netdev - Re: [PATCH] net: guard against coalescing packets from buggy network drivers

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Date:	Fri, 25 Apr 2014 21:32:27 +0200
From:	Svenning Sørensen <sss@...omea.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	<linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: guard against coalescing packets from buggy network
 drivers


On 25-04-2014 18:49, Eric Dumazet wrote:
> On Fri, 2014-04-25 at 18:01 +0200, Svenning Sørensen wrote:
>
>> You are right, of course, there are more effective ways to catch buggy
>> drivers.
>> But they will probably also be much more expensive.
>> This one is very cheap, being in a relatively cold path, especially
>> compared to the memcpy in the same path.
> It seems you missed the recent tipc thread then.
Yes, I'm not subscribed to the mailing list at the moment - too much 
traffic and too little time..

>
> They are many ways this code path can be abused, your patch only takes
> care of one case.
>
> In tipc case, they added skb to shinfo->frag_list without changing
> skb->data_len.
>
> So skb_tailroom(to) was not returning 0 as it should.
>
> The invariant should be more strict.
>
> BUG_ON(to->data + to->len != skb_tail_pointer(to));
>
> I still think a BUG() to catch this is better.
>
> There is no guarantee the developer/user will catch a WARN(),
> the bug could sit there a long time.
>
> I have no pity for serious bugs like that.
Neither do I.
Either BUG or WARN would have worked for me and would have saved me a 
lot of time in chasing this bug in a monster size (almost 800 files!) 
third party driver for this unfamiliar chip on newly developed embedded 
hardware (so I couldn't be sure it wasn't a hardware bug) with nothing 
else than a serial port and a few MB flash to store log files on.


>
> Since you spot buggy drivers, please provide their fixes or at least
> name them so that we can take care of them.
>
> Thanks
>
>
Strange, I was almost certain that I saw at least one with the same bug, 
but I can't seem to find it any longer.
Maybe I looked too quick, or maybe it's been fixed by the git pull I've 
made in the meantime.
I'll take a closer look if I get some spare time some day..
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