[FFmpeg-devel] [PATCH] avutil: add av_memcpy() to avoid undefined behavior with NULL, NULL, 0

[James Almer]

On 7/2/2019 5:56 PM, Michael Niedermayer wrote:
> Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> ---
>  doc/APIchanges      |  3 +++
>  libavutil/mem.h     | 13 +++++++++++++
>  libavutil/version.h |  2 +-
>  3 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/doc/APIchanges b/doc/APIchanges
> index b5fadc2a48..65b8ed74ad 100644
> --- a/doc/APIchanges
> +++ b/doc/APIchanges
> @@ -15,6 +15,9 @@ libavutil:     2017-10-21
>  
>  API changes, most recent first:
>  
> +2019-07-XX - XXXXXXXXXX - lavu 56.31.100 - mem.h
> +  Add av_memcpy()
> +
>  2019-06-21 - XXXXXXXXXX - lavu 56.30.100 - frame.h
>    Add FF_DECODE_ERROR_DECODE_SLICES
>  
> diff --git a/libavutil/mem.h b/libavutil/mem.h
> index 5fb1a02dd9..a35230360d 100644
> --- a/libavutil/mem.h
> +++ b/libavutil/mem.h
> @@ -506,6 +506,19 @@ void *av_memdup(const void *p, size_t size);
>   */
>  void av_memcpy_backptr(uint8_t *dst, int back, int cnt);
>  
> +/**
> + * memcpy() implementation without a NULL pointer special case
> + *
> + * @param dst  Destination buffer
> + * @param src  Source buffer
> + * @param cnt  Number of bytes to copy; must be >= 0
> + */
> +static inline void av_memcpy(void *dst, const void *src, size_t cnt)

How many cases are there in the codebase where cnt can be 0, and dst or
src NULL, without it having been checked before calling memcpy? And from
those, which would not be from situations where the code should have
instead aborted and returned ENOMEM, or EINVAL if either of them are
function arguments?

> +{
> +    if (cnt)
> +        memcpy(dst, src, cnt);
> +}
> +
>  /**
>   * @}
>   */
> diff --git a/libavutil/version.h b/libavutil/version.h
> index e16b93e877..24ca8ab7db 100644
> --- a/libavutil/version.h
> +++ b/libavutil/version.h
> @@ -79,7 +79,7 @@
>   */
>  
>  #define LIBAVUTIL_VERSION_MAJOR  56
> -#define LIBAVUTIL_VERSION_MINOR  30
> +#define LIBAVUTIL_VERSION_MINOR  31
>  #define LIBAVUTIL_VERSION_MICRO 100
>  
>  #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \
>