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Date:   Thu, 14 Mar 2019 07:56:10 -0700
From:   Eric Dumazet <>
To:     Sabrina Dubroca <>
Cc:, Jianlin Shi <>,
        Stefano Brivio <>
Subject: Re: [PATCH net] net: enforce xmit_recursion for devices with a queue

On 03/14/2019 07:15 AM, Sabrina Dubroca wrote:
> 2019-03-14, 05:58:03 -0700, Eric Dumazet wrote:
>> On 03/14/2019 03:15 AM, Sabrina Dubroca wrote:
>>> Commit 745e20f1b626 ("net: add a recursion limit in xmit path")
>>> introduced a recursion limit, but it only applies to devices without a
>>> queue. Virtual devices with a queue (either because they don't have
>>> the IFF_NO_QUEUE flag, or because the administrator added one) can
>>> still cause an unbounded recursion, via __dev_queue_xmit ->
>>> __dev_xmit_skb -> qdisc_run -> __qdisc_run -> qdisc_restart ->
>>> sch_direct_xmit -> dev_hard_start_xmit . Jianlin reported this in a
>>> setup with 16 gretap devices stacked on top of one another.
>>> This patch prevents the stack overflow by incrementing xmit_recursion in
>>> code paths that can call dev_hard_start_xmit() (like commit 745e20f1b626
>>> did). If the recursion limit is exceeded, the packet is enqueued and the
>>> qdisc is scheduled.
>>> Reported-by: Jianlin Shi <>
>>> Signed-off-by: Sabrina Dubroca <>
>>> Reviewed-by: Stefano Brivio <>
>> Hi Sabrina, thanks for the patch.
>> Can't we detect this in the control path instead ?
> I don't see how. You could have a perfectly reasonable set of gretap
> devices that trigger this situation from simply reshuffling the IP
> addresses:
> gretap$x remote 1.1.$((x-1)).{1,2}
> (all those addresses set on a single veth device)
> Then you move those addresses to the corresponding device
> (1.1.${x}.{1,2} on gretap$x), and your machine crashes.

If this only can be done with gretap, why gretap cant implement the protection,
outside of the fast path ?


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