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

Reviewed-by: Nick Desaulniers ndesaulniers@google.com

---

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

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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