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

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

Andreas Rheinhardt:
> 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.
> 
>  libavutil/bprint.c | 5 +++++
>  libavutil/bprint.h | 3 +++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/libavutil/bprint.c b/libavutil/bprint.c
> index 23998a8b02..4e9571715c 100644
> --- a/libavutil/bprint.c
> +++ b/libavutil/bprint.c
> @@ -84,6 +84,11 @@ void av_bprint_init(AVBPrint *buf, unsigned size_init, unsigned size_max)
>  
>  void av_bprint_init_for_buffer(AVBPrint *buf, char *buffer, unsigned size)
>  {
> +    if (size == 0) {
> +        av_bprint_init(buf, 0, AV_BPRINT_SIZE_COUNT_ONLY);
> +        return;
> +    }
> +
>      buf->str      = buffer;
>      buf->len      = 0;
>      buf->size     = size;
> diff --git a/libavutil/bprint.h b/libavutil/bprint.h
> index f27d30f723..8559745478 100644
> --- a/libavutil/bprint.h
> +++ b/libavutil/bprint.h
> @@ -144,6 +144,9 @@ void av_bprint_init(AVBPrint *buf, unsigned size_init, unsigned size_max);
>   * Init a print buffer using a pre-existing buffer.
>   *
>   * The buffer will not be reallocated.
> + * In case size equals zero, the AVBPrint will be initialized to use
> + * the internal buffer as if using AV_BPRINT_SIZE_COUNT_ONLY with
> + * av_bprint_init().
>   *
>   * @param buf     buffer structure to init
>   * @param buffer  byte buffer to use for the string data

Ping for this patch (and the rest of this patchset). Will apply patches
2-5 in two days unless there are objections; will apply the rest today.

- Andreas

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