[PATCHv2] vfs: Handle file systems without ->parse_params better

[Laura Abbott <labbott@redhat.com>]

The new mount API relies on file systems to provide a ->parse_params
function to handle parsing of arguments. If a file system doesn't
have a ->parse_param function, it falls back to parsing the source
option and rejecting all other options. This is a change in behavior
for some file systems which would just quietly ignore extra options
and mount successfully. This was noticed by users as squashfs failing
to mount with extra options after the conversion to the new mount
API.

File systems with a ->parse_params function rely on the top level
to parse the "source" param so we can't easily move that around. To
get around this, introduce a default parsing functions for file
systems that take no arguments. This parses only the "source" option
and only logs an error for other arguments. Update the comment
to reflect this expected behavior for "source" parsing as well.

Fixes: 3e1aeb00e6d1 ("vfs: Implement a filesystem superblock
creation/configuration context")
Link: https://lore.kernel.org/lkml/20191130181548.GA28459@gentoo-tp.home/
Reported-by: Jeremi Piotrowski <jeremi.piotrowski@gmail.com>
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1781863
Signed-off-by: Laura Abbott <labbott@redhat.com>
---
v2: Dropped most the boiler plate for parsing and just compared
against "source". Renamed to ignore_unknown_parse_param.
---
 fs/fs_context.c | 36 +++++++++++++++++++++++++++++-------
 1 file changed, 29 insertions(+), 7 deletions(-)

diff --git a/fs/fs_context.c b/fs/fs_context.c
index 138b5b4d621d..086ade29b811 100644
--- a/fs/fs_context.c
+++ b/fs/fs_context.c
@@ -107,6 +107,22 @@ static int vfs_parse_sb_flag(struct fs_context *fc, const char *key)
 	return -ENOPARAM;
 }
 
+/**
+ * ignore_unknowns_parse_param - ->parse_param function for a file system that
+ * takes no arguments
+ * @fc: The filesystem context
+ * @param: The parameter.
+ */
+static int ignore_unknown_parse_param(struct fs_context *fc, struct fs_parameter *param)
+{
+
+	if (strcmp(param->key, "source") == 0)
+		return -ENOPARAM;
+	/* Just log an error for backwards compatibility */
+	errorf(fc, "%s: Unknown parameter '%s'", fc->fs_type->name, param->key);
+	return 0;
+}
+
 /**
  * vfs_parse_fs_param - Add a single parameter to a superblock config
  * @fc: The filesystem context to modify
@@ -126,6 +142,7 @@ static int vfs_parse_sb_flag(struct fs_context *fc, const char *key)
 int vfs_parse_fs_param(struct fs_context *fc, struct fs_parameter *param)
 {
 	int ret;
+	int (*parse_param)(struct fs_context *, struct fs_parameter *);
 
 	if (!param->key)
 		return invalf(fc, "Unnamed parameter\n");
@@ -141,14 +158,19 @@ int vfs_parse_fs_param(struct fs_context *fc, struct fs_parameter *param)
 		 */
 		return ret;
 
-	if (fc->ops->parse_param) {
-		ret = fc->ops->parse_param(fc, param);
-		if (ret != -ENOPARAM)
-			return ret;
-	}
+	parse_param = fc->ops->parse_param;
+	if (!parse_param)
+		parse_param = ignore_unknown_parse_param;
+
+	ret = parse_param(fc, param);
+	if (ret != -ENOPARAM)
+		return ret;
 
-	/* If the filesystem doesn't take any arguments, give it the
-	 * default handling of source.
+	/*
+	 * File systems may have a ->parse_param function but rely on
+	 * the top level to parse the source function. File systems
+	 * may have their own source parsing though so this needs
+	 * to come after the call to parse_param above.
 	 */
 	if (strcmp(param->key, "source") == 0) {
 		if (param->type != fs_value_is_string)
-- 
2.21.0