[PATCH 06/34] 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.

Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
---
 include/linux/fs.h | 75 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 8314cd351673..8a891b80d0b4 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 +1575,80 @@ 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)
+{
+#ifdef CONFIG_IDMAP_MOUNTS
+	return make_kuid(to, __kuid_val(kuid));
+#else
+	return kuid;
+#endif
+}
+
+static inline kgid_t kgid_into_mnt(struct user_namespace *to, kgid_t kgid)
+{
+#ifdef CONFIG_IDMAP_MOUNTS
+	return make_kgid(to, __kgid_val(kgid));
+#else
+	return kgid;
+#endif
+}
+
+static inline kuid_t i_uid_into_mnt(struct user_namespace *to,
+				    const struct inode *inode)
+{
+#ifdef CONFIG_IDMAP_MOUNTS
+	return kuid_into_mnt(to, inode->i_uid);
+#else
+	return inode->i_uid;
+#endif
+}
+
+static inline kgid_t i_gid_into_mnt(struct user_namespace *to,
+				    const struct inode *inode)
+{
+#ifdef CONFIG_IDMAP_MOUNTS
+	return kgid_into_mnt(to, inode->i_gid);
+#else
+	return inode->i_gid;
+#endif
+}
+
+static inline kuid_t kuid_from_mnt(struct user_namespace *to, kuid_t kuid)
+{
+#ifdef CONFIG_IDMAP_MOUNTS
+	return KUIDT_INIT(from_kuid(to, kuid));
+#else
+	return kuid;
+#endif
+}
+
+static inline kgid_t kgid_from_mnt(struct user_namespace *to, kgid_t kgid)
+{
+#ifdef CONFIG_IDMAP_MOUNTS
+	return KGIDT_INIT(from_kgid(to, kgid));
+#else
+	return kgid;
+#endif
+}
+
+static inline kuid_t fsuid_into_mnt(struct user_namespace *to)
+{
+#ifdef CONFIG_IDMAP_MOUNTS
+	return kuid_from_mnt(to, current_fsuid());
+#else
+	return current_fsuid();
+#endif
+}
+
+static inline kgid_t fsgid_into_mnt(struct user_namespace *to)
+{
+#ifdef CONFIG_IDMAP_MOUNTS
+	return kgid_from_mnt(to, current_fsgid());
+#else
+	return current_fsgid();
+#endif
+}
+
 extern struct timespec64 current_time(struct inode *inode);
 
 /*
-- 
2.29.0