (not recommended) you have to link libav* as
> diff --git a/libavutil/mem_internal.h b/libavutil/mem_internal.h
> index 2448c606f1..e2911b5610 100644
> --- a/libavutil/mem_internal.h
> +++ b/libavutil/mem_internal.h
> @@ -75,18 +75,20 @@
>    * @param v Name of the variable
>    */
>   
> +#define MAX_ALIGNMENT (HAVE_SIMD_ALIGN_64 ? 64 : (HAVE_SIMD_ALIGN_32 ? 32 : 16))
> +
>   #if defined(__INTEL_COMPILER) && __INTEL_COMPILER < 1110 || defined(__SUNPRO_C)
> -    #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned (n))) v
> -    #define DECLARE_ASM_ALIGNED(n,t,v)  t __attribute__ ((aligned (n))) v
> -    #define DECLARE_ASM_CONST(n,t,v)    const t __attribute__ ((aligned (n))) v
> +    #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned (FFMIN(n, MAX_ALIGNMENT)))) v
> +    #define DECLARE_ASM_ALIGNED(n,t,v)  t __attribute__ ((aligned (FFMIN(n, MAX_ALIGNMENT)))) v
> +    #define DECLARE_ASM_CONST(n,t,v)    const t __attribute__ ((aligned (FFMIN(n, MAX_ALIGNMENT)))) v
>   #elif defined(__DJGPP__)
>       #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned (FFMIN(n, 16)))) v
>       #define DECLARE_ASM_ALIGNED(n,t,v)  t av_used __attribute__ ((aligned (FFMIN(n, 16)))) v
>       #define DECLARE_ASM_CONST(n,t,v)    static const t av_used __attribute__ ((aligned (FFMIN(n, 16)))) v
>   #elif defined(__GNUC__) || defined(__clang__)
> -    #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned (n))) v
> -    #define DECLARE_ASM_ALIGNED(n,t,v)  t av_used __attribute__ ((aligned (n))) v
> -    #define DECLARE_ASM_CONST(n,t,v)    static const t av_used __attribute__ ((aligned (n))) v
> +    #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned (FFMIN(n, MAX_ALIGNMENT)))) v
> +    #define DECLARE_ASM_ALIGNED(n,t,v)  t av_used __attribute__ ((aligned (FFMIN(n, MAX_ALIGNMENT)))) v
> +    #define DECLARE_ASM_CONST(n,t,v)    static const t av_used __attribute__ ((aligned (FFMIN(n, MAX_ALIGNMENT)))) v
>   #elif defined(_MSC_VER)
>       #define DECLARE_ALIGNED(n,t,v)      __declspec(align(n)) t v
>       #define DECLARE_ASM_ALIGNED(n,t,v)  __declspec(align(n)) t v

ping

We really should fix this before 7.0 (and probably also backport it, 
since UB is UB).

I'm fine with whatever approach, as long as the UB is gone.
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