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

[James Bottomley]

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