fscrypto: make filename crypto functions return 0 on success

Several filename crypto functions: fname_decrypt(),
fscrypt_fname_disk_to_usr(), and fscrypt_fname_usr_to_disk(), returned
the output length on success or -errno on failure.  However, the output
length was redundant with the value written to 'oname->len'.  It is also
potentially error-prone to make callers have to check for '< 0' instead
of '!= 0'.

Therefore, make these functions return 0 instead of a length, and make
the callers who cared about the return value being a length use
'oname->len' instead.  For consistency also make other callers check for
a nonzero result rather than a negative result.

This change also fixes the inconsistency of fname_encrypt() actually
already returning 0 on success, not a length like the other filename
crypto functions and as documented in its function comment.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Acked-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/ext4/dir.c

@@ -260,11 +260,12 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
 					/* Directory is encrypted */
 					err = fscrypt_fname_disk_to_usr(inode,
 						0, 0, &de_name, &fstr);
+					de_name = fstr;
 					fstr.len = save_len;
-					if (err < 0)
+					if (err)
 						goto errout;
 					if (!dir_emit(ctx,
-					    fstr.name, err,
+					    de_name.name, de_name.len,
 					    le32_to_cpu(de->inode),
 					    get_dtype(sb, de->file_type)))
 						goto done;

fs/ext4/namei.c

@@ -639,7 +639,7 @@ static struct stats dx_show_leaf(struct inode *dir,
 					res = fscrypt_fname_alloc_buffer(
 						dir, len,
 						&fname_crypto_str);
-					if (res < 0)
+					if (res)
 						printk(KERN_WARNING "Error "
 							"allocating crypto "
 							"buffer--skipping "
@@ -647,7 +647,7 @@ static struct stats dx_show_leaf(struct inode *dir,
 					res = fscrypt_fname_disk_to_usr(dir,
 						0, 0, &de_name,
 						&fname_crypto_str);
-					if (res < 0) {
+					if (res) {
 						printk(KERN_WARNING "Error "
 							"converting filename "
 							"from disk to usr"
@@ -1011,7 +1011,7 @@ static int htree_dirblock_to_tree(struct file *dir_file,
 			err = fscrypt_fname_disk_to_usr(dir, hinfo->hash,
 					hinfo->minor_hash, &de_name,
 					&fname_crypto_str);
-			if (err < 0) {
+			if (err) {
 				count = err;
 				goto errout;
 			}
@@ -3144,7 +3144,7 @@ static int ext4_symlink(struct inode *dir,
 		istr.name = (const unsigned char *) symname;
 		istr.len = len;
 		err = fscrypt_fname_usr_to_disk(inode, &istr, &ostr);
-		if (err < 0)
+		if (err)
 			goto err_drop_inode;
 		sd->len = cpu_to_le16(ostr.len);
 		disk_link.name = (char *) sd;

fs/ext4/symlink.c

@@ -68,12 +68,11 @@ static const char *ext4_encrypted_get_link(struct dentry *dentry,
 	paddr = pstr.name;
 
 	res = fscrypt_fname_disk_to_usr(inode, 0, 0, &cstr, &pstr);
-	if (res < 0)
+	if (res)
 		goto errout;
 
 	/* Null-terminate the name */
-	if (res <= pstr.len)
-		paddr[res] = '\0';
+	paddr[pstr.len] = '\0';
 	if (cpage)
 		put_page(cpage);
 	set_delayed_call(done, kfree_link, paddr);