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Date:   Tue, 6 Apr 2021 14:15:01 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Christian Brauner <christian.brauner@ubuntu.com>
Cc:     Jens Axboe <axboe@kernel.dk>,
        syzbot <syzbot+c88a7030da47945a3cc3@syzkaller.appspotmail.com>,
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Subject: Re: [syzbot] WARNING in mntput_no_expire (2)
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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...