fs: add vfs_parse_fs_param_source() helper

Add a simple helper that filesystems can use in their parameter parser
to parse the "source" parameter. A few places open-coded this function
and that already caused a bug in the cgroup v1 parser that we fixed.
Let's make it harder to get this wrong by introducing a helper which
performs all necessary checks.

Link: https://syzkaller.appspot.com/bug?id=6312526aba5beae046fdae8f00399f87aab48b12
Cc: Christoph Hellwig <hch@lst.de>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/fs_context.c

@@ -79,6 +79,35 @@ static int vfs_parse_sb_flag(struct fs_context *fc, const char *key)
 	return -ENOPARAM;
 }
 
+/**
+ * vfs_parse_fs_param_source - Handle setting "source" via parameter
+ * @fc: The filesystem context to modify
+ * @param: The parameter
+ *
+ * This is a simple helper for filesystems to verify that the "source" they
+ * accept is sane.
+ *
+ * Returns 0 on success, -ENOPARAM if this is not  "source" parameter, and
+ * -EINVAL otherwise. In the event of failure, supplementary error information
+ *  is logged.
+ */
+int vfs_parse_fs_param_source(struct fs_context *fc, struct fs_parameter *param)
+{
+	if (strcmp(param->key, "source") != 0)
+		return -ENOPARAM;
+
+	if (param->type != fs_value_is_string)
+		return invalf(fc, "Non-string source");
+
+	if (fc->source)
+		return invalf(fc, "Multiple sources");
+
+	fc->source = param->string;
+	param->string = NULL;
+	return 0;
+}
+EXPORT_SYMBOL(vfs_parse_fs_param_source);
+
 /**
  * vfs_parse_fs_param - Add a single parameter to a superblock config
  * @fc: The filesystem context to modify
@@ -122,15 +151,9 @@ int vfs_parse_fs_param(struct fs_context *fc, struct fs_parameter *param)
 	/* If the filesystem doesn't take any arguments, give it the
 	 * default handling of source.
 	 */
-	if (strcmp(param->key, "source") == 0) {
-		if (param->type != fs_value_is_string)
-			return invalf(fc, "VFS: Non-string source");
-		if (fc->source)
-			return invalf(fc, "VFS: Multiple sources");
-		fc->source = param->string;
-		param->string = NULL;
-		return 0;
-	}
+	ret = vfs_parse_fs_param_source(fc, param);
+	if (ret != -ENOPARAM)
+		return ret;
 
 	return invalf(fc, "%s: Unknown parameter '%s'",
 		      fc->fs_type->name, param->key);
@@ -504,16 +527,11 @@ static int legacy_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	struct legacy_fs_context *ctx = fc->fs_private;
 	unsigned int size = ctx->data_size;
 	size_t len = 0;
+	int ret;
 
-	if (strcmp(param->key, "source") == 0) {
-		if (param->type != fs_value_is_string)
-			return invalf(fc, "VFS: Legacy: Non-string source");
-		if (fc->source)
-			return invalf(fc, "VFS: Legacy: Multiple sources");
-		fc->source = param->string;
-		param->string = NULL;
-		return 0;
-	}
+	ret = vfs_parse_fs_param_source(fc, param);
+	if (ret != -ENOPARAM)
+		return ret;
 
 	if (ctx->param_type == LEGACY_FS_MONOLITHIC_PARAMS)
 		return invalf(fc, "VFS: Legacy: Can't mix monolithic and individual options");

include/linux/fs_context.h

@@ -139,6 +139,8 @@ extern int vfs_parse_fs_string(struct fs_context *fc, const char *key,
 extern int generic_parse_monolithic(struct fs_context *fc, void *data);
 extern int vfs_get_tree(struct fs_context *fc);
 extern void put_fs_context(struct fs_context *fc);
+extern int vfs_parse_fs_param_source(struct fs_context *fc,
+				     struct fs_parameter *param);
 
 /*
  * sget() wrappers to be called from the ->get_tree() op.

kernel/cgroup/cgroup-v1.c

@@ -911,15 +911,11 @@ int cgroup1_parse_param(struct fs_context *fc, struct fs_parameter *param)
 
 	opt = fs_parse(fc, cgroup1_fs_parameters, param, &result);
 	if (opt == -ENOPARAM) {
-		if (strcmp(param->key, "source") == 0) {
-			if (param->type != fs_value_is_string)
-				return invalf(fc, "Non-string source");
-			if (fc->source)
-				return invalf(fc, "Multiple sources not supported");
-			fc->source = param->string;
-			param->string = NULL;
-			return 0;
-		}
+		int ret;
+
+		ret = vfs_parse_fs_param_source(fc, param);
+		if (ret != -ENOPARAM)
+			return ret;
 		for_each_subsys(ss, i) {
 			if (strcmp(param->key, ss->legacy_name))
 				continue;