On 03/27/2012 00:38, Casey Schaufler wrote:
On 3/26/2012 7:22 AM, David Howells wrote:
J. R. Okajima <hooanon05@xxxxxxxxxxx> wrote:
(4) Added some code to override the credentials around upper
inode
creation to make sure the inode gets the right UID/GID. This
doesn't
help if the lower inode has some sort of foreign user
identifier.
Also, I'm not sure whether the LSM xattrs should be blindly
copied up.
Should the LSM policies applicable to the lower fs's apply to
the upper
fs too?
Obviously the xattr entry may not have its meanings on the upper
fs,
True. I'm not sure what's the best way to deal with that. Possibly
add an
extra flag to vfs_setxattr() and have the fs vet them... OTOH, this
gives us
files on the lowerfs that may well differ in appearance to files on
the
upperfs with respect to their xattrs.
Are you suggesting that you would have heterogeneous filesystems
with different xattr behavior stacked at the same time as you're
using an LSM that cares about the behavior of xattrs? Oh dear. As
I typed that sentence I identified a viable use case.
The most common use case for union mounts is a livecd. In this case the
livecd iso has a tmpfs rw mount above it for copyup. I don't know how
useful having a smart xattr copyup mechanism is in that place as most of
the labels will be whatever is assigned to the iso. Another usecase is
snapshotting of a filesystem for rollback. In this case it is probably
important to retain the existing label as the copyup is supposed to be
to the same location. The last case which is similar to the snapshotting
case but with a persistant upper branch would be to have an NFS mount
and have local configuration changes stored locally. In that case as
well I can see simple copy up as being ok to do since you're using it
more as an overlay than a union mount.
Now the tricky case is when you have two filesystems with two different
directories with the same name. You're going to merge those namespaces.
The problem occurs when the parent directories have different labels.
For example /mnt/upper/foo is labeled foo_t and /mnt/lower/foo is
labeled foo2_t. What do you use for the label for files created in
/mnt/union/foo?
or the upper fs may return an error when setting the xattr.
Additionally the
returned errno may not follow the generic semantics (ENOTSUP,
ENOSPC, or
EDQUOT) since the fs may return fs-specific error.
Also true. It's possible that the best way is just to ignore
everything but a
medium-related error such as EIO, ENOSPC and EDQUOT: We tried... Oh
well.
On the other hand, users may expect that the all xattrs are
copied-up,
particulary when he knows that the xattrs works well on the upper
fs too.
In copy-up, it will be hard to support all cases.
Yeah. Ideally, the copied-up file would be indistinguishable from
the lower
file, but in practice that's not possible unless inode numbers and
other
physical characteristics of the lower file are recorded in the upper
fs (say
on an xattr).
In order to leave users how to handle the xattrs, I'd suggest
introducing some mount options, which are similar to cp(1).
cp(1) has several options
--preserve=mode,ownership,timestamps,context,links,xattr,all
('mode' includes acl which are based upon xattr)
Since the mode (without acl), ownership and timestamps should
always be
copied-up, the new mount options will be something like
cpup-xattr=acl,context,all
I would suggest 'cpyup' or 'copyup' rather than 'cpup' - the latter
looks like
something to do with CPUs, but yes, it's worth considering.
And only when the option is specfied, the xattrs are copied up. No
special error handling is necessary, all the errors should be
returned
to users unconditionally.
That's not necessarily good enough. What if and LSM, say SELinux,
is in
force? Now SELinux will happily label the files for you - but
there's a
reasonable chance they won't be correct. OTOH, they may not be
correct even
if they are copied up.
The underlying storage (the "real" file) has to have the xattr
attached
to it.
Any other behavior is incorrect. If the underlying filesystem does
not
support
xattrs that lack of support has to be propagated upwards even if the
higher
layer filesystem supports xattrs.
In the case of Smack filesystems that don't support xattrs are still
given
labels based on mount options. If the lower filesystem is NTFS and
the upper
ext4 you've got to respect the NTFS labeling behavior for files
there.
Does union-mount preserve mtime? If not, it is critical for some
applications such like "make" I am afraid.
Ummm... Interesting question. If it copies up a file, then that
file must
have been opened for writing. Is the mtime altered by such an
event, or only
by a write() having been issued? Also, what about ctime? That
doesn't seem
to have been copied up either.
What is the expected behavior of union mounts for all attributes? Is
it
specified anywhere?
I would assume its under their documentation patch. I tried to lookup
up what we did for our copyup semantics for file attributes. Its not
simple or straight forward in the original UnionFS. I think its safe to
say all the first class attributes need to be copied up. When possible
we also copied the xattrs as well. Its important to realize though that
union mounts are namespace unification and not fileunification so if you
copy one xattr up you need to copy them all because any that don't get
copied will get left behind.
David
--
To unsubscribe from this list: send the line "unsubscribe
linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
--
This message was distributed to subscribers of the selinux mailing
list.
If you no longer wish to subscribe, send mail to
majordomo@xxxxxxxxxxxxx with
the words "unsubscribe selinux" without quotes as the message.
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
