Re: [FFmpeg-devel] [PATCH] libavcodec/utils: Ensure allocated buffer is zero-initialized

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

> On Jun 26, 2025, at 12:07, xjdeng <micro6947-at-gmail.com@ffmpeg.org> wrote:
> 
> In `av_fast_padded_mallocz`, the allocated buffer's zero-initialization is not guaranteed.
> This is because it calls `av_fast_malloc`, which in turn calls `fast_malloc` with `zero_realloc=0`.
> Consequently, the memory returned by the underlying `av_malloc` (used within `fast_malloc`)
> is not guaranteed to be zero-initialized.
> 
> Furthermore, if `*size` is adjusted to be greater than `min_size + AV_INPUT_BUFFER_PADDING_SIZE`,
> the subsequent `memset` operation will not cover the entire allocated buffer,
> leaving a portion of it uninitialized.
> 
> To ensure the entire allocated buffer is properly zero-initialized, we should use `FFMAX`
> to adjust the `memset` range.

I think memset size is enough. size >= min_size + AV_INPUT_BUFFER_PADDING_SIZE.

> 
> Signed-off-by: xjdeng <micro6947@gmail.com>
> 
> ---
> libavcodec/utils.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/libavcodec/utils.c b/libavcodec/utils.c
> index f2686b6863..e2afce71ef 100644
> --- a/libavcodec/utils.c
> +++ b/libavcodec/utils.c
> @@ -72,8 +72,8 @@ void av_fast_padded_mallocz(void *ptr, unsigned int *size, size_t min_size)
>         return;
>     }
>     av_fast_malloc(p, size, min_size + AV_INPUT_BUFFER_PADDING_SIZE);
> -    if (*p)
> -        memset(*p, 0, min_size + AV_INPUT_BUFFER_PADDING_SIZE);
> +    if (*p) 
> +        memset(*p, 0, FFMAX(*size, min_size + AV_INPUT_BUFFER_PADDING_SIZE));
> }
> 
> int av_codec_is_encoder(const AVCodec *avcodec)
> -- 
> 2.27.0.windows.1
> 
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