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From:   Nathan Chancellor <nathan@kernel.org>
To:     Alain Volmat <alain.volmat@foss.st.com>
Cc:     dri-devel@lists.freedesktop.org,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>, Kees Cook <keescook@chromium.org>,
        Sami Tolvanen <samitolvanen@google.com>, llvm@lists.linux.dev,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev,
        Nathan Chancellor <nathan@kernel.org>
Subject: [PATCH] drm/sti: Fix return type of sti_{dvo,hda,hdmi}_connector_mode_valid()
Date:   Wed,  2 Nov 2022 08:56:23 -0700
Message-Id: <20221102155623.3042869-1-nathan@kernel.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

With clang's kernel control flow integrity (kCFI, CONFIG_CFI_CLANG),
indirect call targets are validated against the expected function
pointer prototype to make sure the call target is valid to help mitigate
ROP attacks. If they are not identical, there is a failure at run time,
which manifests as either a kernel panic or thread getting killed. A
proposed warning in clang aims to catch these at compile time, which
reveals:

  drivers/gpu/drm/sti/sti_hda.c:637:16: error: incompatible function pointer types initializing 'enum drm_mode_status (*)(struct drm_connector *, struct drm_display_mode *)' with an expression of type 'int (struct drm_connector *, struct drm_display_mode *)' [-Werror,-Wincompatible-function-pointer-types-strict]
          .mode_valid = sti_hda_connector_mode_valid,
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
  drivers/gpu/drm/sti/sti_dvo.c:376:16: error: incompatible function pointer types initializing 'enum drm_mode_status (*)(struct drm_connector *, struct drm_display_mode *)' with an expression of type 'int (struct drm_connector *, struct drm_display_mode *)' [-Werror,-Wincompatible-function-pointer-types-strict]
          .mode_valid = sti_dvo_connector_mode_valid,
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
  drivers/gpu/drm/sti/sti_hdmi.c:1035:16: error: incompatible function pointer types initializing 'enum drm_mode_status (*)(struct drm_connector *, struct drm_display_mode *)' with an expression of type 'int (struct drm_connector *, struct drm_display_mode *)' [-Werror,-Wincompatible-function-pointer-types-strict]
          .mode_valid = sti_hdmi_connector_mode_valid,
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

->mode_valid() in 'struct drm_connector_helper_funcs' expects a return
type of 'enum drm_mode_status', not 'int'. Adjust the return type of
sti_{dvo,hda,hdmi}_connector_mode_valid() to match the prototype's to
resolve the warning and CFI failure.

Link: https://github.com/ClangBuiltLinux/linux/issues/1750
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 drivers/gpu/drm/sti/sti_dvo.c  | 5 +++--
 drivers/gpu/drm/sti/sti_hda.c  | 5 +++--
 drivers/gpu/drm/sti/sti_hdmi.c | 5 +++--
 3 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/sti/sti_dvo.c b/drivers/gpu/drm/sti/sti_dvo.c
index b6ee8a82e656..076d5f30a09c 100644
--- a/drivers/gpu/drm/sti/sti_dvo.c
+++ b/drivers/gpu/drm/sti/sti_dvo.c
@@ -346,8 +346,9 @@ static int sti_dvo_connector_get_modes(struct drm_connector *connector)
 
 #define CLK_TOLERANCE_HZ 50
 
-static int sti_dvo_connector_mode_valid(struct drm_connector *connector,
-					struct drm_display_mode *mode)
+static enum drm_mode_status
+sti_dvo_connector_mode_valid(struct drm_connector *connector,
+			     struct drm_display_mode *mode)
 {
 	int target = mode->clock * 1000;
 	int target_min = target - CLK_TOLERANCE_HZ;
diff --git a/drivers/gpu/drm/sti/sti_hda.c b/drivers/gpu/drm/sti/sti_hda.c
index 03cc401ed593..a53b5a15c2a9 100644
--- a/drivers/gpu/drm/sti/sti_hda.c
+++ b/drivers/gpu/drm/sti/sti_hda.c
@@ -601,8 +601,9 @@ static int sti_hda_connector_get_modes(struct drm_connector *connector)
 
 #define CLK_TOLERANCE_HZ 50
 
-static int sti_hda_connector_mode_valid(struct drm_connector *connector,
-					struct drm_display_mode *mode)
+static enum drm_mode_status
+sti_hda_connector_mode_valid(struct drm_connector *connector,
+			     struct drm_display_mode *mode)
 {
 	int target = mode->clock * 1000;
 	int target_min = target - CLK_TOLERANCE_HZ;
diff --git a/drivers/gpu/drm/sti/sti_hdmi.c b/drivers/gpu/drm/sti/sti_hdmi.c
index cb82622877d2..09e0cadb6368 100644
--- a/drivers/gpu/drm/sti/sti_hdmi.c
+++ b/drivers/gpu/drm/sti/sti_hdmi.c
@@ -1004,8 +1004,9 @@ static int sti_hdmi_connector_get_modes(struct drm_connector *connector)
 
 #define CLK_TOLERANCE_HZ 50
 
-static int sti_hdmi_connector_mode_valid(struct drm_connector *connector,
-					struct drm_display_mode *mode)
+static enum drm_mode_status
+sti_hdmi_connector_mode_valid(struct drm_connector *connector,
+			      struct drm_display_mode *mode)
 {
 	int target = mode->clock * 1000;
 	int target_min = target - CLK_TOLERANCE_HZ;

base-commit: 9abf2313adc1ca1b6180c508c25f22f9395cc780
-- 
2.38.1