Re: [RFC][PATCH] rbind across namespaces

Ram
2005-05-21T07:27:16Z

[-- Attachment #1: Type: text/plain, Size: 1022 bytes --]

On Fri, 2005-05-20 at 23:27, Miklos Szeredi wrote:
> > I have enclosed a patch that allows rbinds across any two namespaces.
> > NOTE: currenly bind from foriegn namespace to current namespace is
> > allowed. This patch now allows:
>
> Note: you are accessing ->mnt_namespace without holding vfsmount_lock.

I realized that just after sending the patch :( . Have corrected it in
the new patch.

>
> Also why check current->namespace? It doesn't hurt to do
> get_namespace() even if it's not strictly needed. And it would
> simplify the code.

however get_namespace() will chew up some extra cycles
sometimes unnecessarily. I did incorporate your comment and
got much simpler code.

>
> In fact all uses of current->namespace and check_mnt() could be
> eliminated from namespace.c. The only use of ->namespace would be in
> copy_namespace() and exit_namespace().

Yes. I will make this a separate patch, which I will send out soon.
Will have to look at each of the cases deeply.

Enclosed the simplified patch,
RP

[-- Attachment #2: rbind_across_namespace1.patch --]
[-- Type: text/x-patch, Size: 2797 bytes --]

Summary:

Allows rbinds across any two namespaces. NOTE: currenly bind from foriegn
namespace to current namespace is allowed. This patch now allows:

binds/rbinds from any namespace to any other namespace, under the assumption
that if a process has access to a namespace, it ought to have permission to
manipulate that namespace.

The patch incorporates ideas from Miklos and Jamie, and is dependent on
Miklos's "fix race in mark_mounts_for_expiry" patch to function correctly. Also
it depends on Miklos's "fix bind mount from foreign namespace" patch, because
without that patch umounts would fail.

Signed off by Ram Pai <linuxram@us.ibm.com>

--- /home/linux/views/linux-2.6.12-rc4/fs/namespace.c 2005-05-06 23:22:29.000000000 -0700
+++ linux-2.6.12-rc4/fs/namespace.c 2005-05-21 00:17:31.000000000 -0700
@@ -616,11 +616,14 @@ out_unlock:
}

/*
- * do loopback mount.
+ * do loopback mount. The loopback mount can be done from any namespace
+ * to any other namespace including the current namespace, as long as
+ * the task acquired rights to manipulate them.
*/
static int do_loopback(struct nameidata *nd, char *old_name, int recurse)
{
struct nameidata old_nd;
+ struct namespace *mntpt_ns, *old_ns;
struct vfsmount *mnt = NULL;
int err = mount_is_safe(nd);
if (err)
@@ -631,16 +634,39 @@ static int do_loopback(struct nameidata
if (err)
return err;

- down_write(&current->namespace->sem);
err = -EINVAL;
- if (check_mnt(nd->mnt) && (!recurse || check_mnt(old_nd.mnt))) {
- err = -ENOMEM;
- if (recurse)
- mnt = copy_tree(old_nd.mnt, old_nd.dentry);
- else
- mnt = clone_mnt(old_nd.mnt, old_nd.dentry);
+ spin_lock(&vfsmount_lock);
+
+ mntpt_ns = nd->mnt->mnt_namespace;
+ old_ns = old_nd.mnt->mnt_namespace;
+ if (!mntpt_ns || !mntpt_ns->root || !old_ns || !old_ns->root) {
+ spin_unlock(&vfsmount_lock);
+ goto out;
+ }
+ get_namespace(mntpt_ns);
+ get_namespace(old_ns);
+
+ spin_unlock(&vfsmount_lock);
+
+ /*
+ * make sure we don't race with some
+ * other thread manipulating the
+ * namespaces.
+ */
+ if (old_ns < mntpt_ns) {
+ down_write(&old_ns->sem);
+ }
+ down_write(&mntpt_ns->sem);
+ if (old_ns > mntpt_ns) {
+ down_write(&old_ns->sem);
}

+ err = -ENOMEM;
+ if (recurse)
+ mnt = copy_tree(old_nd.mnt, old_nd.dentry);
+ else
+ mnt = clone_mnt(old_nd.mnt, old_nd.dentry);
+
if (mnt) {
/* stop bind mounts from expiring */
spin_lock(&vfsmount_lock);
@@ -656,7 +682,17 @@ static int do_loopback(struct nameidata
mntput(mnt);
}

- up_write(&current->namespace->sem);
+ if (old_ns < mntpt_ns) {
+ up_write(&old_ns->sem);
+ }
+ up_write(&mntpt_ns->sem);
+ if (old_ns > mntpt_ns) {
+ up_write(&old_ns->sem);
+ }
+
+ put_namespace(old_ns);
+ put_namespace(mntpt_ns);
+out:
path_release(&old_nd);
return err;
}