fscrypt: only set dentry_operations on ciphertext dentries

Plaintext dentries are always valid, so only set fscrypt_d_ops on
ciphertext dentries.

Besides marginally improved performance, this allows overlayfs to use an
fscrypt-encrypted upperdir, provided that all the following are true:

    (1) The fscrypt encryption key is placed in the keyring before
	mounting overlayfs, and remains while the overlayfs is mounted.

    (2) The overlayfs workdir uses the same encryption policy.

    (3) No dentries for the ciphertext names of subdirectories have been
	created in the upperdir or workdir yet.  (Since otherwise
	d_splice_alias() will reuse the old dentry with ->d_op set.)

One potential use case is using an ephemeral encryption key to encrypt
all files created or changed by a container, so that they can be
securely erased ("crypto-shredded") after the container stops.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
index 2e7498a821a487c83177d0290645521ff66d7126..9d8910e86ee5d57c4ee9f678eb0203c7e1e0dd53 100644 (file)
--- a/fs/crypto/hooks.c
+++ b/fs/crypto/hooks.c
@@ -115,9 +115,8 @@ int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry)
                spin_lock(&dentry->d_lock);
                dentry->d_flags |= DCACHE_ENCRYPTED_NAME;
                spin_unlock(&dentry->d_lock);
+               d_set_d_op(dentry, &fscrypt_d_ops);
        }
-
-       d_set_d_op(dentry, &fscrypt_d_ops);
        return 0;
 }
 EXPORT_SYMBOL_GPL(__fscrypt_prepare_lookup);