[PATCH v2] drm/edid: Distribute switch variables for initialization

[Kees Cook]

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 <kees...@chromium.org>
---
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

[Nick Desaulniers]

On Fri, Mar 6, 2020 at 9:32 AM Kees Cook <kees...@chromium.org> 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

That's not good, have you filed a bug against Clang yet? It should at
least warn when the corresponding stack init flag is set.

> 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 <kees...@chromium.org>

Reviewed-by: Nick Desaulniers <ndesau...@google.com>

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--
Thanks,
~Nick Desaulniers

[Nick Desaulniers]

On Fri, Mar 6, 2020 at 9:36 AM Nick Desaulniers <ndesau...@google.com> wrote:
>
> On Fri, Mar 6, 2020 at 9:32 AM Kees Cook <kees...@chromium.org> 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
>
> That's not good, have you filed a bug against Clang yet? It should at
> least warn when the corresponding stack init flag is set.

D'oh, link is below.

--
Thanks,
~Nick Desaulniers

[Daniel Vetter]

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 <kees...@chromium.org>

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

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