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From: Christian Brauner <brauner@kernel.org>
To: Christoph Hellwig <hch@lst.de>
Cc: Seth Forshee <sforshee@digitalocean.com>,
	Amir Goldstein <amir73il@gmail.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org,
	Christian Brauner <christian.brauner@ubuntu.com>
Subject: [PATCH v2 05/10] docs: update mapping documentation
Date: Tue, 30 Nov 2021 13:10:27 +0100	[thread overview]
Message-ID: <20211130121032.3753852-6-brauner@kernel.org> (raw)
In-Reply-To: <20211130121032.3753852-1-brauner@kernel.org>

From: Christian Brauner <christian.brauner@ubuntu.com>

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)
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
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v2 */
unchanged
---
 Documentation/filesystems/idmappings.rst | 72 ------------------------
 1 file changed, 72 deletions(-)

diff --git a/Documentation/filesystems/idmappings.rst b/Documentation/filesystems/idmappings.rst
index 1229a75ec75d..7a879ec3b6bf 100644
--- a/Documentation/filesystems/idmappings.rst
+++ b/Documentation/filesystems/idmappings.rst
@@ -952,75 +952,3 @@ The raw userspace id that is put on disk is ``u1000`` so when the user takes
 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.
-- 
2.30.2


  parent reply	other threads:[~2021-11-30 12:11 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-30 12:10 [PATCH v2 00/10] Extend and tweak mapping support Christian Brauner
2021-11-30 12:10 ` [PATCH v2 01/10] fs: add is_idmapped_mnt() helper Christian Brauner
2021-12-02 17:09   ` Seth Forshee
2021-11-30 12:10 ` [PATCH v2 02/10] fs: move mapping helpers Christian Brauner
2021-12-02 17:10   ` Seth Forshee
2021-11-30 12:10 ` [PATCH v2 03/10] fs: tweak fsuidgid_has_mapping() Christian Brauner
2021-12-02 17:12   ` Seth Forshee
2021-11-30 12:10 ` [PATCH v2 04/10] fs: account for filesystem mappings Christian Brauner
2021-12-02 17:13   ` Seth Forshee
2021-11-30 12:10 ` Christian Brauner [this message]
2021-12-02 17:27   ` [PATCH v2 05/10] docs: update mapping documentation Seth Forshee
2021-11-30 12:10 ` [PATCH v2 06/10] fs: use low-level mapping helpers Christian Brauner
2021-12-02 17:34   ` Seth Forshee
2021-11-30 12:10 ` [PATCH v2 07/10] fs: remove unused " Christian Brauner
2021-12-02 17:39   ` Seth Forshee
2021-11-30 12:10 ` [PATCH v2 08/10] fs: port higher-level " Christian Brauner
2021-12-02 17:40   ` Seth Forshee
2021-11-30 12:10 ` [PATCH v2 09/10] fs: add i_user_ns() helper Christian Brauner
2021-12-02 17:40   ` Seth Forshee
2021-11-30 12:10 ` [PATCH v2 10/10] fs: support mapped mounts of mapped filesystems Christian Brauner
2021-12-02 17:50   ` Seth Forshee
2021-12-02 19:20     ` Christian Brauner

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