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Date:   Mon, 13 Jan 2020 13:30:47 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Ian Kent <raven@themaw.net>
Cc:     Aleksa Sarai <cyphar@cyphar.com>,
        David Howells <dhowells@redhat.com>,
        Eric Biederman <ebiederm@xmission.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        stable@vger.kernel.org,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Serge Hallyn <serge@hallyn.com>, dev@opencontainers.org,
        containers@lists.linux-foundation.org, linux-api@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC 0/1] mount: universally disallow mounting over
 symlinks
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On Mon, Jan 13, 2020 at 02:03:00PM +0800, Ian Kent wrote:

> Oh wait, for systemd I was actually looking at:
> https://github.com/systemd/systemd/blob/master/src/shared/switch-root.c
> 
> > 
> > Mind you, that's not the actual systemd repo. either I probably
> > need to look a lot deeper (and at the actual systemd repo) to
> > work out what's actually being called.
> > 
> > > Sigh...  Guess I'll have to dig that Fedora KVM image out and
> > > try to see what it's about... ;-/  Here comes a couple of hours
> > > of build...

D'oh...  And yes, that would've been a bisect hazard - switch to
path_lookupat() later in the series gets rid of that.  Incremental
(to be foldede, of course):

diff --git a/fs/namei.c b/fs/namei.c
index 1793661c3342..204677c37751 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2634,7 +2634,7 @@ path_mountpoint(struct nameidata *nd, unsigned flags, struct path *path)
 		(err = lookup_last(nd)) > 0) {
 		s = trailing_symlink(nd);
 	}
-	if (!err)
+	if (!err && (nd->flags & LOOKUP_RCU))
 		err = unlazy_walk(nd);
 	if (!err)
 		err = handle_lookup_down(nd);