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

[Kacper Michajlow]

n Thu, 26 Jun 2025 at 16:07, Zhao Zhili
<quinkblack-at-foxmail.com at ffmpeg.org> wrote:
>
>
>
> > On Jun 26, 2025, at 12:07, xjdeng <micro6947-at-gmail.com at 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 at 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));
> > }

I think the current code is intentional. In av_fast_malloc() and
siblings the `*size` is the real allocation size, but your working
area that you should be using is `
min_size` and hence why this area (+padding) is zeroed. You shouldn't
be using anything beyond that, as it is allocated space, but "not
active" right now.

These functions are meant to be `fast` by not reallocating when there
is enough space for user payload, and part of the `fast` part is also
not zeroing the memory area that you as a user don't request in
`min_size` parameter.

- Kacper