netdev - Re: PROBLEM: System call 'sendmsg' of process ospfd (quagga) causes kernel oops

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Message-ID: <1318932093.2657.26.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Tue, 18 Oct 2011 12:01:33 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Elmar Vonlanthen <evonlanthen@...il.com>,
	linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org>,
	Timo Teräs <timo.teras@....fi>
Subject: Re: PROBLEM: System call 'sendmsg' of process ospfd (quagga) causes
 kernel oops

Le mardi 18 octobre 2011 à 11:34 +0200, Herbert Xu a écrit :
> On Tue, Oct 18, 2011 at 04:30:32AM +0200, Eric Dumazet wrote:
> >
> > If David/Herbert/Timo agree, then patch should find its way into current
> > kernel, then to stable trees as well.
> 
> Actually, I think we should instead fix the users of needed_headroom
> to not read it twice which is causing problems here.
> 
> GRE tunnels by their nature do not have a fixed value for
> needed_headroom.  As the underlying routes change the necessary
> headroom may need to be adjusted due to further encapsulation such
> as IPsec.
> 
> Keeping it constant from tunnel creation may result in suboptimal
> performance due to unnecessary header reallocations.
> 
> However, until we audit the stack to see if there are further
> instances of double-readings such as the one causing the crash
> here, I'm fine with your patch making it constant.
> 
> Once we're sure that all of the double-readings are gone we
> can revert to a dynamic needed_headroom.
> 

Sure, we can work on this path for future kernels.

Adding an RCU protected structure to hold hard_header_len /
needed_headroom / needed_tailroom should be possible, but this adds yet
another pointer dereference...

Thanks !


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