[RESEND v2 3/4] kmod: Return directly if module name is empty in request_module()

[Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>]

If module name is empty, it is better to return directly at the beginning
of request_module() without doing the needless call_modprobe() operation.

Call trace:

request_module()
      |
      |
__request_module()
      |
      |
call_modprobe()
      |
      |
call_usermodehelper_exec() -- retval = sub_info->retval;
      |
      |
call_usermodehelper_exec_work()
      |
      |
call_usermodehelper_exec_sync() -- sub_info->retval = ret;
      |
      | --> call_usermodehelper_exec_async() --> do_execve()
      |
kernel_wait4(pid, (int __user *)&ret, 0, NULL);

sub_info->retval is 256 after call kernel_wait4(), the function
call_usermodehelper_exec() returns sub_info->retval which is 256,
then call_modprobe() and __request_module() returns 256.

Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
---

v2:
  - update the commit message to explain the detailed reason

 kernel/kmod.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 3cd075c..5851444 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -28,6 +28,8 @@
 
 #include <trace/events/module.h>
 
+#define MODULE_NOT_FOUND 256
+
 /*
  * Assuming:
  *
@@ -144,6 +146,9 @@ int __request_module(bool wait, const char *fmt, ...)
 	if (ret >= MODULE_NAME_LEN)
 		return -ENAMETOOLONG;
 
+	if (strlen(module_name) == 0)
+		return MODULE_NOT_FOUND;
+
 	ret = security_kernel_module_request(module_name);
 	if (ret)
 		return ret;
-- 
2.1.0