fscrypt: introduce fscrypt_prepare_readdir()

The last remaining use of fscrypt_get_encryption_info() from filesystems
is for readdir (->iterate_shared()).  Every other call is now in
fs/crypto/ as part of some other higher-level operation.

We need to add a new argument to fscrypt_get_encryption_info() to
indicate whether the encryption policy is allowed to be unrecognized or
not.  Doing this is easier if we can work with high-level operations
rather than direct filesystem use of fscrypt_get_encryption_info().

So add a function fscrypt_prepare_readdir() which wraps the call to
fscrypt_get_encryption_info() for the readdir use case.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Link: https://lore.kernel.org/r/20201203022041.230976-6-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/hooks.c

@@ -114,6 +114,12 @@ int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry,
 }
 EXPORT_SYMBOL_GPL(__fscrypt_prepare_lookup);
 
+int __fscrypt_prepare_readdir(struct inode *dir)
+{
+	return fscrypt_get_encryption_info(dir);
+}
+EXPORT_SYMBOL_GPL(__fscrypt_prepare_readdir);
+
 /**
  * fscrypt_prepare_setflags() - prepare to change flags with FS_IOC_SETFLAGS
  * @inode: the inode on which flags are being changed

fs/ext4/dir.c

@@ -118,11 +118,9 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
 	struct buffer_head *bh = NULL;
 	struct fscrypt_str fstr = FSTR_INIT(NULL, 0);
 
-	if (IS_ENCRYPTED(inode)) {
-		err = fscrypt_get_encryption_info(inode);
-		if (err)
-			return err;
-	}
+	err = fscrypt_prepare_readdir(inode);
+	if (err)
+		return err;
 
 	if (is_dx_dir(inode)) {
 		err = ext4_dx_readdir(file, ctx);

fs/ext4/namei.c

@@ -1004,7 +1004,7 @@ static int htree_dirblock_to_tree(struct file *dir_file,
 					   EXT4_DIR_REC_LEN(0));
 	/* Check if the directory is encrypted */
 	if (IS_ENCRYPTED(dir)) {
-		err = fscrypt_get_encryption_info(dir);
+		err = fscrypt_prepare_readdir(dir);
 		if (err < 0) {
 			brelse(bh);
 			return err;

fs/f2fs/dir.c

@@ -1022,7 +1022,7 @@ static int f2fs_readdir(struct file *file, struct dir_context *ctx)
 	int err = 0;
 
 	if (IS_ENCRYPTED(inode)) {
-		err = fscrypt_get_encryption_info(inode);
+		err = fscrypt_prepare_readdir(inode);
 		if (err)
 			goto out;
 

fs/ubifs/dir.c

@@ -514,7 +514,7 @@ static int ubifs_readdir(struct file *file, struct dir_context *ctx)
 		return 0;
 
 	if (encrypted) {
-		err = fscrypt_get_encryption_info(dir);
+		err = fscrypt_prepare_readdir(dir);
 		if (err)
 			return err;
 

include/linux/fscrypt.h

@@ -242,6 +242,7 @@ int __fscrypt_prepare_rename(struct inode *old_dir, struct dentry *old_dentry,
 			     unsigned int flags);
 int __fscrypt_prepare_lookup(struct inode *dir, struct dentry *dentry,
 			     struct fscrypt_name *fname);
+int __fscrypt_prepare_readdir(struct inode *dir);
 int fscrypt_prepare_setflags(struct inode *inode,
 			     unsigned int oldflags, unsigned int flags);
 int fscrypt_prepare_symlink(struct inode *dir, const char *target,
@@ -537,6 +538,11 @@ static inline int __fscrypt_prepare_lookup(struct inode *dir,
 	return -EOPNOTSUPP;
 }
 
+static inline int __fscrypt_prepare_readdir(struct inode *dir)
+{
+	return -EOPNOTSUPP;
+}
+
 static inline int fscrypt_prepare_setflags(struct inode *inode,
 					   unsigned int oldflags,
 					   unsigned int flags)
@@ -795,6 +801,24 @@ static inline int fscrypt_prepare_lookup(struct inode *dir,
 	return 0;
 }
 
+/**
+ * fscrypt_prepare_readdir() - prepare to read a possibly-encrypted directory
+ * @dir: the directory inode
+ *
+ * If the directory is encrypted and it doesn't already have its encryption key
+ * set up, try to set it up so that the filenames will be listed in plaintext
+ * form rather than in no-key form.
+ *
+ * Return: 0 on success; -errno on error.  Note that the encryption key being
+ *	   unavailable is not considered an error.
+ */
+static inline int fscrypt_prepare_readdir(struct inode *dir)
+{
+	if (IS_ENCRYPTED(dir))
+		return __fscrypt_prepare_readdir(dir);
+	return 0;
+}
+
 /**
  * fscrypt_prepare_setattr() - prepare to change a possibly-encrypted inode's
  *			       attributes