linux-kernel - Re: [PATCH v3 3/4] kmod: Return directly if module name is empty in request

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Date:   Mon, 20 Apr 2020 18:19:31 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc:     Shuah Khan <shuah@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jessica Yu <jeyu@...nel.org>, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Markus Elfring <Markus.Elfring@....de>,
        Xuefeng Li <lixuefeng@...ngson.cn>
Subject: Re: [PATCH v3 3/4] kmod: Return directly if module name is empty in
 request_module()

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@...ngson.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