fs, fscrypt: add an S_ENCRYPTED inode flag

Introduce a flag S_ENCRYPTED which can be set in ->i_flags to indicate
that the inode is encrypted using the fscrypt (fs/crypto/) mechanism.

Checking this flag will give the same information that
inode->i_sb->s_cop->is_encrypted(inode) currently does, but will be more
efficient.  This will be useful for adding higher-level helper functions
for filesystems to use.  For example we'll be able to replace this:

	if (ext4_encrypted_inode(inode)) {
		ret = fscrypt_get_encryption_info(inode);
		if (ret)
			return ret;
		if (!fscrypt_has_encryption_key(inode))
			return -ENOKEY;
	}

with this:

	ret = fscrypt_require_key(inode);
	if (ret)
		return ret;

... since we'll be able to retain the fast path for unencrypted files as
a single flag check, using an inline function.  This wasn't possible
before because we'd have had to frequently call through the
->i_sb->s_cop->is_encrypted function pointer, even when the encryption
support was disabled or not being used.

Note: we don't define S_ENCRYPTED to 0 if CONFIG_FS_ENCRYPTION is
disabled because we want to continue to return an error if an encrypted
file is accessed without encryption support, rather than pretending that
it is unencrypted.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -4103,8 +4103,11 @@ void ext4_set_inode_flags(struct inode *inode)
 		new_fl |= S_DIRSYNC;
 	if (test_opt(inode->i_sb, DAX))
 		new_fl |= S_DAX;
+	if (flags & EXT4_ENCRYPT_FL)
+		new_fl |= S_ENCRYPTED;
 	inode_set_flags(inode, new_fl,
-			S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC|S_DAX);
+			S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC|S_DAX|
+			S_ENCRYPTED);
 }
 
 /* Propagate flags from i_flags to EXT4_I(inode)->i_flags */

fs/f2fs/f2fs.h

@@ -3147,6 +3147,7 @@ static inline void f2fs_set_encrypted_inode(struct inode *inode)
 {
 #ifdef CONFIG_F2FS_FS_ENCRYPTION
 	file_set_encrypt(inode);
+	inode->i_flags |= S_ENCRYPTED;
 #endif
 }
 

fs/f2fs/inode.c

@@ -43,8 +43,11 @@ void f2fs_set_inode_flags(struct inode *inode)
 		new_fl |= S_NOATIME;
 	if (flags & FS_DIRSYNC_FL)
 		new_fl |= S_DIRSYNC;
+	if (f2fs_encrypted_inode(inode))
+		new_fl |= S_ENCRYPTED;
 	inode_set_flags(inode, new_fl,
-			S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC);
+			S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC|
+			S_ENCRYPTED);
 }
 
 static void __get_inode_rdev(struct inode *inode, struct f2fs_inode *ri)

include/linux/fs.h

@@ -1778,6 +1778,7 @@ struct super_operations {
 #else
 #define S_DAX		0	/* Make all the DAX code disappear */
 #endif
+#define S_ENCRYPTED	16384	/* Encrypted file (using fs/crypto/) */
 
 /*
  * Note that nosuid etc flags are inode-specific: setting some file-system
@@ -1816,6 +1817,7 @@ struct super_operations {
 #define IS_AUTOMOUNT(inode)	((inode)->i_flags & S_AUTOMOUNT)
 #define IS_NOSEC(inode)		((inode)->i_flags & S_NOSEC)
 #define IS_DAX(inode)		((inode)->i_flags & S_DAX)
+#define IS_ENCRYPTED(inode)	((inode)->i_flags & S_ENCRYPTED)
 
 #define IS_WHITEOUT(inode)	(S_ISCHR(inode->i_mode) && \
 				 (inode)->i_rdev == WHITEOUT_DEV)