linux-kernel - Re: [syzbot] WARNING in mntput_no

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Date:   Tue, 6 Apr 2021 14:23:28 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        syzbot <syzbot+c88a7030da47945a3cc3@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, io-uring@...r.kernel.org
Subject: Re: [syzbot] WARNING in mntput_no_expire (2)

On Tue, Apr 06, 2021 at 02:15:01PM +0000, Al Viro wrote:

> I'm referring to the fact that your diff is with an already modified path_lookupat()
> _and_ those modifications have managed to introduce a bug your patch reverts.
> No terminate_walk() paired with that path_init() failure, i.e. path_init() is
> responsible for cleanups on its (many) failure exits...

I can't tell without seeing the variant your diff is against, but at a guess
it had a non-trivial amount of trouble with missed rcu_read_unlock() in
cases when path_init() fails after having done rcu_read_lock().  For trivial
testcase, consider passing -1 for dfd, so that it would fail with -EBADF.
Or passing 0 for dfd and "blah" for name (assuming your stdin is not a directory).
Sure, you could handle those in path_init() (or delay grabbing rcu_read_lock()
in there, spreading it in a bunch of branches), but duplicated cleanup logics
for a bunch of failure exits is asking for trouble.