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()
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__request_module()
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call_modprobe()
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call_usermodehelper_exec() -- retval = sub_info->retval;
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call_usermodehelper_exec_work()
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call_usermodehelper_exec_sync() -- sub_info->retval = ret;
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| --> call_usermodehelper_exec_async() --> do_execve()
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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>
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/kernel/exit.c#n1576
