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

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

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()?

>      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.

>  }
>  
>  char *av_strtok(char *s, const char *delim, char **saveptr)

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