Re: [FFmpeg-devel] [PATCH] avutil/avstring: shrink allocation from av_get_token to fit token

[Kacper Michajlow <kasper93-at-gmail.com@ffmpeg.org>]

On Fri, 4 Jul 2025 at 20:22, Andreas Rheinhardt
<andreas.rheinhardt@outlook.com> wrote:
>
> Kacper Michajłow:
> > av_get_token() allocates an output buffer with the same size as the
> > input. Generally, this is harmless, but when the input string is large
> > and consists of many small tokens, calling av_get_token() repeatedly to
> > extract all tokens will significantly amplify memory allocations.
> >
> > To fix this, after obtaining the return value, simply realloc the buffer
> > to the actual size needed for output string.
> >
> > Fixes OOM when parsing filter graph string.
> > Fixes OSS-Fuzz: 394983446
> >
> > Signed-off-by: Kacper Michajłow <kasper93@gmail.com>
> > ---
> >  libavutil/avstring.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/libavutil/avstring.c b/libavutil/avstring.c
> > index 875eb691db..b4266aefe5 100644
> > --- a/libavutil/avstring.c
> > +++ b/libavutil/avstring.c
> > @@ -142,7 +142,7 @@ end:
> >
> >  char *av_get_token(const char **buf, const char *term)
> >  {
> > -    char *out     = av_malloc(strlen(*buf) + 1);
> > +    char *out     = av_realloc(NULL, strlen(*buf) + 1);
>
> I thought pointers provided by av_malloc could be used with
> av_realloc()? Or is it to avoid the unnecessary alignment provided by
> av_malloc()?

I was thinking about this. But the docs, of av_realloc() say

 * @param ptr  Pointer to a memory block already allocated with
 *             av_realloc() or `NULL`

so to honor this, I changed to av_realloc(NULL). I've looked at other
uses and it seems to be a pattern that is used for cases like this
one.

> >      char *ret     = out, *end = out;
> >      const char *p = *buf;
> >      if (!out)
> > @@ -172,7 +172,7 @@ char *av_get_token(const char **buf, const char *term)
> >
> >      *buf = p;
> >
> > -    return ret;
> > +    return av_realloc(ret, out - ret + 2);
>
> You seem to presume that av_realloc() can't fail when used for
> shrinking. But this is not documented.

This function returns a pointer. So if it fails, returning NULL is
valid. Unless you mean that we add another possible point of failure?
I don't think it matters, if we are already close to OOM state, this
small alloc wouldn't be a difference maker.

- Kacper

> >  }
> >
> >  char *av_strtok(char *s, const char *delim, char **saveptr)
>
> _______________________________________________
> ffmpeg-devel mailing list
> ffmpeg-devel@ffmpeg.org
> https://ffmpeg.org/mailman/listinfo/ffmpeg-devel
>
> To unsubscribe, visit link above, or email
> ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".
_______________________________________________
ffmpeg-devel mailing list
ffmpeg-devel@ffmpeg.org
https://ffmpeg.org/mailman/listinfo/ffmpeg-devel

To unsubscribe, visit link above, or email
ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".