fs: introduce two inode i_{u,g}id initialization helpers

Give filesystem two little helpers that do the right thing when
initializing the i_uid and i_gid fields on idmapped and non-idmapped
mounts. Filesystems shouldn't have to be concerned with too many
details.

Link: https://lore.kernel.org/r/20210320122623.599086-5-christian.brauner@ubuntu.com
Inspired-by: Vivek Goyal <vgoyal@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Author: Christian Brauner

fs/ext4/ialloc.c

@@ -970,7 +970,7 @@ struct inode *__ext4_new_inode(struct user_namespace *mnt_userns,
 		i_gid_write(inode, owner[1]);
 	} else if (test_opt(sb, GRPID)) {
 		inode->i_mode = mode;
-		inode->i_uid = mapped_fsuid(mnt_userns);
+		inode_fsuid_set(inode, mnt_userns);
 		inode->i_gid = dir->i_gid;
 	} else
 		inode_init_owner(mnt_userns, inode, dir, mode);

fs/inode.c

@@ -2148,7 +2148,7 @@ EXPORT_SYMBOL(init_special_inode);
 void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
 		      const struct inode *dir, umode_t mode)
 {
-	inode->i_uid = mapped_fsuid(mnt_userns);
+	inode_fsuid_set(inode, mnt_userns);
 	if (dir && dir->i_mode & S_ISGID) {
 		inode->i_gid = dir->i_gid;
 
@@ -2160,7 +2160,7 @@ void inode_init_owner(struct user_namespace *mnt_userns, struct inode *inode,
 			 !capable_wrt_inode_uidgid(mnt_userns, dir, CAP_FSETID))
 			mode &= ~S_ISGID;
 	} else
-		inode->i_gid = mapped_fsgid(mnt_userns);
+		inode_fsgid_set(inode, mnt_userns);
 	inode->i_mode = mode;
 }
 EXPORT_SYMBOL(inode_init_owner);

fs/xfs/xfs_inode.c

@@ -812,7 +812,7 @@ xfs_init_new_inode(
 
 	if (dir && !(dir->i_mode & S_ISGID) &&
 	    (mp->m_flags & XFS_MOUNT_GRPID)) {
-		inode->i_uid = mapped_fsuid(mnt_userns);
+		inode_fsuid_set(inode, mnt_userns);
 		inode->i_gid = dir->i_gid;
 		inode->i_mode = mode;
 	} else {

include/linux/fs.h

@@ -1692,6 +1692,34 @@ static inline kgid_t mapped_fsgid(struct user_namespace *mnt_userns)
 	return kgid_from_mnt(mnt_userns, current_fsgid());
 }
 
+/**
+ * inode_fsuid_set - initialize inode's i_uid field with callers fsuid
+ * @inode: inode to initialize
+ * @mnt_userns: user namespace of the mount the inode was found from
+ *
+ * Initialize the i_uid field of @inode. If the inode was found/created via
+ * an idmapped mount map the caller's fsuid according to @mnt_users.
+ */
+static inline void inode_fsuid_set(struct inode *inode,
+				   struct user_namespace *mnt_userns)
+{
+	inode->i_uid = mapped_fsuid(mnt_userns);
+}
+
+/**
+ * inode_fsgid_set - initialize inode's i_gid field with callers fsgid
+ * @inode: inode to initialize
+ * @mnt_userns: user namespace of the mount the inode was found from
+ *
+ * Initialize the i_gid field of @inode. If the inode was found/created via
+ * an idmapped mount map the caller's fsgid according to @mnt_users.
+ */
+static inline void inode_fsgid_set(struct inode *inode,
+				   struct user_namespace *mnt_userns)
+{
+	inode->i_gid = mapped_fsgid(mnt_userns);
+}
+
 /**
  * fsuidgid_has_mapping() - check whether caller's fsuid/fsgid is mapped
  * @sb: the superblock we want a mapping in