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From:   Nathan Chancellor <nathan@kernel.org>
To:     Alexandra Winter <wintera@linux.ibm.com>,
        Wenjia Zhang <wenjia@linux.ibm.com>
Cc:     Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        linux-s390@vger.kernel.org, netdev@vger.kernel.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 1/3] s390/ctcm: Fix return type of ctc{mp,}m_tx()
Date:   Wed,  2 Nov 2022 09:32:50 -0700
Message-Id: <20221102163252.49175-1-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/s390/net/ctcm_main.c:1064:21: error: incompatible function pointer types initializing 'netdev_tx_t (*)(struct sk_buff *, struct net_device *)' (aka 'enum netdev_tx (*)(struct sk_buff *, struct net_device *)') with an expression of type 'int (struct sk_buff *, struct net_device *)' [-Werror,-Wincompatible-function-pointer-types-strict]
          .ndo_start_xmit         = ctcm_tx,
                                    ^~~~~~~
  drivers/s390/net/ctcm_main.c:1072:21: error: incompatible function pointer types initializing 'netdev_tx_t (*)(struct sk_buff *, struct net_device *)' (aka 'enum netdev_tx (*)(struct sk_buff *, struct net_device *)') with an expression of type 'int (struct sk_buff *, struct net_device *)' [-Werror,-Wincompatible-function-pointer-types-strict]
          .ndo_start_xmit         = ctcmpc_tx,
                                    ^~~~~~~~~

->ndo_start_xmit() in 'struct net_device_ops' expects a return type of
'netdev_tx_t', not 'int'. Adjust the return type of ctc{mp,}m_tx() to
match the prototype's to resolve the warning and potential CFI failure,
should s390 select ARCH_SUPPORTS_CFI_CLANG in the future.

Link: https://github.com/ClangBuiltLinux/linux/issues/1750
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 drivers/s390/net/ctcm_main.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/s390/net/ctcm_main.c b/drivers/s390/net/ctcm_main.c
index 37b551bd43bf..4eea7d0285c1 100644
--- a/drivers/s390/net/ctcm_main.c
+++ b/drivers/s390/net/ctcm_main.c
@@ -834,7 +834,7 @@ static int ctcmpc_transmit_skb(struct channel *ch, struct sk_buff *skb)
  *               the generic network layer.
  */
 /* first merge version - leaving both functions separated */
-static int ctcm_tx(struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t ctcm_tx(struct sk_buff *skb, struct net_device *dev)
 {
 	struct ctcm_priv *priv = dev->ml_priv;
 
@@ -877,7 +877,7 @@ static int ctcm_tx(struct sk_buff *skb, struct net_device *dev)
 }
 
 /* unmerged MPC variant of ctcm_tx */
-static int ctcmpc_tx(struct sk_buff *skb, struct net_device *dev)
+static netdev_tx_t ctcmpc_tx(struct sk_buff *skb, struct net_device *dev)
 {
 	int len = 0;
 	struct ctcm_priv *priv = dev->ml_priv;

base-commit: 9abf2313adc1ca1b6180c508c25f22f9395cc780
-- 
2.38.1