On Tue, 2020-11-03 at 14:53 -0500, Ilia Mirkin wrote:
On Tue, Nov 3, 2020 at 2:47 PM Lyude Paul lyude@redhat.com wrote:
Sorry! Thought I had responded to this but apparently not, comments down below
On Thu, 2020-10-22 at 14:04 -0400, Ilia Mirkin wrote:
On Thu, Oct 22, 2020 at 12:55 PM Lyude Paul lyude@redhat.com wrote:
Noticed this when trying to compile with -Wall on a kernel fork. We potentially don't set width here, which causes the compiler to complain about width potentially being uninitialized in drm_cvt_modes(). So, let's fix that.
Signed-off-by: Lyude Paul lyude@redhat.com
Cc: stable@vger.kernel.org # v5.9+ Fixes: 3f649ab728cd ("treewide: Remove uninitialized_var() usage") Signed-off-by: Lyude Paul lyude@redhat.com
drivers/gpu/drm/drm_edid.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c index 631125b46e04..2da158ffed8e 100644 --- a/drivers/gpu/drm/drm_edid.c +++ b/drivers/gpu/drm/drm_edid.c @@ -3094,6 +3094,7 @@ static int drm_cvt_modes(struct drm_connector *connector,
for (i = 0; i < 4; i++) { int width, height; + u8 cvt_aspect_ratio;
cvt = &(timing->data.other_data.data.cvt[i]);
@@ -3101,7 +3102,8 @@ static int drm_cvt_modes(struct drm_connector *connector, continue;
height = (cvt->code[0] + ((cvt->code[1] & 0xf0) << 4) +
2; - switch (cvt->code[1] & 0x0c) { + cvt_aspect_ratio = cvt->code[1] & 0x0c; + switch (cvt_aspect_ratio) { case 0x00: width = height * 4 / 3; break; @@ -3114,6 +3116,10 @@ static int drm_cvt_modes(struct drm_connector *connector, case 0x0c: width = height * 15 / 9; break; + default:
What value would cvt->code[1] have such that this gets hit?
Or is this a "compiler is broken, so let's add more code" situation? If so, perhaps the code added could just be enough to silence the compiler (unreachable, etc)?
I mean, this information comes from the EDID which inherently means it's coming from an untrusted source so the value could be literally anything as long as the EDID has a valid checksum. Note (assuming I'm understanding this code correctly):
drm_add_edid_modes() → add_cvt_modes() → drm_for_each_detailed_block() → do_cvt_mode() → drm_cvt_modes()
So afaict this isn't a broken compiler but a legitimate uninitialized variable.
The value can be anything, but it has to be something. The switch is on "unknown & 0x0c", so only 4 cases are possible, which are enumerated in the switch.
oops, you're completely right lol. will figure out what the unreachable macro in the kernel is and use that in a respin of this patch
-ilia