Re: [PATCH v1 2/9] fs/fuse: add FUSE_OWNER_UID_GID_EXT extension

[Christian Brauner <brauner@kernel.org>]

On Thu, Aug 29, 2024 at 10:24:42AM GMT, Miklos Szeredi wrote:
> On Thu, 18 Jul 2024 at 21:12, Aleksandr Mikhalitsyn
> <aleksandr.mikhalitsyn@canonical.com> wrote:
> 
> > This was a first Christian's idea when he originally proposed a
> > patchset for cephfs [2]. The problem with this
> > approach is that we don't have an idmapping provided in all
> > inode_operations, we only have it where it is supposed to be.
> > To workaround that, Christian suggested applying a mapping only when
> > we have mnt_idmap, but if not just leave uid/gid as it is.
> > This, of course, leads to inconsistencies between different
> > inode_operations, for example ->lookup (idmapping is not applied) and
> > ->symlink (idmapping is applied).
> > This inconsistency, really, is not a big deal usually, but... what if
> > a server does UID/GID-based permission checks? Then it is a problem,
> > obviously.
> 
> Is it even sensible to do UID/GID-based permission checks in the
> server if idmapping is enabled?

It really makes no sense.

> 
> If not, then we should just somehow disable that configuration (i.e.
> by the server having to opt into idmapping), and then we can just use
> the in_h.[ugi]d for creates, no?

Fwiw, that's what the patchset is doing. It's only supported if the
server sets "default_permissions".