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

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Date:   Tue, 6 Apr 2021 13:13:13 +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 02:35:05PM +0200, Christian Brauner wrote:

> And while we're at it might I bring up the possibility of an additional
> cleanup of how we currently call path_init().
> Right now we pass the return value from path_init() directly into e.g.
> link_path_walk() which as a first thing checks for error. Which feels
> rather wrong and has always confused me when looking at these codepaths.

Why?

> I get that it might make sense for reasons unrelated to path_init() that
> link_path_walk() checks its first argument for error but path_init()
> should be checked for error right away especially now that we return
> early when LOOKUP_CACHED is set without LOOKUP_RCU.

But you are making the _callers_ of path_init() do that, for no good
reason.

> thing especially in longer functions such as path_lookupat() where it
> gets convoluted pretty quickly. I think it would be cleaner to have
> something like [1]. The early exists make the code easier to reason
> about imho. But I get that that's a style discussion.

Your variant is a lot more brittle, actually.

> @@ -2424,33 +2424,49 @@ static int path_lookupat(struct nameidata *nd, unsigned flags, struct path *path
>         int err;
> 
>         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.