[Mesa-dev] [PATCH 2/2] gallium/u_vbuf: Protect against overflow with large instance divisors.

[Brian Paul]

Reviewed-by: Brian Paul <brianp at vmware.com>

On 03/22/2018 04:18 PM, Eric Anholt wrote:
> GTF-GLES3.gtf.GL3Tests.instanced_arrays.instanced_arrays_divisor uses -1
> as a divisor, so we would overflow to count=0 and upload no data,
> triggering the assert below.  We want to upload 1 element in this case,
> fixing the test on VC5.
> 
> v2: Use some more obvious logic, and explain why we don't use the normal
>      round_up().
> ---
>   src/gallium/auxiliary/util/u_vbuf.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/src/gallium/auxiliary/util/u_vbuf.c b/src/gallium/auxiliary/util/u_vbuf.c
> index 95d7990c6ca4..8a680d60a687 100644
> --- a/src/gallium/auxiliary/util/u_vbuf.c
> +++ b/src/gallium/auxiliary/util/u_vbuf.c
> @@ -936,7 +936,16 @@ u_vbuf_upload_buffers(struct u_vbuf *mgr,
>            size = mgr->ve->src_format_size[i];
>         } else if (instance_div) {
>            /* Per-instance attrib. */
> -         unsigned count = (num_instances + instance_div - 1) / instance_div;
> +
> +         /* Figure out how many instances we'll render given instance_div.  We
> +          * can't use the typical div_round_up() pattern because the CTS uses
> +          * instance_div = ~0 for a test, which overflows div_round_up()'s
> +          * addition.
> +          */
> +         unsigned count = num_instances / instance_div;
> +         if (count * instance_div != num_instances)
> +            count++;
> +
>            first += vb->stride * start_instance;
>            size = vb->stride * (count - 1) + mgr->ve->src_format_size[i];
>         } else {
>