[PATCH 3/8] fs: use mount types in iattr

[Christian Brauner <brauner@kernel.org>]

Add ia_mnt{g,u}id members of type kmnt{g,u}id_t to struct iattr. We use
an anonymous union (similar to what we do in struct file) around
ia_{g,u}id and ia_mnt{g,u}id.

At the end of this series ia_{g,u}id and ia_mnt{g,u}id will always
contain the same value independent of whether struct iattr is
initialized from an idmapped mount. This is a change from how this is
done today.

Wrapping this in a anonymous unions has a few advantages. It allows us
to avoid needlessly increasing struct iattr. Since the types for
ia_{g,u}id and ia_mnt{g,u}id are structures with overlapping/identical
members they are covered by 6.5.2.3/6 of the C standard and it is safe
to initialize and access them.

Filesystems that raise FS_ALLOW_IDMAP and thus support idmapped mounts
will have to use ia_mnt{g,u}id and the associated helpers. And will be
ported at the end of this series. They will immediately benefit from the
type safe new helpers.

Filesystems that do not support FS_ALLOW_IDMAP can continue to use
ia_{g,u}id for now. The aim is to convert every filesystem to always use
ia_mnt{g,u}id and thus ultimately remove the ia_{g,u}id members.

Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
CC: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
---
 include/linux/fs.h            | 18 ++++++++++++++++--
 include/linux/mnt_idmapping.h |  5 +++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8724a31b95e5..0da6c0481dbd 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -221,8 +221,22 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 struct iattr {
 	unsigned int	ia_valid;
 	umode_t		ia_mode;
-	kuid_t		ia_uid;
-	kgid_t		ia_gid;
+	/*
+	 * The two anonymous unions wrap structures with the same member.
+	 *
+	 * Filesystems raising FS_ALLOW_IDMAP need to use ia_mnt{g,u}id which
+	 * are a dedicated type requiring the filesystem to use the dedicated
+	 * helpers. Other filesystem can continue to use ia_{g,u}id until they
+	 * have been ported.
+	 */
+	union {
+		kuid_t		ia_uid;
+		kmntuid_t	ia_mntuid;
+	};
+	union {
+		kgid_t		ia_gid;
+		kmntgid_t	ia_mntgid;
+	};
 	loff_t		ia_size;
 	struct timespec64 ia_atime;
 	struct timespec64 ia_mtime;
diff --git a/include/linux/mnt_idmapping.h b/include/linux/mnt_idmapping.h
index 8dbaef494e02..8f555c746cf4 100644
--- a/include/linux/mnt_idmapping.h
+++ b/include/linux/mnt_idmapping.h
@@ -21,6 +21,11 @@ typedef struct {
 	gid_t val;
 } kmntgid_t;
 
+static_assert(sizeof(kmntuid_t) == sizeof(kuid_t));
+static_assert(sizeof(kmntgid_t) == sizeof(kgid_t));
+static_assert(offsetof(kmntuid_t, val) == offsetof(kuid_t, val));
+static_assert(offsetof(kmntgid_t, val) == offsetof(kgid_t, val));
+
 #ifdef CONFIG_MULTIUSER
 static inline uid_t __kmntuid_val(kmntuid_t uid)
 {
-- 
2.34.1