Re: [syzbot] INFO: rcu detected stall in sys_recvmmsg

[Paolo Abeni]

On Tue, 2021-09-21 at 17:13 -0700, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    1f77990c4b79 Add linux-next specific files for 20210920
> git tree:       linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=1383891d300000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=ab1346371f2e6884
> dashboard link: https://syzkaller.appspot.com/bug?extid=3360da629681aa0d22fe
> compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1625f1ab300000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17eb1b3b300000
> 
> The issue was bisected to:
> 
> commit 020250f31c4c75ac7687a673e29c00786582a5f4
> Author: Dmitry Kadashev <dkadashev@xxxxxxxxx>
> Date:   Thu Jul 8 06:34:43 2021 +0000
> 
>     namei: make do_linkat() take struct filename
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15f5ef77300000
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=17f5ef77300000
> console output: https://syzkaller.appspot.com/x/log.txt?x=13f5ef77300000
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+3360da629681aa0d22fe@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 020250f31c4c ("namei: make do_linkat() take struct filename")

I'm unsure why the bisection points to this commit. This looks like an
MPTCP specific issue, due to bad handling of MSG_WAITALL recvmsg()
flag.

I'll test a fix with syzbot to confirm the above.

Cheers,

Paolo