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Merge tag 'fs.idmapped.v5.17' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux
Pull fs idmapping updates from Christian Brauner:
"This contains the work to enable the idmapping infrastructure to
support idmapped mounts of filesystems mounted with an idmapping.
In addition this contains various cleanups that avoid repeated
open-coding of the same functionality and simplify the code in quite a
few places.
We also finish the renaming of the mapping helpers we started a few
kernel releases back and move them to a dedicated header to not
continue polluting the fs header needlessly with low-level idmapping
helpers. With this series the fs header only contains idmapping
helpers that interact with fs objects.
Currently we only support idmapped mounts for filesystems mounted
without an idmapping themselves. This was a conscious decision
mentioned in multiple places (cf. [1]).
As explained at length in [3] it is perfectly fine to extend support
for idmapped mounts to filesystem's mounted with an idmapping should
the need arise. The need has been there for some time now (cf. [2]).
Before we can port any filesystem that is mountable with an idmapping
to support idmapped mounts in the coming cycles, we need to first
extend the mapping helpers to account for the filesystem's idmapping.
This again, is explained at length in our documentation at [3] and
also in the individual commit messages so here's an overview.
Currently, the low-level mapping helpers implement the remapping
algorithms described in [3] in a simplified manner as we could rely on
the fact that all filesystems supporting idmapped mounts are mounted
without an idmapping.
In contrast, filesystems mounted with an idmapping are very likely to
not use an identity mapping and will instead use a non-identity
mapping. So the translation step from or into the filesystem's
idmapping in the remapping algorithm cannot be skipped for such
filesystems.
Non-idmapped filesystems and filesystems not supporting idmapped
mounts are unaffected by this change as the remapping algorithms can
take the same shortcut as before. If the low-level helpers detect that
they are dealing with an idmapped mount but the underlying filesystem
is mounted without an idmapping we can rely on the previous shortcut
and can continue to skip the translation step from or into the
filesystem's idmapping. And of course, if the low-level helpers detect
that they are not dealing with an idmapped mount they can simply
return the relevant id unchanged; no remapping needs to be performed
at all.
These checks guarantee that only the minimal amount of work is
performed. As before, if idmapped mounts aren't used the low-level
helpers are idempotent and no work is performed at all"
Link: 2ca4dcc490 ("fs/mount_setattr: tighten permission checks") [1]
Link: https://github.com/containers/podman/issues/10374 [2]
Link: Documentations/filesystems/idmappings.rst [3]
Link: a65e58e791 ("fs: document and rename fsid helpers") [4]
* tag 'fs.idmapped.v5.17' of git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux:
fs: support mapped mounts of mapped filesystems
fs: add i_user_ns() helper
fs: port higher-level mapping helpers
fs: remove unused low-level mapping helpers
fs: use low-level mapping helpers
docs: update mapping documentation
fs: account for filesystem mappings
fs: tweak fsuidgid_has_mapping()
fs: move mapping helpers
fs: add is_idmapped_mnt() helper
5dfbfe71e3