linux-kernel - Re: [PATCH 01/34] VFS: Make clone_mnt() and copy

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Message-ID: <20101006183140.GD18479@shell>
Date:	Wed, 6 Oct 2010 14:31:41 -0400
From:	Valerie Aurora <vaurora@...hat.com>
To:	Szeredi Miklos <miklos@...redi.hu>
Cc:	Ram Pai <linuxram@...ibm.com>, mszeredi2@...il.com,
	viro@...iv.linux.org.uk, hch@...radead.org, agruen@...e.de,
	npiggin@...nel.dk, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 01/34] VFS: Make clone_mnt() and copy_tree() return error codes

On Fri, Oct 01, 2010 at 11:12:48AM +0200, Szeredi Miklos wrote:
> On Fri, Oct 1, 2010 at 3:58 AM, Ram Pai <linuxram@...ibm.com> wrote:
> > > > > > @@ -1212,11 +1216,12 @@ struct vfsmount *copy_tree(struct vfsmount *mnt, struct dentry *dentry,
> > > > > > ? ? ? ? struct path path;
> > > > > >
> > > > > > ? ? ? ? if (!(flag & CL_COPY_ALL) && IS_MNT_UNBINDABLE(mnt))
> > > > > > - ? ? ? ? ? ? ? return NULL;
> > > > > > + ? ? ? ? ? ? ? return ERR_PTR(-EINVAL);
> > > >
> > > > Ram, do you remember how this worked?
> > >
> > > Oops. That should be a OR condition. there is one other check in that
> > > function that should also be a OR condition.
> >
> > I may be wrong here. Can't exactly recollect what CL_COPY_ALL flag means. Al Viro
> > might remember? ?If CL_COPY_ALL means, to clone everything irrespective of any other
> > flags, then the above code seems right.
> 
> CL_COPY_ALL means clone the mount despite MNT_UNBINDABLE.  It is used
> for cloning the whole namespace and for collect_mounts(), both of
> which ignore MNT_UNBINDABLE.
> 
> Of the two remaining callers of copy_tree() do_loopback already checks
> MNT_UNBINDABLE on the root of the tree to be copied.

I reviewed and tested and agree.

But I don't think this change should go into stable.  It doesn't fix
any existing bug and I don't like perturbing the code in stable for a
code cleanup.

> So that leaves the one in pnode.c.   That one will be called when
> attaching a new mount or mount tree.  If the root of that tree is
> unbindable then the propagation will fail with -ENOMEM which is wrong,
> it should simply skip the whole tree and not try to propagate.   Calls

Not try to propagate - and return an error?  Or succeed and ignore?

> which result in propagation are  do_loopback, do_move_mount and
> do_add_mount.  Of this do_loopback and do_move_mount already check for
> MNT_UNBINDABLE, do_add_mount doesn't check, but should probably just
> mask out MNT_UNBINDABLE.

Hm, if we stop trusting callers of do_add_mount(), we should probably
do a lot more than just mask this out.  Interestingly, most out-of-VFS
callers just seem to add MNT_SHRINKABLE, maybe we should export
do_add_shrinkable() instead or something like that?

> So in the end that check in copy_tree() should never actually trigger
> and can be turned into a WARN_ON

WARN_ON() makes sense.

> Additionally the propagation code should perhaps be more defensive and
> skip MNT_UNBINDABLE source mounts.

Maybe WARN_ON() here too?

I'm not going to get around to revamping this part of mount
propagation until after I finish union mount copyup rewrite, anyone
else up for doing it sooner?

-VAL
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