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

From: Kees Cook
Date: Thu Feb 20 2020 - 01:22:47 EST

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>
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: {

+ 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;
+ }
/* Speaker Allocation Data Block */
if (dbl >= 1)