Re: [PATCH] drm/edid: Distribute switch variables for initialization

[Kees Cook]

On Wed, Feb 19, 2020 at 10:22:29PM -0800, Kees Cook wrote:
> Variables declared in a switch statement before any case statements
> cannot be automatically initialized with compiler instrumentation (as
> they are not part of any execution flow). With GCC's proposed automatic
> stack variable initialization feature, this triggers a warning (and they
> don't get initialized). Clang's automatic stack variable initialization
> (via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
> doesn't initialize such variables[1]. Note that these warnings (or silent
> skipping) happen before the dead-store elimination optimization phase,
> so even when the automatic initializations are later elided in favor of
> direct initializations, the warnings remain.
> 
> To avoid these problems, move such variables into the "case" where
> they're used or lift them up into the main function body.
> 
> drivers/gpu/drm/drm_edid.c: In function âdrm_edid_to_eldâ:
> drivers/gpu/drm/drm_edid.c:4395:9: warning: statement will never be executed [-Wswitch-unreachable]
>  4395 |     int sad_count;
>       |         ^~~~~~~~~
> 
> [1] https://bugs.llvm.org/show_bug.cgi?id=44916
> 
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Ping. Can someone pick this up, please?

Thanks!

-Kees

> ---
>  drivers/gpu/drm/drm_edid.c |    5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index 805fb004c8eb..2941b65b427f 100644
> --- a/drivers/gpu/drm/drm_edid.c
> +++ b/drivers/gpu/drm/drm_edid.c
> @@ -4392,9 +4392,9 @@ static void drm_edid_to_eld(struct drm_connector *connector, struct edid *edid)
>  			dbl = cea_db_payload_len(db);
>  
>  			switch (cea_db_tag(db)) {
> -				int sad_count;
> +			case AUDIO_BLOCK: {
>  
> -			case AUDIO_BLOCK:
> +				int sad_count;
>  				/* Audio Data Block, contains SADs */
>  				sad_count = min(dbl / 3, 15 - total_sad_count);
>  				if (sad_count >= 1)
> @@ -4402,6 +4402,7 @@ static void drm_edid_to_eld(struct drm_connector *connector, struct edid *edid)
>  					       &db[1], sad_count * 3);
>  				total_sad_count += sad_count;
>  				break;
> +			}
>  			case SPEAKER_BLOCK:
>  				/* Speaker Allocation Data Block */
>  				if (dbl >= 1)
> 

-- 
Kees Cook