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@loongson.cn ---
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;