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Subject[PATCH 5.15 17/28] docs: update mapping documentation
Date
From: Christian Brauner <christian.brauner@ubuntu.com>

commit 8cc5c54de44c5e8e104d364a627ac4296845fc7f upstream.

Now that we implement the full remapping algorithms described in our
documentation remove the section about shortcircuting them.

Link: https://lore.kernel.org/r/20211123114227.3124056-6-brauner@kernel.org (v1)
Link: https://lore.kernel.org/r/20211130121032.3753852-6-brauner@kernel.org (v2)
Link: https://lore.kernel.org/r/20211203111707.3901969-6-brauner@kernel.org
Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
CC: linux-fsdevel@vger.kernel.org
Reviewed-by: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Documentation/filesystems/idmappings.rst | 72 -------------------------------
1 file changed, 72 deletions(-)

--- a/Documentation/filesystems/idmappings.rst
+++ b/Documentation/filesystems/idmappings.rst
@@ -952,75 +952,3 @@ The raw userspace id that is put on disk
their home directory back to their home computer where they are assigned
``u1000`` using the initial idmapping and mount the filesystem with the initial
idmapping they will see all those files owned by ``u1000``.
-
-Shortcircuting
---------------
-
-Currently, the implementation of idmapped mounts enforces that the filesystem
-is mounted with the initial idmapping. The reason is simply that none of the
-filesystems that we targeted were mountable with a non-initial idmapping. But
-that might change soon enough. As we've seen above, thanks to the properties of
-idmappings the translation works for both filesystems mounted with the initial
-idmapping and filesystem with non-initial idmappings.
-
-Based on this current restriction to filesystem mounted with the initial
-idmapping two noticeable shortcuts have been taken:
-
-1. We always stash a reference to the initial user namespace in ``struct
- vfsmount``. Idmapped mounts are thus mounts that have a non-initial user
- namespace attached to them.
-
- In order to support idmapped mounts this needs to be changed. Instead of
- stashing the initial user namespace the user namespace the filesystem was
- mounted with must be stashed. An idmapped mount is then any mount that has
- a different user namespace attached then the filesystem was mounted with.
- This has no user-visible consequences.
-
-2. The translation algorithms in ``mapped_fs*id()`` and ``i_*id_into_mnt()``
- are simplified.
-
- Let's consider ``mapped_fs*id()`` first. This function translates the
- caller's kernel id into a kernel id in the filesystem's idmapping via
- a mount's idmapping. The full algorithm is::
-
- mapped_fsuid(kid):
- /* Map the kernel id up into a userspace id in the mount's idmapping. */
- from_kuid(mount-idmapping, kid) = uid
-
- /* Map the userspace id down into a kernel id in the filesystem's idmapping. */
- make_kuid(filesystem-idmapping, uid) = kuid
-
- We know that the filesystem is always mounted with the initial idmapping as
- we enforce this in ``mount_setattr()``. So this can be shortened to::
-
- mapped_fsuid(kid):
- /* Map the kernel id up into a userspace id in the mount's idmapping. */
- from_kuid(mount-idmapping, kid) = uid
-
- /* Map the userspace id down into a kernel id in the filesystem's idmapping. */
- KUIDT_INIT(uid) = kuid
-
- Similarly, for ``i_*id_into_mnt()`` which translated the filesystem's kernel
- id into a mount's kernel id::
-
- i_uid_into_mnt(kid):
- /* Map the kernel id up into a userspace id in the filesystem's idmapping. */
- from_kuid(filesystem-idmapping, kid) = uid
-
- /* Map the userspace id down into a kernel id in the mounts's idmapping. */
- make_kuid(mount-idmapping, uid) = kuid
-
- Again, we know that the filesystem is always mounted with the initial
- idmapping as we enforce this in ``mount_setattr()``. So this can be
- shortened to::
-
- i_uid_into_mnt(kid):
- /* Map the kernel id up into a userspace id in the filesystem's idmapping. */
- __kuid_val(kid) = uid
-
- /* Map the userspace id down into a kernel id in the mounts's idmapping. */
- make_kuid(mount-idmapping, uid) = kuid
-
-Handling filesystems mounted with non-initial idmappings requires that the
-translation functions be converted to their full form. They can still be
-shortcircuited on non-idmapped mounts. This has no user-visible consequences.

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