netdev - [PATCH] net: core: Distribute switch variables for initialization

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Date:   Wed, 19 Feb 2020 22:23:04 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Kees Cook <keescook@...omium.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] net: core: Distribute switch variables for initialization

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.

net/core/skbuff.c: In function ‘skb_checksum_setup_ip’:
net/core/skbuff.c:4809:7: warning: statement will never be executed [-Wswitch-unreachable]
 4809 |   int err;
      |       ^~~

[1] https://bugs.llvm.org/show_bug.cgi?id=44916

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 net/core/skbuff.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 864cb9e9622f..03d5bb1d469e 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -4805,9 +4805,9 @@ static __sum16 *skb_checksum_setup_ip(struct sk_buff *skb,
 				      typeof(IPPROTO_IP) proto,
 				      unsigned int off)
 {
-	switch (proto) {
-		int err;
+	int err;

+	switch (proto) {
 	case IPPROTO_TCP:
 		err = skb_maybe_pull_tail(skb, off + sizeof(struct tcphdr),
 					  off + MAX_TCP_HDR_LEN);