Re: [PATCH v2] tty: Add comments for tty-ldisc module loading logic

[quic_zijuhu]

On 12/16/2023 1:51 AM, Elliot Berman wrote:
> 
> 
> On 12/15/2023 9:26 AM, Jiri Slaby wrote:
>> On 15. 12. 23, 15:19, Vijaya Krishna Nivarthi wrote:
>>> Hi,
>>>
>>>
>>> On 12/15/2023 7:11 PM, Zijun Hu wrote:
>>>> Current tty-ldisc module loading logic within tty_ldisc_get()
>>>> is prone to mislead beginner that the module is able to be loaded
>>>> by a user without capability CAP_SYS_MODULE, add comments to make
>>>> the logic easy to undertand.
>>>>
>>>> Signed-off-by: Zijun Hu <quic_zijuhu@xxxxxxxxxxx>
>>>> ---
>>>> Changes in v2:
>>>> - Remove condition checking changes
>>>>
>>>>   drivers/tty/tty_ldisc.c | 4 ++++
>>>>   1 file changed, 4 insertions(+)
>>>>
>>>> diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
>>>> index 3f68e213df1f..34526ffaccbc 100644
>>>> --- a/drivers/tty/tty_ldisc.c
>>>> +++ b/drivers/tty/tty_ldisc.c
>>>> @@ -150,6 +150,10 @@ static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc)
>>>>        */
>>>>       ldops = get_ldops(disc);
>>>>       if (IS_ERR(ldops)) {
>>>> +        /*
>>>> +         * Always request tty-ldisc module regardless of user's
>>>> +         * CAP_SYS_MODULE if autoload is enabled.
>>>> +         */
> 
> The added comment confused me more :-)
> 
> "Request tty-ldisc if process has CAP_SYS_MODULE or autoload is enabled"
> 
got it, please ignore my comments and changes.
>>>
>>> Without much knowledge of this file...
>>>
>>>
>>> What the if condition below accomplishes is evident,
>>
>> After a bit of thinking, sure.
>>
>>> it probably doesn't require a comment.
>>
>> I would not add a comment there at all. I would rewrite the code so it is obvious to everyone. Like:
>>
>> static inline bool tty_ldisc_can_autoload(void)
>> {
>>   return capable(CAP_SYS_MODULE) || tty_ldisc_autoload;
>> }
>>
>> And then:
>> if (!tty_ldisc_can_autoload())
>>   return ERR_PTR(-EPERM);
>>
if you want to remain current logic, suggest think about below question:

for a user without module loading permission CAP_SYS_MODULE, kernel should not allow module to be loaded for the user,
even if kernel calls request_module() to load a module for the user, the loading operation will be refused by permission
checking triggered by request_module(). right?

i have no concern about current design if your answer is NO.

it maybe be worth double checking current logic introduced by below commit if your answer is YES
7c0cca7c847e "tty: ldisc: add sysctl to prevent autoloading of ldiscs"
i also don't understand why above commit will introduce extra capable(CAP_SYS_MODULE) checking.

>>> A more useful comment would be why it does so?
>>
>> From an insider, the reason is obvious. But maybe not so much for newcomers. Well, one could document the new inline above. Like:
>> ""
>> We allow loads for capable users or when autoloading is explicitly enabled.
>> ""
>> or alike...
> 
> I agree with Vijaya that it seems evident after a few moments of analysis, but we're
> also maybe used to reading kernel code more. I don't think we should be opposed
> to changes that make code easier to grok, even if they're trivial.
> 
> If we want to make it clearer, I like Jiri's suggestion. One other thing I'd add
> is to give a reference to read config LDISC_AUTOLOAD's help text.
> 
> Zijun,
> 
> Please send future revisions of the patch to our internal pre-submit review list
> before sending to kernel.org. Qualcommers can visit go/upstream.
> 
got it, will follow go/upstream for further patch upstream.
> - Elliot