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Date:   Mon, 16 Mar 2020 10:54:17 +0100
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Kees Cook <keescook@chromium.org>
Cc:     Daniel Vetter <daniel@ffwll.ch>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        David Airlie <airlied@linux.ie>,
        dri-devel@lists.freedesktop.org,
        clang-built-linux@googlegroups.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] drm/edid: Distribute switch variables for
 initialization
Message-ID: <20200316095417.GJ2363188@phenom.ffwll.local>
Mail-Followup-To: Kees Cook <keescook@chromium.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>, David Airlie <airlied@linux.ie>,
        dri-devel@lists.freedesktop.org, clang-built-linux@googlegroups.com,
        linux-kernel@vger.kernel.org
References: <202003060930.DDCCB6659@keescook>
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On Fri, Mar 06, 2020 at 09:32:13AM -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, lift such variables up into the next code
> block.
> 
> drivers/gpu/drm/drm_edid.c: In function ‘drm_edid_to_eld’:
> 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@chromium.org>

Thanks for your patch, applied to drm-misc-next.
-Daniel

> ---
> v2: move into function block instead being switch-local (Ville Syrjälä)
> ---
>  drivers/gpu/drm/drm_edid.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
> index 805fb004c8eb..46cee78bc175 100644
> --- a/drivers/gpu/drm/drm_edid.c
> +++ b/drivers/gpu/drm/drm_edid.c
> @@ -4381,6 +4381,7 @@ static void drm_edid_to_eld(struct drm_connector *connector, struct edid *edid)
>  
>  	if (cea_revision(cea) >= 3) {
>  		int i, start, end;
> +		int sad_count;
>  
>  		if (cea_db_offsets(cea, &start, &end)) {
>  			start = 0;
> @@ -4392,8 +4393,6 @@ 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:
>  				/* Audio Data Block, contains SADs */
>  				sad_count = min(dbl / 3, 15 - total_sad_count);
> -- 
> 2.20.1
> 
> 
> -- 
> Kees Cook

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch