Re: [PATCH RFC 4/7] fs: support FD_FAILFS_ROOT in fchroot()

[Christian Brauner]

On Thu, Jul 23, 2026 at 05:49:06AM -0700, Andy Lutomirski wrote:
> On Thu, Jul 23, 2026 at 4:37 AM Christian Brauner <brauner@xxxxxxxxxx> wrote:
> >
> > Once entered, failfs is a throw-away-the-key moment. The task is
> > considered chrooted so it cannot create user namespaces to regain
> > CAP_SYS_CHROOT. chroot()/fchroot() back out require CAP_SYS_CHROOT.
> 
> I kind of alluded to this earlier, but I don't think we should promise
> this.  In particular, I still think we should eventually allow a
> non-chrooted process with no_new_privs to chroot to any valid
> directory, and I think we should consider changing the definition of
> current_chrooted() such that failfs-as-root would cause it to return
> false.  I'm also not sure why it's useful -- if I want to throw away

I had considered that but introducing this change alongside this series
felt too spicy for me.

> the key to accessing the normal contents of my mountns without
> changing my mountns, I need to chroot somewhere (like failfs) *and* I
> need to somehow prevent myself from getting an fd or a magic link back
> to the ordinary mountns contents.  If I somehow get such an fd,
> preventing my from fchrooting to it doesn't seem to accomplish
> anything, since I could traverse the fd with openat, etc just as
> easily, or even fchdir to it and use relative paths.

I think I generally agree with you. My main concern here is mostly about
changing very long-standing behavior here and making failfs special here
feels like it could easily misused.

> 
> My understanding of the exact workings of the mount tree is possibly
> not good enough to specify the semantics I think are correct quite
> exactly, but I think a decent approximation is that a task should be
> considered to be not chrooted if its root is at the top of its mount
> tree or is not in a mount tree at all.

If we could altering this definition from the patchset I would be in
favor of it. I'm curious what your take is though.