Re: Faulty interaction between open_tree_clone and OPEN_TREE_NAMESPACE?

[Christian Brauner <brauner@kernel.org>]

On Thu, Apr 30, 2026 at 12:33:47AM -0400, Gabriel Sakash wrote:
> Hello.
> 
> Christian Brauner, I have CC'd you since you're the author of
> https://lore.kernel.org/all/20251229-work-empty-namespace-v1-0-
> bfb24c7b061f@kernel.org/.
> 
> Due to Arch Linux updating the kernel to 7.0.2 today, I have been able to test
> the new OPEN_TREE_NAMESPACE open_tree flag on my main workstation.
> However, I have found that it and open_tree_attr do not work together.
> 
> At the end is the C program I used to test the following.
> I expect that the output is something like:
> TARGET SOURCE                    FSTYPE OPTIONS PROPAGATION
> /               /dev/nvme0n1p2[/@] btrfs         <options> private
> But I get:
> open_tree_attr /: Operation not permitted
> 
> Using "plain" open_tree works as expected. I have narrowed down 3 issues:
> 
> 1:
> fs/namespace.c:5196 => wants_mount_setattr(...) to check if the attribute
> changes are sound.
> fs/namespace.c:5101 => may_mount() to check if the permission are met
> 
> may_mount is:
> return ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN);
> 
> Because this is from open_tree_attr, the process has not entered the new mount
> namespace yet. This will check if the process has CAP_SYS_ADMIN in the init
> user namespace, which it won't.
> 
> 2:
> fs/namespace.c:5198 => do_mount_setattr(&file->f_path, ...)
> fs/namespace.c:5198 => path_mounted(path)
> 
> This attempts to make the mount attribute changes on the path of the namespace
> "file", and not to the (root) mount in the new mount namespace. It will fail on
> path_mounted because the namespace "file" won't be the root of the NS FS.
> 
> 3:
> fs/namespace.c:4952 => !anon_ns_root(mnt) && !check_mnt(mnt)
> 
> This checks if the mount is anonymous (it won't be), and if not,that
> it's in the same
> mount namespace as the process (also false).
> It appears that the current mount attribute calls aren't set up to handle
> mount namespaces different from that of the current process.
> 
> This issue is still prevalent when running a VM with a kernel built off of
> Linus's master branch. Apologies if this is already known about.
> 
> Thank you for your time and good day.

Thank you for this bug report. It has all the details we need. This is a
missing feature for open_tree_attr() for sure and should have been
merged alongside the original implementation. Thanks for pointing that
out. I'll send a fix for this.