[PATCH v2 06/39] fs: add id translation helpers

[Christian Brauner]

Add simple helpers to make it easy to map kuids into and from idmapped
mounts. We provide simple wrappers that filesystems can use to
e.g. initialize inodes similar to i_{uid,gid}_read() and
i_{uid,gid}_write(). Accessing an inode through an idmapped mount will
require the inode to be mapped according to the mount's user namespace.
If the fsids are used to compare against inodes or to initialize inodes
they are required to be shifted from the mount's user namespace. Passing
the initial user namespace to these helpers makes them a nop and so any
non-idmapped paths will not be impacted.

Cc: Christoph Hellwig <hch at lst.de>
Cc: David Howells <dhowells at redhat.com>
Cc: Al Viro <viro at zeniv.linux.org.uk>
Cc: linux-fsdevel at vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
---
/* v2 */
- Christoph Hellwig <hch at lst.de>:
  - Get rid of the ifdefs and the config option that hid idmapped mounts.
---
 include/linux/fs.h | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 21cc971fd960..9e487cbf0f5c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -39,6 +39,7 @@
 #include <linux/fs_types.h>
 #include <linux/build_bug.h>
 #include <linux/stddef.h>
+#include <linux/cred.h>
 
 #include <asm/byteorder.h>
 #include <uapi/linux/fs.h>
@@ -1574,6 +1575,48 @@ static inline void i_gid_write(struct inode *inode, gid_t gid)
 	inode->i_gid = make_kgid(inode->i_sb->s_user_ns, gid);
 }
 
+static inline kuid_t kuid_into_mnt(struct user_namespace *to, kuid_t kuid)
+{
+	return make_kuid(to, __kuid_val(kuid));
+}
+
+static inline kgid_t kgid_into_mnt(struct user_namespace *to, kgid_t kgid)
+{
+	return make_kgid(to, __kgid_val(kgid));
+}
+
+static inline kuid_t i_uid_into_mnt(struct user_namespace *to,
+				    const struct inode *inode)
+{
+	return kuid_into_mnt(to, inode->i_uid);
+}
+
+static inline kgid_t i_gid_into_mnt(struct user_namespace *to,
+				    const struct inode *inode)
+{
+	return kgid_into_mnt(to, inode->i_gid);
+}
+
+static inline kuid_t kuid_from_mnt(struct user_namespace *to, kuid_t kuid)
+{
+	return KUIDT_INIT(from_kuid(to, kuid));
+}
+
+static inline kgid_t kgid_from_mnt(struct user_namespace *to, kgid_t kgid)
+{
+	return KGIDT_INIT(from_kgid(to, kgid));
+}
+
+static inline kuid_t fsuid_into_mnt(struct user_namespace *to)
+{
+	return kuid_from_mnt(to, current_fsuid());
+}
+
+static inline kgid_t fsgid_into_mnt(struct user_namespace *to)
+{
+	return kgid_from_mnt(to, current_fsgid());
+}
+
 extern struct timespec64 current_time(struct inode *inode);
 
 /*
-- 
2.29.2