[PATCH v2 00/28] user_namespace: introduce fsid mappings

[Christian Brauner]

On Mon, Feb 17, 2020 at 01:06:08PM -0800, James Bottomley wrote:
> On Fri, 2020-02-14 at 19:35 +0100, Christian Brauner wrote:
> [...]
> > People not as familiar with user namespaces might not be aware that
> > fsid mappings already exist. Right now, fsid mappings are always
> > identical to id mappings. Specifically, the kernel will lookup fsuids
> > in the uid mappings and fsgids in the gid mappings of the relevant
> > user namespace.
> 
> This isn't actually entirely true: today we have the superblock user
> namespace, which can be used for fsid remapping on filesystems that
> support it (currently f2fs and fuse).  Since this is a single shift,

Note that this states "the relevant" user namespace not the caller's
user namespace. And the point is true even for such filesystems. fuse
does call make_kuid(fc->user_ns, attr->uid) and hence looks up the
mapping in the id mappings.. This would be replaced by make_kfsuid().

> how is it going to play with s_user_ns?  Do you have to understand the
> superblock mapping to use this shift, or are we simply using this to
> replace s_user_ns?

I'm not sure what you mean by understand the superblock mapping. The
case is not different from the devpts patch in this series.
Fuse needs to be changed to call make_kfsuid() since it is mountable
inside user namespaces at which point everthing just works.