netdev - [PATCH] Fix RCU warning in rt_cache_seq

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Message-Id: <1312909360-2675-1-git-send-email-mark.rutland@arm.com>
Date:	Tue,  9 Aug 2011 18:02:40 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	netdev@...r.kernel.org
Cc:	Mark Rutland <mark.rutland@....com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Gergely Kalman <synapse@...py.csoma.elte.hu>
Subject: [PATCH] Fix RCU warning in rt_cache_seq_show

Commit f2c31e32 ("net: fix NULL dereferences in check_peer_redir()")
added rcu protection to dst neighbour, and updated callsites for
dst_{get,set}_neighbour. Unfortunately, it missed rt_cache_seq_show.

This produces a warning on v3.1-rc1 (on a preemptible kernel, on an
ARM Vexpress A9x4):

===================================================
[ INFO: suspicious rcu_dereference_check() usage. ]
---------------------------------------------------
include/net/dst.h:91 invoked rcu_dereference_check() without protection!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 0
2 locks held by proc01/32159:

stack backtrace:
[<80014880>] (unwind_backtrace+0x0/0xf8) from [<802e5c78>] (rt_cache_seq_show+0x18c/0x1c4)
[<802e5c78>] (rt_cache_seq_show+0x18c/0x1c4) from [<800e0c5c>] (seq_read+0x324/0x4a4)
[<800e0c5c>] (seq_read+0x324/0x4a4) from [<8010786c>] (proc_reg_read+0x70/0x94)
[<8010786c>] (proc_reg_read+0x70/0x94) from [<800c0ba8>] (vfs_read+0xb0/0x144)
[<800c0ba8>] (vfs_read+0xb0/0x144) from [<800c0ea8>] (sys_read+0x40/0x70)
[<800c0ea8>] (sys_read+0x40/0x70) from [<8000e0c0>] (ret_fast_syscall+0x0/0x3c)

This patch adds calls to rcu_read_{lock,unlock} in rt_cache_seq_show,
protecting the dereferenced variable, and clearing the warning.

Signed-off-by: Mark Rutland <mark.rutland@....com>
Cc: David S. Miller <davem@...emloft.net>
Cc: Eric Dumazet <eric.dumazet@...il.com>
Cc: Gergely Kalman <synapse@...py.csoma.elte.hu>
---
 net/ipv4/route.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index e3dec1c..6699ef7 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -419,6 +419,7 @@ static int rt_cache_seq_show(struct seq_file *seq, void *v)
 		struct neighbour *n;
 		int len;
 
+		rcu_read_lock();
 		n = dst_get_neighbour(&r->dst);
 		seq_printf(seq, "%s\t%08X\t%08X\t%8X\t%d\t%u\t%d\t"
 			      "%08X\t%d\t%u\t%u\t%02X\t%d\t%1d\t%08X%n",
@@ -435,6 +436,7 @@ static int rt_cache_seq_show(struct seq_file *seq, void *v)
 			-1,
 			(n && (n->nud_state & NUD_CONNECTED)) ? 1 : 0,
 			r->rt_spec_dst, &len);
+		rcu_read_unlock();
 
 		seq_printf(seq, "%*s\n", 127 - len, "");
 	}
-- 
1.7.0.4


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