netdev - Re: ipv6_mc_check_mld - kernel BUG at net/core/skbuff.c:1128

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Message-ID: <20150811214725.GE4402@odroid>
Date:	Tue, 11 Aug 2015 23:47:25 +0200
From:	Linus Lüssing <linus.luessing@...3.blue>
To:	Brenden Blanco <bblanco@...mgrid.com>
Cc:	netdev@...r.kernel.org
Subject: Re: ipv6_mc_check_mld - kernel BUG at net/core/skbuff.c:1128

On Tue, Aug 11, 2015 at 10:51:40PM +0200, Linus Lüssing wrote:
> On Mon, Aug 10, 2015 at 02:56:12PM -0700, Brenden Blanco wrote:
> > Doing some code reading with Alexei, we found a suspect commit, which
> > introduces an skb_get and skb_may_pull of the same skb, which leads to the BUG
> > when skb->len == len.
> 
> Urgh, didn't know that pskb_may_pull() doesn't like an skb with a
> reference count greater than one... But yes, the BUG() call in
> skbuff.c:1128 / pskb_expand_head() says that (though in this case
> the BUG() in skbuff.c call actually seems kinda weird (/"wrong"?), as
> it isn't shared between different code paths).

The more I think about it, I'm tending to remove the BUG() call in
pskb_expand_head() as in this case it obviously isn't a bug.

The skb_get() allows a simple and in my opinion easy to read cleanup
part of skb_trimmed for any caller of ip{v6,}_mc_check_mld(). No need
to check whether skb == skb_trimmed for a caller for instance,
simply checking whether skb_trimmed exists is enough.

Any objections to remove the "if (skb_shared(skb)) BUG()" part in
pskb_expand_head()? Or would there be any other undesired side
effects in utilising skb_get() like that?

Cheers, Linus
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