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

[Xingjing Deng <micro6947-at-gmail.com@ffmpeg.org>]

 Thank you very much — I think you've convinced me. This was my first time
asking a question in the FFmpeg community, and my limited understanding of
the code led to a false positive. I apologize for the delayed response, and
I truly appreciate your time and help.

Kacper Michajlow <kasper93@gmail.com> 于2025年6月27日周五 02:16写道：

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