blobmsg: fix attrs iteration in the blobmsg_check_array_len()

Starting with 75e300aeec25 ("blobmsg: fix wrong payload len passed from
blobmsg_check_array") blobmsg_check_array_len() gets *blob* length
passed as argument. It cannot be used with __blobmsg_for_each_attr()
which expects *data* length.

Use blobmsg_for_each_attr() which calculates *data* length on its own.

The same bug was already reported in the past and there was fix attempt
in the commit cd75136b1342 ("blobmsg: fix wrong payload len passed from
blobmsg_check_array"). That change made blobmsg_check_attr_len() calls
fail however.

This is hopefully the correct & complete fix:
1. blobmsg_check_array_len() gets *blob* length
2. It calls blobmsg_check_attr_len() which requires *blob* length
3. It uses blobmsg_for_each_attr() which gets *data* length

This fixes iterating over random memory treated as attrs. That was
resulting in check failing randomly for totally correct blobs. It's
critical e.g. for procd project with its instance_fill_array() failing
and procd not starting services.

Fixes: 75e300aeec25 ("blobmsg: fix wrong payload len passed from blobmsg_check_array")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

blobmsg.c

@@ -117,16 +117,18 @@ int blobmsg_check_array(const struct blob_attr *attr, int type)
 	return blobmsg_check_array_len(attr, type, blob_len(attr));
 }
 
-int blobmsg_check_array_len(const struct blob_attr *attr, int type, size_t len)
+int blobmsg_check_array_len(const struct blob_attr *attr, int type,
+			    size_t blob_len)
 {
 	struct blob_attr *cur;
+	size_t rem;
 	bool name;
 	int size = 0;
 
 	if (type > BLOBMSG_TYPE_LAST)
 		return -1;
 
-	if (!blobmsg_check_attr_len(attr, false, len))
+	if (!blobmsg_check_attr_len(attr, false, blob_len))
 		return -1;
 
 	switch (blobmsg_type(attr)) {
@@ -140,11 +142,11 @@ int blobmsg_check_array_len(const struct blob_attr *attr, int type, size_t len)
 		return -1;
 	}
 
-	__blobmsg_for_each_attr(cur, attr, len) {
+	blobmsg_for_each_attr(cur, attr, rem) {
 		if (type != BLOBMSG_TYPE_UNSPEC && blobmsg_type(cur) != type)
 			return -1;
 
-		if (!blobmsg_check_attr_len(cur, name, len))
+		if (!blobmsg_check_attr_len(cur, name, rem))
 			return -1;
 
 		size++;