netdev - [PATCH] net, llc: Avoid undefined behaviour in llc_conn_ac_inc_vr_by

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Date:	Sun, 26 Jun 2011 00:11:12 +0200 (CEST)
From:	Jesper Juhl <jj@...osbits.net>
To:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] net, llc: Avoid undefined behaviour in
 llc_conn_ac_inc_vr_by_1()

Introduce a sequence point (;) between two writes to llc_sk(sk)->vR in
net/llc/llc_c_ac.c:llc_conn_ac_inc_vr_by_1() so that the order in
which the writes happen become well defined.

While the code may work fine now it may break at any time with a
different compiler, a new version of current compiler or even just a
different optimization level of the current compiler. Much better to
clearly express what's intended in a way that guarantees the result.

Signed-off-by: Jesper Juhl <jj@...osbits.net>
---
 net/llc/llc_c_ac.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/net/llc/llc_c_ac.c b/net/llc/llc_c_ac.c
index ea225bd..e535ca4 100644
--- a/net/llc/llc_c_ac.c
+++ b/net/llc/llc_c_ac.c
@@ -1296,7 +1296,14 @@ int llc_conn_ac_set_vr_0(struct sock *sk, struct sk_buff *skb)
 
 int llc_conn_ac_inc_vr_by_1(struct sock *sk, struct sk_buff *skb)
 {
-	llc_sk(sk)->vR = PDU_GET_NEXT_Vr(llc_sk(sk)->vR);
+	/* Do not consolidate this on one line. Since the PDU_GET_NEXT_Vr
+	   macro increments its argument which is the same as what we are
+	   writing to, then we'll have two writes to the same variable
+	   without an intervening sequence point, which leads to the
+	   situation where we can't really know what gets stored as the
+	   result since the compiler is free to do those in any order. */
+	const u8 new_vr = PDU_GET_NEXT_Vr(llc_sk(sk)->vR);
+	llc_sk(sk)->vR = new_vr;
 	return 0;
 }
 
-- 
1.7.5.2


-- 
Jesper Juhl <jj@...osbits.net>       http://www.chaosbits.net/
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