linux-kernel - Re: [PATCH v18 24/25] x86/cet/shstk: Add arch

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Message-ID: <a7ef2df2-815f-9994-11b7-026afe45094f@intel.com>
Date:   Wed, 3 Feb 2021 14:11:40 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Yu, Yu-cheng" <yu-cheng.yu@...el.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v18 24/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack

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.