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SubjectRe: [PATCH v3 3/4] kmod: Return directly if module name is empty in request_module()
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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/kernel/exit.c#n1576

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