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

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,
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?

James