[FFmpeg-devel] [PATCH] libswscale: avoid UB nullptr-with-offset.

[Michael Niedermayer]

On Mon, Dec 21, 2020 at 01:52:08PM +0000, jleconte wrote:
> ---
>  libswscale/slice.c            | 12 +++++-------
>  libswscale/swscale_unscaled.c |  5 +++--
>  2 files changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/libswscale/slice.c b/libswscale/slice.c
> index 7849b70f4d..b1cfc0e506 100644
> --- a/libswscale/slice.c
> +++ b/libswscale/slice.c
> @@ -158,14 +158,12 @@ int ff_init_slice_from_src(SwsSlice * s, uint8_t *src[4], int stride[4], int src
>                          chrY + chrH,
>                          lumY + lumH};
>  
> -    uint8_t *const src_[4] = {src[0] + (relative ? 0 : start[0]) * stride[0],
> -                              src[1] + (relative ? 0 : start[1]) * stride[1],
> -                              src[2] + (relative ? 0 : start[2]) * stride[2],
> -                              src[3] + (relative ? 0 : start[3]) * stride[3]};
> -
>      s->width = srcW;
>  
>      for (i = 0; i < 4; ++i) {
> +        if (!src[i])
> +          continue;

indention


> +        uint8_t *const src_ = src[i] + (relative ? 0 : start[i]) * stride[i];

please call this src_i, its more clear then what it is


>          int j;
>          int first = s->plane[i].sliceY;
>          int n = s->plane[i].available_lines;
> @@ -175,13 +173,13 @@ int ff_init_slice_from_src(SwsSlice * s, uint8_t *src[4], int stride[4], int src
>          if (start[i] >= first && n >= tot_lines) {
>              s->plane[i].sliceH = FFMAX(tot_lines, s->plane[i].sliceH);
>              for (j = 0; j < lines; j+= 1)
> -                s->plane[i].line[start[i] - first + j] = src_[i] +  j * stride[i];
> +                s->plane[i].line[start[i] - first + j] = src_ +  j * stride[i];
>          } else {
>              s->plane[i].sliceY = start[i];
>              lines = lines > n ? n : lines;
>              s->plane[i].sliceH = lines;
>              for (j = 0; j < lines; j+= 1)
> -                s->plane[i].line[j] = src_[i] +  j * stride[i];
> +                s->plane[i].line[j] = src_ +  j * stride[i];
>          }
>  
>      }
> diff --git a/libswscale/swscale_unscaled.c b/libswscale/swscale_unscaled.c
> index 563de39696..39fb7cc87f 100644
> --- a/libswscale/swscale_unscaled.c
> +++ b/libswscale/swscale_unscaled.c
> @@ -1806,6 +1806,9 @@ static int planarCopyWrapper(SwsContext *c, const uint8_t *src[],
>      const AVPixFmtDescriptor *desc_dst = av_pix_fmt_desc_get(c->dstFormat);
>      int plane, i, j;
>      for (plane = 0; plane < 4; plane++) {
> +        if (!dst[plane])
> +            continue;
> +
>          int length = (plane == 0 || plane == 3) ? c->srcW  : AV_CEIL_RSHIFT(c->srcW,   c->chrDstHSubSample);
>          int y =      (plane == 0 || plane == 3) ? srcSliceY: AV_CEIL_RSHIFT(srcSliceY, c->chrDstVSubSample);
>          int height = (plane == 0 || plane == 3) ? srcSliceH: AV_CEIL_RSHIFT(srcSliceH, c->chrDstVSubSample);

This produces new warnings:
libswscale/swscale_unscaled.c: In function ‘planarCopyWrapper’:
libswscale/swscale_unscaled.c:1812:9: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
         int length = (plane == 0 || plane == 3) ? c->srcW  : AV_CEIL_RSHIFT(c->srcW,   c->chrDstHSubSample);
         ^~~


thx

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I am the wisest man alive, for I know one thing, and that is that I know
nothing. -- Socrates
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