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

[Tao Cui <cui.tao@linux.dev>]

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.

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,
Tao

> Cheers,
> Matt