Re: åå: INFO: rcu detected stall in tc_modify_qdisc

[Eric Dumazet]

On 7/30/20 10:44 AM, Vinicius Costa Gomes wrote:
> Hi,
> 
> Dmitry Vyukov <dvyukov@xxxxxxxxxx> writes:
> 
>> On Wed, Jul 29, 2020 at 9:13 PM Vinicius Costa Gomes
>> <vinicius.gomes@xxxxxxxxx> wrote:
>>>
>>> Hi,
>>>
>>> "Zhang, Qiang" <Qiang.Zhang@xxxxxxxxxxxxx> writes:
>>>
>>>> ________________________________________
>>>> åää: linux-kernel-owner@xxxxxxxxxxxxxxx <linux-kernel-owner@xxxxxxxxxxxxxxx> äè syzbot <syzbot+9f78d5c664a8c33f4cce@xxxxxxxxxxxxxxxxxxxxxxxxx>
>>>> åéæé: 2020å7æ29æ 13:53
>>>> æää: davem@xxxxxxxxxxxxx; fweisbec@xxxxxxxxx; jhs@xxxxxxxxxxxx; jiri@xxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; mingo@xxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; syzkaller-bugs@xxxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx; vinicius.gomes@xxxxxxxxx; xiyou.wangcong@xxxxxxxxx
>>>> äé: INFO: rcu detected stall in tc_modify_qdisc
>>>>
>>>> Hello,
>>>>
>>>> syzbot found the following issue on:
>>>>
>>>> HEAD commit:    181964e6 fix a braino in cmsghdr_from_user_compat_to_kern()
>>>> git tree:       net
>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=12925e38900000
>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=f87a5e4232fdb267
>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=9f78d5c664a8c33f4cce
>>>> compiler:       gcc (GCC) 10.1.0-syz 20200507
>>>> syz repro:
>>>> https://syzkaller.appspot.com/x/repro.syz?x=16587f8c900000
>>>
>>> It seems that syzkaller is generating an schedule with too small
>>> intervals (3ns in this case) which causes a hrtimer busy-loop which
>>> starves other kernel threads.
>>>
>>> We could put some limits on the interval when running in software mode,
>>> but I don't like this too much, because we are talking about users with
>>> CAP_NET_ADMIN and they have easier ways to do bad things to the system.
>>
>> Hi Vinicius,
>>
>> Could you explain why you don't like the argument if it's for CAP_NET_ADMIN?
>> Good code should check arguments regardless I think and it's useful to
>> protect root from, say, programming bugs rather than kill the machine
>> on any bug and misconfiguration. What am I missing?
> 
> I admit that I am on the fence on that argument: do not let even root
> crash the system (the point that my code is crashing the system gives
> weight to this side) vs. root has great powers, they need to know what
> they are doing.
> 
> The argument that I used to convince myself was: root can easily create
> a bunch of processes and give them the highest priority and do
> effectively the same thing as this issue, so I went with a the "they
> need to know what they are doing side".
> 
> A bit more on the specifics here:
> 
>   - Using a small interval size, is only a limitation of the taprio
>   software mode, when using hardware offloads (which I think most users
>   do), any interval size (supported by the hardware) can be used;
> 
>   - Choosing a good lower limit for this seems kind of hard: something
>   below 1us would never work well, I think, but things 1us < x < 100us
>   will depend on the hardware/kernel config/system load, and this is the
>   range includes "useful" values for many systems.
> 
> Perhaps a middle ground would be to impose a limit based on the link
> speed, the interval can never be smaller than the time it takes to send
> the minimum ethernet frame (for 1G links this would be ~480ns, should be
> enough to catch most programming mistakes). I am going to add this and
> see how it looks like.
> 
> Sorry for the brain dump :-)


I do not know taprio details, but do you really need a periodic timer ?

Presumably there is no need to fire a timer before next packet departure time ?