[PATCH v4 3/4] kmod: Return directly if module name is empty in request_module()

[Tiezhu Yang]

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

(1) In the kernel space

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);

The exit status of child process is wrote to the address of variable
"ret" after call kernel_wait4(), its value is 256 if module name is
empty, then sub_info->retval is assigned to "ret" which is 256, the
function call_usermodehelper_exec() returns 256, call_modprobe() and
__request_module() also return 256.

(2)In the user space

when build and execute the following application, we can see the exit
status is 256.

$ ./system
exit status = 256

$ ./execl
exit status = 256

$ cat system.c
#include <stdio.h>
#include <stdlib.h>

int main()
{
	int status = 0;

	status = system("modprobe '' > /dev/null 2>&1");
	printf("exit status = %d\n", status);

	return status;
}

$ cat execl.c
#include <sys/wait.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>

int main()
{
	pid_t pid, w;
	int status;

	pid = fork();
	if (pid == -1) {
		perror("fork");
		exit(EXIT_FAILURE);
	}

	if (pid == 0) {
		execl("/bin/sh", "sh", "-c",
		      "modprobe '' > /dev/null 2>&1", (char *) 0);
	} else {
		w = waitpid(pid, &status, 0);
		if (w == -1) {
			perror("waitpid");
			exit(EXIT_FAILURE);
		}

		printf("exit status = %d\n", status);
		exit(EXIT_SUCCESS);
	}

	return 0;
}

The exit status of child process is wrote to the address of variable
"status" after call waitpid() in the user space that corresponds with
kernel_wait4() in the kernel space.

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

v4:
  - update the commit message with application

v3:
  - no changes

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