Re: [PATCH v18 24/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

[Dave Hansen]

On 2/3/21 1:54 PM, Yu, Yu-cheng wrote:
> On 1/29/2021 10:56 AM, Yu, Yu-cheng wrote:
>> On 1/29/2021 9:07 AM, Dave Hansen wrote:
>>> On 1/27/21 1:25 PM, Yu-cheng Yu wrote:
>>>> +    if (!IS_ENABLED(CONFIG_X86_CET))
>>>> +        return -EOPNOTSUPP;
>>>
>>> Let's ignore glibc for a moment.  What error code *should* the kernel be
>>> returning here?  errno(3) says:
>>>
>>>         EOPNOTSUPP      Operation not supported on socket (POSIX.1)
>>> ...
>>>         ENOTSUP         Operation not supported (POSIX.1)
>>>
>>
>> Yeah, other places in kernel use ENOTSUPP.  This seems to be out of
>> line.  And since the issue is long-existing, applications already know
>> how to deal with it.  I should have made that argument.  Change it to
>> ENOTSUPP.
> 
> When I make the change, checkpatch says...
> 
> WARNING: ENOTSUPP is not a SUSV4 error code, prefer EOPNOTSUPP
> #128: FILE: arch/x86/kernel/cet_prctl.c:33:
> +        return -ENOTSUPP;
> 
> Do we want to reconsider?

I'm not sure I trust checkpatch over manpages.  I had to google "SUSV4".
 I'm not sure it matters at *all* for a 100% Linux-specific interface.

ENOTSUPP does seem less popular lately:

> $ git diff v5.0.. kernel/ arch/ drivers/ | grep ^+.*return.*E.*NO.*SUP.*\; | grep -o -- -E.*\; | sort | uniq -c | sort -n
> ... noise
>      61 -EOPNOTSUPP);
>     260 -ENOTSUPP;
>    1577 -EOPNOTSUPP;

but far from unused.  That might be due to checkpatch spew more than
anything.