Re: [RFC][PATCH] rbind across namespaces

Ram
2005-05-22T20:26:50Z

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On Sat, 2005-05-21 at 06:12, Miklos Szeredi wrote:
> > Ok. look at the enclosed patch. Does it look any better? The special
> > casing for detached mounts ate up some brain cells and made the code
> > less simpler.
>
> Yes, this isn't trivial stuff.
>
> I realized one more thing: nd->mnt (the destination vfsmount) might be
> detached while waiting for the semaphore. So that needs to be
> rechecked after taking the semaphores.

Ok. fixed that. that was surprisingly trivial though initially it looked
like some complex locking.

>
> And the same for old_nd->mnt in case of rbind. Though I'm
> not sure what the semantics should be in this case:
>
> 1) rbind always fails if the source is detached
> 2) rbind always succeeds, and if the source is detached it just
> copies that single mount

> I like 2) better. Is there anything against it?

sure. as much functionality as we can get. I have incorporated (2).

Take a look at the enclosed patch,
RP
>
> Miklos
> -
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[-- Attachment #2: rbind_across_namespace3.patch --]
[-- Type: text/x-patch, Size: 2589 bytes --]

--- /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 2005-05-22 13:22:46.000000000 -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)
@@ -631,15 +634,54 @@ 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);
+
+ /*
+ * Allow all kinds of binds and rbinds except
+ * recursive bind to a detached mount
+ */
+ mntpt_ns = nd->mnt->mnt_namespace;
+ old_ns = old_nd.mnt->mnt_namespace;
+ if (!mntpt_ns || !mntpt_ns->root ||
+ (recurse && (!old_ns || !old_ns->root))) {
+ spin_unlock(&vfsmount_lock);
+ goto out;
}
+ get_namespace(mntpt_ns);
+ if (recurse && old_ns != mntpt_ns)
+ get_namespace(old_ns);
+
+ spin_unlock(&vfsmount_lock);
+
+ /*
+ * make sure we don't race with some other thread manipulating the
+ * namespaces.
+ */
+ if (recurse && (old_ns < mntpt_ns))
+ down_write(&old_ns->sem);
+ down_write(&mntpt_ns->sem);
+ if (recurse && (old_ns > mntpt_ns))
+ down_write(&old_ns->sem);
+
+
+ /*
+ * well... mounts might have detached while we acquired
+ * the semaphores. Revalidate that the destination mount
+ * is still attached.
+ */
+ if (!nd->mnt->mnt_namespace)
+ goto error_out;
+
+ /*
+ * and if the source mount is detached, than just do
+ * a bind, instead of a rbind
+ */
+ err = -ENOMEM;
+ if (recurse && old_nd.mnt->mnt_namespace)
+ 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 */
@@ -656,7 +698,15 @@ static int do_loopback(struct nameidata
mntput(mnt);
}

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