linux-kernel - [PATCH 4.4 21/36] ipv6 sit: work around bogus gcc-8 -Wrestrict warning

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Message-Id: <20180310001808.517143778@linuxfoundation.org>
Date:   Fri,  9 Mar 2018 16:18:37 -0800
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Martin Sebor <msebor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        "David S. Miller" <davem@...emloft.net>
Subject: [PATCH 4.4 21/36] ipv6 sit: work around bogus gcc-8 -Wrestrict warning

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Arnd Bergmann <arnd@...db.de>

[ Upstream commit ca79bec237f5809a7c3c59bd41cd0880aa889966 ]

gcc-8 has a new warning that detects overlapping input and output arguments
in memcpy(). It triggers for sit_init_net() calling ipip6_tunnel_clone_6rd(),
which is actually correct:

net/ipv6/sit.c: In function 'sit_init_net':
net/ipv6/sit.c:192:3: error: 'memcpy' source argument is the same as destination [-Werror=restrict]

The problem here is that the logic detecting the memcpy() arguments finds them
to be the same, but the conditional that tests for the input and output of
ipip6_tunnel_clone_6rd() to be identical is not a compile-time constant.

We know that netdev_priv(t->dev) is the same as t for a tunnel device,
and comparing "dev" directly here lets the compiler figure out as well
that 'dev == sitn->fb_tunnel_dev' when called from sit_init_net(), so
it no longer warns.

This code is old, so Cc stable to make sure that we don't get the warning
for older kernels built with new gcc.

Cc: Martin Sebor <msebor@...il.com>
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83456
Signed-off-by: Arnd Bergmann <arnd@...db.de>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 net/ipv6/sit.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/net/ipv6/sit.c
+++ b/net/ipv6/sit.c
@@ -176,7 +176,7 @@ static void ipip6_tunnel_clone_6rd(struc
 #ifdef CONFIG_IPV6_SIT_6RD
 	struct ip_tunnel *t = netdev_priv(dev);

-	if (t->dev == sitn->fb_tunnel_dev) {
+	if (dev == sitn->fb_tunnel_dev) {
 		ipv6_addr_set(&t->ip6rd.prefix, htonl(0x20020000), 0, 0, 0);
 		t->ip6rd.relay_prefix = 0;
 		t->ip6rd.prefixlen = 16;