[PATCH v4 08/40] 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: 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>
Reviewed-by: Christoph Hellwig <hch at lst.de>
---
/* v2 */
- Christoph Hellwig <hch at lst.de>:
  - Get rid of the ifdefs and the config option that hid idmapped mounts.

/* v3 */
unchanged

/* v4 */
- Serge Hallyn <serge at hallyn.com>:
  - Use "mnt_userns" to refer to a vfsmount's userns everywhere to make
    terminology consistent.
---
 include/linux/fs.h | 47 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8667d0cdc71e..0c820837d240 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 +1575,52 @@ 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 *mnt_userns,
+				   kuid_t kuid)
+{
+	return make_kuid(mnt_userns, __kuid_val(kuid));
+}
+
+static inline kgid_t kgid_into_mnt(struct user_namespace *mnt_userns,
+				   kgid_t kgid)
+{
+	return make_kgid(mnt_userns, __kgid_val(kgid));
+}
+
+static inline kuid_t i_uid_into_mnt(struct user_namespace *mnt_userns,
+				    const struct inode *inode)
+{
+	return kuid_into_mnt(mnt_userns, inode->i_uid);
+}
+
+static inline kgid_t i_gid_into_mnt(struct user_namespace *mnt_userns,
+				    const struct inode *inode)
+{
+	return kgid_into_mnt(mnt_userns, inode->i_gid);
+}
+
+static inline kuid_t kuid_from_mnt(struct user_namespace *mnt_userns,
+				   kuid_t kuid)
+{
+	return KUIDT_INIT(from_kuid(mnt_userns, kuid));
+}
+
+static inline kgid_t kgid_from_mnt(struct user_namespace *mnt_userns,
+				   kgid_t kgid)
+{
+	return KGIDT_INIT(from_kgid(mnt_userns, kgid));
+}
+
+static inline kuid_t fsuid_into_mnt(struct user_namespace *mnt_userns)
+{
+	return kuid_from_mnt(mnt_userns, current_fsuid());
+}
+
+static inline kgid_t fsgid_into_mnt(struct user_namespace *mnt_userns)
+{
+	return kgid_from_mnt(mnt_userns, current_fsgid());
+}
+
 extern struct timespec64 current_time(struct inode *inode);
 
 /*
-- 
2.29.2