[Mesa-dev] [PATCH v2] i915: fixing driver crashes if too few vertices are submitted

[Ian Romanick]

On 09/09/2015 11:16 AM, Marius Predut wrote:
> Comparison with a signed expression and unsigned value
> is converted to unsigned value, reason for minus value is interpreted
> as a big unsigned value. For this case the "for" loop
> is going into unexpected behavior.
> 
> v1:Brian Paul: code style fix.

I don't think you really did...

> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=38109
> Signed-off-by: Marius Predut <marius.predut at intel.com>
> ---
>  src/mesa/tnl_dd/t_dd_dmatmp.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/src/mesa/tnl_dd/t_dd_dmatmp.h b/src/mesa/tnl_dd/t_dd_dmatmp.h
> index 7be3954..79de224 100644
> --- a/src/mesa/tnl_dd/t_dd_dmatmp.h
> +++ b/src/mesa/tnl_dd/t_dd_dmatmp.h
> @@ -627,6 +627,9 @@ static void TAG(render_quads_verts)( struct gl_context *ctx,
>        LOCAL_VARS;
>        GLuint j;
>  
> +      if(count % 4 != 0)
           ^
...because I'm quite sure Brian's code had a space here, per Mesa's
coding standards.

Also, this change is incorrect.  If an application does
glDrawArrays(GL_QUADS, 0, (n * 4) + 3), this change will cause zero
quads to be drawn when n quads should be drawn.

Page 18 (page 32 of the PDF) of the OpenGL 2.1 spec says:

    "The total number of vertices between Begin and End is 4n + k,
    where 0 ≤ k ≤ 3; if k is not zero, the final k vertices are
    ignored."

We probably don't have a piglit test for this scenario, so one should be
added.  You can CC me on that patch. :)

I think the correct change is to trim count such that (count % 4) == 0.
 If the modified value of count is zero, bail out.  With that change,
the other hunk (below) is unnecessary.

> +          return;
> +
>        INIT(GL_TRIANGLES);
>  
>        for (j = start; j < count-3; j += 4) {
> @@ -1248,7 +1251,7 @@ static GLboolean TAG(validate_render)( struct gl_context *ctx,
>  	    ok = (GLint) count < GET_SUBSEQUENT_VB_MAX_ELTS();
>  	 }
>  	 else {
> -	    ok = HAVE_TRIANGLES; /* flatshading is ok. */
> +	    ok = HAVE_TRIANGLES && (count % 4 == 0); /* flatshading is ok. */
>  	 }
>  	 break;
>        default:
>