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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Fri, 21 May 2021 10:23:47 +0200
Message-ID: <CAJfpegt8JS-9LsRhmw9W2VMNG+tc+gGNsSwy1ULLoSFeYpHueA@mail.gmail.com>
Subject: Re: [PATCH v4 1/5] fuse: Fix leak in fuse_dentry_automount() error path
To:     Greg Kurz <groug@kaod.org>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        virtualization@lists.linux-foundation.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        virtio-fs-list <virtio-fs@redhat.com>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        Max Reitz <mreitz@redhat.com>, Vivek Goyal <vgoyal@redhat.com>
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On Fri, 21 May 2021 at 10:15, Greg Kurz <groug@kaod.org> wrote:
>
> On Fri, 21 May 2021 09:54:19 +0200
> Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> > On Thu, 20 May 2021 at 21:45, Al Viro <viro@zeniv.linux.org.uk> wrote:
> > >
> > > On Thu, May 20, 2021 at 05:46:50PM +0200, Greg Kurz wrote:
> > > > Some rollback was forgotten during the addition of crossmounts.
> > >
> > > Have you actually tested that?  Because I strongly suspect that
> > > by that point the ownership of fc and fm is with sb and those
> > > should be taken care of by deactivate_locked_super().
> >
> > Not quite.  Patch looks correct because destruction of fm is done in
> > fuse_put_super(), which only gets called if the sb initialization gets
> > as far as setting up sb->s_root, which only happens after the
> > successful fuse_fill_super_submount() call in this case.
> >
> > Doing the destruction from the various ->kill_sb() instances instead
> > of from ->put_super() would also fix this, but I'm not quite sure that
> > that would be any cleaner.
> >
>
> As saying in the answer I've just posted, a failure in
> fuse_fill_super_submount() causes an actual crash because
> fuse_mount_remove() logically assumes fm to already be in
> fc->mounts, which isn't the case at this point.
>
> In the root mount case, this is handled by taking back
> the ownership on fm, i.e. do the rollback *and* clear
> sb->s_fs_info. It seems that the same should be done
> for submounts.

Agreed.  Thanks for verifying.

Miklos