[Mesa-dev] [PATCH] i965/ir: Fix invalid bblock_t pointer dereference in dead_control_flow_eliminate.

[Matt Turner]

On Wed, Mar 16, 2016 at 10:40 AM, Francisco Jerez <currojerez at riseup.net> wrote:
> For the first basic block in the program 'block->prev()' would return
> an incorrectly cast list head sentinel and the subsequent
> 'prev_block->end()' dereference would read invalid memory.  Introduced
> in c7deee69ea6f64ea5b563985bf19d9deebe73b5b.
>
> CC: Ian Romanick <ian.d.romanick at intel.com>
> ---
>  src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp b/src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp
> index 75c7be3..73aa1a9 100644
> --- a/src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_dead_control_flow.cpp
> @@ -43,9 +43,10 @@ dead_control_flow_eliminate(backend_shader *s)
>     bool progress = false;
>
>     foreach_block_safe (block, s->cfg) {
> -      bblock_t *prev_block = block->prev();
> +      bblock_t *const prev_block = block->num ? block->prev() : NULL;
>        backend_instruction *const inst = block->start();
> -      backend_instruction *const prev_inst = prev_block->end();
> +      backend_instruction *const prev_inst =
> +         prev_block ? prev_block->end() : NULL;
>
>        /* ENDIF instructions, by definition, can only be found at the start of
>         * basic blocks.
> --
> 2.7.0

Reviewed-by: Matt Turner <mattst88 at gmail.com>