Re: [FFmpeg-devel] [PATCH 2/7] avutil/bprint: Allow size == 0 in av_bprint_init_for_buffer()

From: Nicolas George <george@nsup.org>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Cc: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
Subject: Re: [FFmpeg-devel] [PATCH 2/7] avutil/bprint: Allow size == 0 in av_bprint_init_for_buffer()
Date: Wed, 9 Aug 2023 12:08:29 +0200
Message-ID: <ZNNlndOtJUSRrauw@phare.normalesup.org> (raw)
In-Reply-To: <AS8P250MB07441B7917A010E04C642A5D8F0FA@AS8P250MB0744.EURP250.PROD.OUTLOOK.COM>

Andreas Rheinhardt (12023-08-06):
> The AVBPrint API guarantees that the string buffer is always
> zero-terminated; in order to honour this guarantee, there
> obviously must be a string buffer at all and it must have
> a size >= 1. Therefore av_bprint_init_for_buffer() treats
> passing a NULL buffer or size == 0 as invalid data that
> leads to undefined behaviour, namely NPD in case NULL is provided
> or a write to a buffer of size 0 in case size == 0.
> 
> But it would be easy to support this, namely by using the internal
> buffer with AV_BPRINT_SIZE_COUNT_ONLY in case size == 0.
> 
> There is a reason to allow this: Several functions like
> av_channel_(description|name) are actually wrappers
> around corresponding AVBPrint functions. They accept user
> provided buffers and are supposed to return the required
> size of the buffer, which would allow the user to call
> it once to get the required buffer size and call it once
> more after having allocated the buffer.
> If av_bprint_init_for_buffer() treats size == 0 as invalid,
> all these users would need to check for this themselves
> and basically add the same codeblock that this patch
> adds to av_bprint_init_for_buffer().
> 
> This change is in line with e.g. snprintf() which also allows
> the pointer to be NULL in case size is zero.
> 
> This fixes Coverity issues #1503074, #1503076 and #1503082;
> all of these issues are about providing NULL to the channel-layout
> functions that are wrappers around AVBPrint versions.
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
> ---
> Missing lavu minor version bump.

Looks good to me.

The other patches in the series too, but I do not maintain the channel
layouts.

Regards,

-- 
  Nicolas George
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