linux-kernel - [PATCH] tcp: Replace infinite loop on recvmsg bug with proper crash

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Message-Id: <1352247335-10396-1-git-send-email-jwerner@chromium.org>
Date:	Tue,  6 Nov 2012 16:15:35 -0800
From:	Julius Werner <jwerner@...omium.org>
To:	linux-kernel@...r.kernel.org
Cc:	netdev@...r.kernel.org, Patrick McHardy <kaber@...sh.net>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	James Morris <jmorris@...ei.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	"David S. Miller" <davem@...emloft.net>,
	Sameer Nanda <snanda@...omium.org>,
	Mandeep Singh Baines <msb@...omium.org>,
	Eric Dumazet <edumazet@...omium.org>,
	Julius Werner <jwerner@...omium.org>
Subject: [PATCH] tcp: Replace infinite loop on recvmsg bug with proper crash

tcp_recvmsg contains a sanity check that WARNs when there is a gap
between the socket's copied_seq and the first buffer in the
sk_receive_queue. In theory, the TCP stack makes sure that This Should
Never Happen (TM)... however, practice shows that there are still a few
bug reports from it out there (and one in my inbox).

Unfortunately, when it does happen for whatever reason, the situation
is not handled very well: the kernel logs a warning and breaks out of
the loop that walks the receive queue. It proceeds to find nothing else
to do on the socket and hits sk_wait_data, which cannot block because
the receive queue is not empty. As no data was read, the outer while
loop repeats (logging the same warning again) ad infinitum until the
system's syslog exhausts all available hard drive capacity.

This patch improves that behavior by going straight to a proper kernel
crash. The cause of the error can be identified right away and the
system's hard drive is not unnecessarily strained.

Signed-off-by: Julius Werner <jwerner@...omium.org>
---
 net/ipv4/tcp.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 197c000..fcb0927 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1628,7 +1628,7 @@ int tcp_recvmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg,
 				 "recvmsg bug: copied %X seq %X rcvnxt %X fl %X\n",
 				 *seq, TCP_SKB_CB(skb)->seq, tp->rcv_nxt,
 				 flags))
-				break;
+				BUG();

 			offset = *seq - TCP_SKB_CB(skb)->seq;
 			if (tcp_hdr(skb)->syn)
-- 
1.7.8.6

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