[Mesa-dev] [PATCH 5/6] glsl/linker: simplify xfb_offset vs xfb_stride overflow check

[Timothy Arceri]

On 2/2/19 5:05 am, Andres Gomez wrote:
> Current implementation uses a complicated calculation which relies in
> an implicit conversion to check the integral part of 2 division
> results.
> 
> However, the calculation actually checks that the xfb_offset is
> smaller or a multiplier of the xfb_stride. For example, while this is
> expected to fail, it actually succeeds:
> 
>    "
> 
>      ...
> 
>      layout(xfb_buffer = 2, xfb_stride = 12) out block3 {
>        layout(xfb_offset = 0) vec3 c;
>        layout(xfb_offset = 12) vec3 d; // ERROR, requires stride of 24

Why does this require a stride of 24?

vec3 c uses bytes 0-11. So there is no issue with vec3 d having an 
offset of 12. Its been a long time since I worked on this but I think 
this change is wrong. I see no reason this should fail compilation.


>      };
> 
>      ...
> 
>    "
> 
> Fixes: 2fab85aaea5 ("glsl: add xfb_stride link time validation")
> Cc: Timothy Arceri <tarceri at itsqueeze.com>
> Signed-off-by: Andres Gomez <agomez at igalia.com>
> ---
>   src/compiler/glsl/link_varyings.cpp | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/src/compiler/glsl/link_varyings.cpp b/src/compiler/glsl/link_varyings.cpp
> index 6cebc5b3c5a..ab66ceb0d00 100644
> --- a/src/compiler/glsl/link_varyings.cpp
> +++ b/src/compiler/glsl/link_varyings.cpp
> @@ -1213,8 +1213,7 @@ tfeedback_decl::store(struct gl_context *ctx, struct gl_shader_program *prog,
>            return false;
>         }
>   
> -      if ((this->offset / 4) / info->Buffers[buffer].Stride !=
> -          (xfb_offset - 1) / info->Buffers[buffer].Stride) {
> +      if (xfb_offset > info->Buffers[buffer].Stride) {
>            linker_error(prog, "xfb_offset (%d) overflows xfb_stride (%d) for "
>                         "buffer (%d)", xfb_offset * 4,
>                         info->Buffers[buffer].Stride * 4, buffer);
>