netdev - Re: PROBLEM: BUG (NULL ptr dereference in ipv4_dst

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Date:	Fri, 29 Jul 2011 16:36:24 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	synapse <synapse@...py.csoma.elte.hu>
Cc:	netdev@...r.kernel.org
Subject: Re: PROBLEM: BUG (NULL ptr dereference in ipv4_dst_check)

Le vendredi 29 juillet 2011 à 16:26 +0200, synapse a écrit :
> On 07/29/11 15:33, Eric Dumazet wrote:
> > Le vendredi 29 juillet 2011 à 15:18 +0200, synapse a écrit :
> >> Hello guys,
> >>
> >> I have a problem that I hope you can help me resolv. This is my first
> >> real bug report, so please be
> >> patient :)
> >>
> >> ### Description:
> >> 3.0.0-rc4 routinely locks up with BUG: unable to handle kernel NULL
> >> pointer dereference at 000000000000002c
> >> I have an intel sr2600 machine with a 10Gbit interface, it periodically
> >> locks up after a few days.
> >> It serves a lot of traffic. The trace is at the end of the mail.
> >> ###
> >>
> >> ### My efforts:
> >> I've traced the error back from atomic_dec_and_test() to:
> >>
> >> ipv4_dst_check()
> >> check_peer_redir()
> >> neigh_release()
> >> atomic_dec_and_test()
> >>
> >> The parameter to atomic_dec_and_test() is NULL (&neigh->refcnt in
> >> neigh_release), so atomic_dec_and_test()
> >> at /arch/x86/include/asm/atomic.h dies at offset 0xffffffff8140f56f.
> >>
> >> ffffffff8140f560:       48 8b 15 19 47 2f 00    mov
> >> 0x2f4719(%rip),%rdx        # 0xffffffff81703c80
> >> ffffffff8140f567:       48 89 50 18             mov    %rdx,0x18(%rax)
> >> ffffffff8140f56b:       48 8b 7b 40             mov    0x40(%rbx),%rdi
> >> ffffffff8140f56f:       f0 ff 4f 2c             lock decl 0x2c(%rdi)
> >> ffffffff8140f573:       0f 94 c0                sete   %al
> >> ffffffff8140f576:       84 c0                   test   %al,%al
> >> ffffffff8140f578:       0f 85 ab 00 00 00       jne    0xffffffff8140f629
> >>
> >>   From what I've seen is that this code is responsible for pmtu related
> >> things. The refcount member of struct neighbour
> >> is NULL and the neigh pointer (struct neighbour *) in neigh_release() is
> >> not. I have no clue how this might happen,
> >> though I suspect somebody releases the data structure somehow. Note that
> >> this code is invoked when redirect_learned.a4
> >> is set and is different from rt_gateway in ipv4_dst_check().
> >>
> >> Is it possible that two packets go to two different cores for processing
> >> and one core invalidates the rt entry
> >> the other is currently working on (meaning the second will try to
> >> dereference a NULL ptr)?
> >> ###
> >>
> >>
> >> This is just my clumsy attempt at tracking this down, I'm not a kernel
> >> expert unfortunately. I'm happy to provide
> >> further info on the matter. If I'm completely on the wrong track please
> >> let me know.
> >>
> >> Thank you for any help,
> >> Gergely Kalman
> >>
> > This bug was probably already fixed.
> >
> > Please try current linux tree
> >
> >
> found no relevant things in the diffs, except for a check against 
> DST_NOCOUNT
> when calling dst_entries_add(opc, 1). Will try with the new kernel, but 
> unfortunately
> it might take days to reproduce.

Hmm, I'll take a look, but check_peer_redir() seems suspicious at first
glance.



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