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Date:   Mon, 2 Oct 2023 15:16:10 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Bob Peterson <rpeterso@redhat.com>
Cc:     linux-fsdevel@vger.kernel.org,
        Christian Brauner <brauner@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Namjae Jeon <linkinjeon@kernel.org>,
        David Sterba <dsterba@suse.com>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Amir Goldstein <amir73il@gmail.com>,
        Trond Myklebust <trond.myklebust@hammerspace.com>,
        Steve French <sfrench@samba.org>,
        Luis Chamberlain <mcgrof@kernel.org>
Subject: Re: [PATCH 08/15] gfs2: fix an oops in gfs2_permission()
Message-ID: <20231002141610.GX800259@ZenIV>
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On Mon, Oct 02, 2023 at 01:59:46PM +0100, Al Viro wrote:
> On Mon, Oct 02, 2023 at 06:46:03AM -0500, Bob Peterson wrote:
> > > diff --git a/fs/gfs2/inode.c b/fs/gfs2/inode.c
> > > index 0eac04507904..e2432c327599 100644
> > > --- a/fs/gfs2/inode.c
> > > +++ b/fs/gfs2/inode.c
> > > @@ -1868,14 +1868,16 @@ int gfs2_permission(struct mnt_idmap *idmap, struct inode *inode,
> > >   {
> > >   	struct gfs2_inode *ip;
> > >   	struct gfs2_holder i_gh;
> > > +	struct gfs2_glock *gl;
> > >   	int error;
> > >   	gfs2_holder_mark_uninitialized(&i_gh);
> > >   	ip = GFS2_I(inode);
> > > -	if (gfs2_glock_is_locked_by_me(ip->i_gl) == NULL) {
> > > +	gl = rcu_dereference(ip->i_gl);
> > > +	if (!gl || gfs2_glock_is_locked_by_me(gl) == NULL) {
> > 
> > This looks wrong. It should be if (gl && ... otherwise the
> > gfs2_glock_nq_init will dereference the null pointer.
> 
> We shouldn't observe NULL ->i_gl unless we are in RCU mode,
> which means we'll bail out without reaching gfs2_glock_nq_init()...

Something like
	if (unlikely(!gl)) {
		/* inode is getting torn down, must be RCU mode */
		WARN_ON_ONCE(!(mask & MAY_NOT_BLOCK));
		return -ECHILD;
	}
might be less confusing way to express that...