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

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Date:   Tue, 6 Apr 2021 16:46:02 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Al Viro <viro@...iv.linux.org.uk>
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:
> On Tue, Apr 06, 2021 at 03:22:05PM +0200, Christian Brauner wrote:
> 
> > Why is a another function in charge of checking the return value of an
> > initialization function. If something like path_init() fails why is the
> > next caller responsible for rejecting it's return value and then we're
> > passing that failure value through the whole function with if (!err)
> > ladders but as I said it's mostly style preferences.
> 
> Because otherwise you either need *all* paths leading to link_path_walk()
> duplicate the logics (and get hurt whenever you miss one) or have "well,
> in some cases link_path_walk() handles ERR_PTR() given to it, in some
> cases its caller do" mess.
> 
> > > >         s = path_init(nd, flags);
> > > > -       if (IS_ERR(s))
> > > > -               return PTR_ERR(s);
> > > 
> > > Where has that come from, BTW?  Currently path_lookupat() does no such thing.
> > 
> > Hm? Are you maybe overlooking path_init() which assigns straight into
> > the variable declaration? Or are you referring to sm else?
> 
> 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...

Note that the paste post the patch was just a doodle to illustrate the
point not sm to review in earnest (I should probably comment prefix
things like this with "untested".).