Re: [PATCH 00/17] VFS: Filesystem information and notifications [ver #17]

Al Viro
2020-03-06T20:56:30Z

On Fri, Mar 06, 2020 at 08:49:26PM +0000, Al Viro wrote:
> On Fri, Mar 06, 2020 at 08:45:23PM +0000, Al Viro wrote:
> > On Fri, Mar 06, 2020 at 08:38:44PM +0000, Al Viro wrote:
> > > On Fri, Mar 06, 2020 at 08:37:05PM +0000, Al Viro wrote:
> > >
> > > > You are misreading mntput_no_expire(), BTW - your get_mount() can
> > > > bloody well race with umount(2), hitting the moment when we are done
> > > > figuring out whether it's busy but hadn't cleaned ->mnt_ns (let alone
> > > > set MNT_DOOMED) yet. If somebody calls umount(2) on a filesystem that
> > > > is not mounted anywhere else, they are not supposed to see the sucker
> > > > return 0 until the filesystem is shut down. You break that.
> > >
> > > While we are at it, d_alloc_parallel() requires i_rwsem on parent held
> > > at least shared.
> >
> > Egads... Let me see if I got it right - you are providing procfs symlinks
> > to objects on the internal mount of that thing. And those objects happen
> > to be directories, so one can get to their parent that way. Or am I misreading
> > that thing?
>
> IDGI. You have (in your lookup) kstrtoul, followed by snprintf, followed
> by strcmp and WARN_ON() in case of mismatch? Is there any point in having
> stat(2) on "00" spew into syslog? Confused...

AFAICS, refcounting in there cannot be right:
+static struct dentry *mountfs_lookup(struct inode *dir, struct dentry *dentry,
+ unsigned int flags)
+{
+ struct mountfs_entry *entry = dir->i_private;
+ int i = 0;
+
+ if (entry) {
+ for (i = 0; i < ARRAY_SIZE(mountfs_attrs); i++)
+ if (strcmp(mountfs_attrs[i], dentry->d_name.name) == 0)
+ break;
+ if (i == ARRAY_SIZE(mountfs_attrs))
+ return ERR_PTR(-ENOMEM);
+ i++;
+ } else {
+ entry = mountfs_get_entry(dentry->d_name.name);
+ if (!entry)
+ return ERR_PTR(-ENOENT);
+ }
+
+ return mountfs_lookup_entry(dentry, entry, i);
+}
ends up consuming a reference in mountfs_lookup_entry() (at the very least,
drops it in case of inode allocation hitting OOM) and nothing in the
that loop in mountfs_lookup() appears to do a matching reference grab.