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

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

Kacper Michajlow:
> 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.
> 

There is a leak in case of reallocation failure. Which is unacceptable.

- Andreas

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