fscrypt: new helper function - fscrypt_prepare_lookup()

Introduce a helper function which prepares to look up the given dentry
in the given directory.  If the directory is encrypted, it handles
loading the directory's encryption key, setting the dentry's ->d_op to
fscrypt_d_ops, and setting DCACHE_ENCRYPTED_WITH_KEY if the directory's
encryption key is available.

Note: once all filesystems switch over to this, we'll be able to move
fscrypt_d_ops and fscrypt_set_encrypted_dentry() to fscrypt_private.h.
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/crypto/hooks.c

@@ -92,3 +92,21 @@ int __fscrypt_prepare_rename(struct inode *old_dir, struct dentry *old_dentry,
 	return 0;
 }
 EXPORT_SYMBOL_GPL(__fscrypt_prepare_rename);
+
+int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry)
+{
+	int err = fscrypt_get_encryption_info(dir);
+
+	if (err)
+		return err;
+
+	if (fscrypt_has_encryption_key(dir)) {
+		spin_lock(&dentry->d_lock);
+		dentry->d_flags |= DCACHE_ENCRYPTED_WITH_KEY;
+		spin_unlock(&dentry->d_lock);
+	}
+
+	d_set_d_op(dentry, &fscrypt_d_ops);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(__fscrypt_prepare_lookup);

include/linux/fscrypt.h

@@ -237,4 +237,32 @@ static inline int fscrypt_prepare_rename(struct inode *old_dir,
 	return 0;
 }
 
+/**
+ * fscrypt_prepare_lookup - prepare to lookup a name in a possibly-encrypted directory
+ * @dir: directory being searched
+ * @dentry: filename being looked up
+ * @flags: lookup flags
+ *
+ * Prepare for ->lookup() in a directory which may be encrypted.  Lookups can be
+ * done with or without the directory's encryption key; without the key,
+ * filenames are presented in encrypted form.  Therefore, we'll try to set up
+ * the directory's encryption key, but even without it the lookup can continue.
+ *
+ * To allow invalidating stale dentries if the directory's encryption key is
+ * added later, we also install a custom ->d_revalidate() method and use the
+ * DCACHE_ENCRYPTED_WITH_KEY flag to indicate whether a given dentry is a
+ * plaintext name (flag set) or a ciphertext name (flag cleared).
+ *
+ * Return: 0 on success, -errno if a problem occurred while setting up the
+ * encryption key
+ */
+static inline int fscrypt_prepare_lookup(struct inode *dir,
+					 struct dentry *dentry,
+					 unsigned int flags)
+{
+	if (IS_ENCRYPTED(dir))
+		return __fscrypt_prepare_lookup(dir, dentry);
+	return 0;
+}
+
 #endif	/* _LINUX_FSCRYPT_H */

include/linux/fscrypt_notsupp.h

@@ -201,4 +201,10 @@ static inline int __fscrypt_prepare_rename(struct inode *old_dir,
 	return -EOPNOTSUPP;
 }
 
+static inline int __fscrypt_prepare_lookup(struct inode *dir,
+					   struct dentry *dentry)
+{
+	return -EOPNOTSUPP;
+}
+
 #endif	/* _LINUX_FSCRYPT_NOTSUPP_H */

include/linux/fscrypt_supp.h

@@ -151,5 +151,6 @@ extern int __fscrypt_prepare_rename(struct inode *old_dir,
 				    struct inode *new_dir,
 				    struct dentry *new_dentry,
 				    unsigned int flags);
+extern int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry);
 
 #endif	/* _LINUX_FSCRYPT_SUPP_H */