Re: [FFmpeg-devel] [PATCH v3] checkasm/h264dsp: Fix stack overflow in check_idct_dequant

[Zhao Zhili <quinkblack-at-foxmail.com@ffmpeg.org>]

> On Jun 16, 2025, at 19:03, Andreas Rheinhardt <andreas.rheinhardt@outlook.com> wrote:
> 
> Zhao Zhili:
>> 
>> 
>>> On Jun 16, 2025, at 17:46, Andreas Rheinhardt <andreas.rheinhardt@outlook.com> wrote:
>>> 
>>> Zhao Zhili:
>>>> From: Zhao Zhili <zhilizhao@tencent.com>
>>>> 
>>>> ---
>>>> tests/checkasm/h264dsp.c | 20 +++++++++++++++-----
>>>> 1 file changed, 15 insertions(+), 5 deletions(-)
>>>> 
>>>> diff --git a/tests/checkasm/h264dsp.c b/tests/checkasm/h264dsp.c
>>>> index f5f9650224..a0f8fd858a 100644
>>>> --- a/tests/checkasm/h264dsp.c
>>>> +++ b/tests/checkasm/h264dsp.c
>>>> @@ -328,25 +328,35 @@ static void check_idct_multiple(void)
>>>> static void check_idct_dequant(void)
>>>> {
>>>>    static const int depths[5] = { 8, 9, 10, 12, 14 };
>>>> -    LOCAL_ALIGNED_16(int16_t, src, [16]);
>>>> -    /* Ensure dst buffers are large enough to hold dctcoefs of all bit-depths. */
>>>> +    /* Ensure buffers are large enough to hold dctcoefs of all bit-depths. */
>>>> +    LOCAL_ALIGNED_16(uint8_t, src_buf, [16 * sizeof(int32_t)]);
>>>>    LOCAL_ALIGNED_16(uint8_t, dst0, [16 * 16 * sizeof(int32_t)]);
>>>>    LOCAL_ALIGNED_16(uint8_t, dst1, [16 * 16 * sizeof(int32_t)]);
>>>> +    int16_t *src = (int16_t *)src_buf;
>>>>    int16_t *dst_ref = (int16_t *)dst0;
>>>>    int16_t *dst_new = (int16_t *)dst1;
>>>>    H264DSPContext h;
>>>>    int bit_depth, i, qmul;
>>>>    declare_func_emms(AV_CPU_FLAG_MMX | AV_CPU_FLAG_SSE2, void, int16_t *output, int16_t *input, int qmul);
>>>> 
>>>> -    for (int j = 0; j < 16; j++)
>>>> -        src[j] = (rnd() % 512) - 256;
>>>> -
>>>>    qmul = rnd() % 4096;
>>>> 
>>>>    for (i = 0; i < FF_ARRAY_ELEMS(depths); i++) {
>>>>        bit_depth = depths[i];
>>>>        ff_h264dsp_init(&h, bit_depth, 1);
>>>> 
>>>> +        if (bit_depth == 8) {
>>>> +            for (size_t j = 0; j < 16; j++) {
>>>> +                int16_t r = (rnd() % 512) - 256;
>>>> +                AV_WN16A(&src_buf[j << 1], r);
>>>> +            }
>>>> +        } else {
>>>> +            for (size_t j = 0; j < 16; j++) {
>>>> +                int32_t r = (rnd() % (1 << (bit_depth + 1))) - (1 << bit_depth);
>>>> +                AV_WN32A(&src_buf[j << 2], r);
>>>> +            }
>>>> +        }
>>>> +
>>>>        memset(dst0, 0, 16 * 16 * SIZEOF_COEF);
>>>>        memset(dst1, 0, 16 * 16 * SIZEOF_COEF);
>>>> 
>>> 
>>> This still has an effective-type violation: src_buf is of type uint8_t,
>>> yet the ff_h264_luma_dc_dequant_idct functions will read it as
>>> int16_t/int32_t. It also still has the downside that buffer overflows
>>> for the 8bit case can go undetected.
>> 
>> A bunch of template has cast like 
>> 
>>    pixel *dst = (pixel *)_dst;
>>    const pixel *src = (const pixel *)_src;
>> 
>> then read and write as int16_t.
>> 
>> And a bunch of checkasm use uint8_t[] array on stack as src and dst,
>> which leading to UB.
>> 
>> This patch isn’t specific. And this patch add zero UB (it’s there before the patch,
>> both src and dst are accessed as int32_t/int16_t while they are int16_t and uint8_t).
>> 
> 
> This patch adds UB: src was int16_t before, so that the accesses in the
> eight bit function were fine, but are not with this patch. Anyway, it is
> irrelevant now.

Why it suddenly becomes a big problem access to properly aligned uint8_t *?

I don’t mind to discuss the rules regarding to these violating of strict aliasing,
especially in checkasm. But why it suddenly becomes a rule blocking a patch
trying to fix a fate failure.

I don’t buy the reason "the accesses in the eight bit function were fine”.

> 
> - Andreas
> 
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