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

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Date:   Mon, 20 Apr 2020 12:08:46 +0800
From:   Tiezhu Yang <yangtiezhu@...ngson.cn>
To:     Luis Chamberlain <mcgrof@...nel.org>, Jessica Yu <jeyu@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Xuefeng Li <lixuefeng@...ngson.cn>
Subject: Re: [PATCH 3/4] kmod: Return directly if module name is empty in
 request_module()

On 04/18/2020 03:19 PM, Luis Chamberlain wrote:
> On Sat, Apr 18, 2020 at 01:58:45PM +0800, Tiezhu Yang wrote:
>> On 04/18/2020 01:48 PM, Luis Chamberlain wrote:
>>> On Fri, Apr 17, 2020 at 11:45 PM Luis Chamberlain <mcgrof@...nel.org> wrote:
>>>> On Sat, Apr 18, 2020 at 01:19:59PM +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.
>>>>>
>>>>> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
>>>>> ---
>>>>>    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;
>>>> I'd rather we just use something standard like -EINVAL.
>>>> What do we return if its not found? Then use that value.
>>> Also, are we testing for this condition yet? If not can we add one?
>> Yes, kmod_test_0001_driver() in tools/testing/selftests/kmod/kmod.sh tests
>> this case and expects result MODULE_NOT_FOUND which is 256.
> OK I see now I had put:
>
> errno_name_to_val()
> {
>          case "$1" in
> 	# kmod calls modprobe and upon of a module not found
> 	# modprobe returns just 1... However in the
> 	# kernel we *sometimes* see 256...
> 	MODULE_NOT_FOUND)
> 		echo 256;;
>
> I found that through testing, however there was nothing set in stone,
> nothing documented. While you are at it, can you find the places where
> this is returned in the kernel code? We should clear this up and
> se things straight. We cannot change what we gave userspace already
> though.

Call Trace:

__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);

__request_module() returns the exist status of child process, if module name
is empty or non-exist, sub_info->retval is 256 after call kernel_wait4().

Should I add this analysis to the commit message?

Thanks,
Tiezhu Yang

>
>    Luis