On 04/21/2020 02:19 AM, Luis Chamberlain wrote:
On Mon, Apr 20, 2020 at 08:33:54PM +0800, Tiezhu Yang wrote:
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@loongson.cn
Thanks for looking into this. I still cannot find where userspace it returns 256. Can you? If I run modprobe without an argument I see 1 returned.
At least kmod [0] has a series of cmd helper structs, the one for modprobe seems to be kmod_cmd_compat_modprobe, and I can see -1 returned which can be converted to 255. It can also return EXIT_FAILURE or EXIT_SUCCESS and /usr/include/stdlib.h defines these as 1 and 0 respectively.
https://git.kernel.org/pub/scm/utils/kernel/kmod/kmod.git/
Luis
Here is my understanding:
When build and execute the following application, we can see the exit status is 256.
$ ./system modprobe: FATAL: Module not found in directory /lib/modules/4.18.0-147.5.1.el8_1.x86_64 exit status = 256
$ ./execl modprobe: FATAL: Module not found in directory /lib/modules/4.18.0-147.5.1.el8_1.x86_64 exit status = 256
$ cat system.c #include <stdio.h> #include <stdlib.h>
int main() { int status = 0;
status = system("modprobe ''"); 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 aaa", (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 correspond with kernel_wait4() [1] in the kernel space.
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kern...