[PATCH] dyndbg: fix use before null check

[Jim Cromie <jim.cromie@gmail.com>]

commit a2d375eda771 ("dyndbg: refine export, rename to dynamic_debug_exec_queries()")

Above commit copies a string before checking for null pointer, fix
this, and add a pr_err.  Also trim comment, and add return val info.

Fixes: a2d375eda771
Cc: stable@vger.kernel.org
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
---
 lib/dynamic_debug.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index bd7b3aaa93c3..711a9def8c83 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -553,17 +553,23 @@ static int ddebug_exec_queries(char *query, const char *modname)
  * @query: query-string described in admin-guide/dynamic-debug-howto
  * @modname: string containing module name, usually &module.mod_name
  *
- * This uses the >/proc/dynamic_debug/control reader, allowing module
- * authors to modify their dynamic-debug callsites. The modname is
- * canonically struct module.mod_name, but can also be null or a
- * module-wildcard, for example: "drm*".
+ * This uses the >control reader, allowing module authors to modify
+ * their dynamic-debug callsites. The modname is canonically struct
+ * module.mod_name, but can also be null or a module-wildcard, for
+ * example: "drm*".
+ * Returns <0 on error, >=0 for callsites changed
  */
 int dynamic_debug_exec_queries(const char *query, const char *modname)
 {
 	int rc;
-	char *qry = kstrndup(query, PAGE_SIZE, GFP_KERNEL);
+	char *qry; /* writable copy of query */
 
-	if (!query)
+	if (!query) {
+		pr_err("non-null query/command string expected\n");
+		return -EINVAL;
+	}
+	qry = kstrndup(query, PAGE_SIZE, GFP_KERNEL);
+	if (!qry)
 		return -ENOMEM;
 
 	rc = ddebug_exec_queries(qry, modname);
-- 
2.28.0