Re: [PATCH mptcp-next 1/2] mptcp: pm: bound extra_subflows admission on userspace PM

[Matthieu Baerts <matttbe@kernel.org>]

Hi Tao,

Thank you for your reply!

On 03/09/2026 10:55, Tao Cui wrote:
> Hi Matt,
> 
> 在 2026/9/3 00:39, Matthieu Baerts 写道:
>> Hi Tao,
>>
>> + cc Quanye
>>
>> On 31/08/2026 11:32, Tao Cui wrote:
>>> From: Tao Cui <cuitao@kylinos.cn>
>>>
>>> mptcp_pm_allow_new_subflow() increments the u8 extra_subflows counter
>>> for every accepted MP_JOIN on sockets using the userspace PM, without
>>> any limit. A peer establishing more than 255 live subflows wraps the
>>> counter back to 0, which then makes the underflow guards warn on the
>>> next subflow close, and permanently corrupts mptcpi_subflows_total
>>> reported to userspace.
>>>
>>> Refuse new MP_JOINs once the counter has reached U8_MAX, so that it
>>> cannot wrap anymore.
>>>
>>> Fixes: e99c1ca89071 ("mptcp: pm: add WARN_ON_ONCE guards on extra_subflows underflow")
>>> Link: https://github.com/multipath-tcp/mptcp_net-next/issues/629
>>
>> Do you have a GitHub account? Just to know if we can assign this task to
>> you (or just leave a message in the comment section).
>>
>>> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
>>> ---
>>>  net/mptcp/pm.c | 11 +++++++++--
>>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/net/mptcp/pm.c b/net/mptcp/pm.c
>>> index 8b68868255c5..3990f6775723 100644
>>> --- a/net/mptcp/pm.c
>>> +++ b/net/mptcp/pm.c
>>> @@ -562,10 +562,17 @@ bool mptcp_pm_allow_new_subflow(struct mptcp_sock *msk)
>>>  
>>>  	if (mptcp_pm_is_userspace(msk)) {
>>>  		if (mptcp_userspace_pm_active(msk)) {
>>> +			bool allow;
>>
>> "ret" could be re-used here.
>>
>>> +
>>> +			/* extra_subflows is a u8: don't let a peer wrap it
>>> +			 * with more than U8_MAX accepted MP_JOINs
>>> +			 */
>>
>> I don't think this comment is needed, "< U8_MAX" is probably clear enough.
>>
>>>  			spin_lock_bh(&pm->lock);
>>> -			pm->extra_subflows++;
>>> +			allow = pm->extra_subflows < U8_MAX;
>>> +			if (allow)
>>> +				pm->extra_subflows++;
>>>  			spin_unlock_bh(&pm->lock);
>>> -			return true;
>>> +			return allow;
>>>  		}
>>>  		return false;
>>>  	}
>>
>> So, at the end, we can re-used the same version as Quanye. Would it be
>> OK for both of you to have a Co-developed-by tag here?
>>
> 
> Thanks for sorting this out with Quanye.
> 
> Sounds good to me. I'll send a v2 taking your two comments into
> account, reusing "ret" and dropping the comment, with Quanye's
> Netlink part as a separate patch with him as author, and
> Co-developed-by tags on the admission patch. I'll switch to
> "Closes:" for the issue as well.

Thanks! Please use "Closes" only for the last patch of the series fixing
the issue. You can use "Link" for the others.

> About the GitHub account: I have one (sharkct235@gmail.com) but
> honestly I don't use it much because of the captcha checks, so
> sorry in advance if I'm slow to react there. I've left a comment on
> the issue, and I'll try to be better at watching it for this kind
> of thing.

Thanks! I guess there shouldn't be new comments over there, but we
appreciate comments simply saying "I'm going to look at it".

Cheers,
Matt
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