Re: 2.6.17-rc3 - fs/namespace.c issue

[Herbert Poetzl <herbert@13thfloor.at>]

On Mon, May 01, 2006 at 02:33:44PM -0700, Andrew Morton wrote:
> Valdis.Kletnieks@vt.edu wrote:
> >
> > There seems to have been a bug introduced in this changeset:
> > 
> > http://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=f6422f17d3a480f21917a3895e2a46b968f56a08
> > 
> > Am running 2.6.17-rc3-mm1.  When this changeset is applied, 'mount --bind'
> > misbehaves:
> > 
> > > # mkdir /foo
> > > # mount -t tmpfs -o rw,nosuid,nodev,noexec,noatime,nodiratime none /foo
> > > # mkdir /foo/bar
> > > # mount --bind /foo/bar /foo
> > > # tail -2 /proc/mounts
> > > none /foo tmpfs rw,nosuid,nodev,noexec,noatime,nodiratime 0 0
> > > none /foo tmpfs rw 0 0
> > 
> > Reverting this changeset causes both mounts to have the same options.
> > 
> > (Thanks to Stephen Smalley for tracking down the changeset...)

well, IMHO there are several open questions regarding semantics

first, what do we expect from --bind mounts regarding
vfs (mount) level flags like noatime, noexec, nodev?

 - should they be propagated from the original mfs/mount?
 - should they only restrict the original set?
 - should they allow to modify the existing flags?
 
IMHO, it makes perfect sense to mount something noatime
and change that rule later for a subtree like this:

 mkdir /foo
 mount -t tmpfs -o rw,noatime none /foo
 mkdir /foo/bar
 mount --bind -o atime /foo/bar /foo/bar

second, has the kernel to decide what flags userspace
can request and/or change, depending on the original?

and finally, how to handle --rbind mounts at a level
deeper than the top?

so I do not consider the example above a misbehaviour. 
what I consider a misbehaviour is that mount (userspace)
blindly assumes that --bind mounts are independant, so
it does not check the existing flags, and thus, does not
preserve them (instead it replaces them with the default)

removing the mnt->mnt_flags = mnt_flags; assignment
is sufficient to _only_ allow the identical attributes
of the original mount, as they are copied in the
clone_mnt() operation, of course, this also makes it
impossible to have any flags/changes to the --bind mounts
over the original

as this patch was torn out of a much larger patch set
to allow for such attribute changes at --bind mount time
I'd sugegst the following untested 'fix'

best,
Herbert

--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -937,7 +937,6 @@ static int do_loopback(struct nameidata 
 		spin_unlock(&vfsmount_lock);
 		release_mounts(&umount_list);
 	}
-	mnt->mnt_flags = mnt_flags;
 
 out:
 	up_write(&namespace_sem);


> (cc's added)

tx

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