[PATCH v5 10/42] capability: handle idmapped mounts

[Christoph Hellwig]

On Tue, Jan 12, 2021 at 11:00:52PM +0100, Christian Brauner wrote:
> In order to determine whether a caller holds privilege over a given
> inode the capability framework exposes the two helpers
> privileged_wrt_inode_uidgid() and capable_wrt_inode_uidgid(). The former
> verifies that the inode has a mapping in the caller's user namespace and
> the latter additionally verifies that the caller has the requested
> capability in their current user namespace.
> If the inode is accessed through an idmapped mount we simply need to map
> it according to the mount's user namespace. Afterwards the checks are
> identical to non-idmapped inodes. If the initial user namespace is
> passed all operations are a nop so non-idmapped mounts will not see a
> change in behavior and will also not see any performance impact.

This adds a bunch of pointless > 80 char lines, that would be nice to
fix up.

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch at lst.de>