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

[Timo Rothenpieler <timo@rothenpieler.org>]

On 26.02.2024 17:58, Timo Rothenpieler wrote:
> On 11/02/2024 18:40, 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          |  2 +-
>>   libavutil/mem_internal.h | 33 ++++++++++++++++++++++++++++-----
>>   2 files changed, 29 insertions(+), 6 deletions(-)
>>
>> diff --git a/libavutil/mem.c b/libavutil/mem.c
>> index 36b8940a0c..62163b4cb3 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_SIMD_ALIGN_64 ? 64 : (HAVE_SIMD_ALIGN_32 ? 32 : 16))
>>   /* 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..b1d89a0605 100644
>> --- a/libavutil/mem_internal.h
>> +++ b/libavutil/mem_internal.h
>> @@ -76,27 +76,50 @@
>>    */
>>   #if defined(__INTEL_COMPILER) && __INTEL_COMPILER < 1110 || 
>> defined(__SUNPRO_C)
>> -    #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned 
>> (n))) v
>> +    #define DECLARE_ALIGNED_T(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
>>   #elif defined(__DJGPP__)
>> -    #define DECLARE_ALIGNED(n,t,v)      t __attribute__ ((aligned 
>> (FFMIN(n, 16)))) v
>> +    #define DECLARE_ALIGNED_T(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_ALIGNED_T(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
>>   #elif defined(_MSC_VER)
>> -    #define DECLARE_ALIGNED(n,t,v)      __declspec(align(n)) t v
>> +    #define DECLARE_ALIGNED_T(n,t,v)    __declspec(align(n)) t v
>>       #define DECLARE_ASM_ALIGNED(n,t,v)  __declspec(align(n)) t v
>>       #define DECLARE_ASM_CONST(n,t,v)    __declspec(align(n)) static 
>> const t v
>>   #else
>> -    #define DECLARE_ALIGNED(n,t,v)      t v
>> +    #define DECLARE_ALIGNED_T(n,t,v)    t v
>>       #define DECLARE_ASM_ALIGNED(n,t,v)  t v
>>       #define DECLARE_ASM_CONST(n,t,v)    static const t v
>>   #endif
>> +#if HAVE_SIMD_ALIGN_64
>> +    #define ALIGN_64 64
>> +    #define ALIGN_32 32
>> +#elif HAVE_SIMD_ALIGN_32
>> +    #define ALIGN_64 32
>> +    #define ALIGN_32 32
>> +#else
>> +    #define ALIGN_64 16
>> +    #define ALIGN_32 16
>> +#endif
>> +
>> +#define DECLARE_ALIGNED(n,t,v) DECLARE_ALIGNED_V(n,t,v)
>> +
>> +// Macro needs to be double-wrapped in order to expand
>> +// possible other macros being passed for n.
>> +#define DECLARE_ALIGNED_V(n,t,v) DECLARE_ALIGNED_##n(t,v)
>> +
>> +#define DECLARE_ALIGNED_4(t,v)  DECLARE_ALIGNED_T(       4, t, v)
>> +#define DECLARE_ALIGNED_8(t,v)  DECLARE_ALIGNED_T(       8, t, v)
>> +#define DECLARE_ALIGNED_16(t,v) DECLARE_ALIGNED_T(      16, t, v)
>> +#define DECLARE_ALIGNED_32(t,v) DECLARE_ALIGNED_T(ALIGN_32, t, v)
>> +#define DECLARE_ALIGNED_64(t,v) DECLARE_ALIGNED_T(ALIGN_64, t, v)
>> +
>>   // Some broken preprocessors need a second expansion
>>   // to be forced to tokenize __VA_ARGS__
>>   #define E1(x) x
> 
> I intend to push this patch soon.

applied
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