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Subject: Re: [PATCH v18 24/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     Dave Hansen <dave.hansen@intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        Pengfei Xu <pengfei.xu@intel.com>
References: <20210127212524.10188-1-yu-cheng.yu@intel.com>
 <20210127212524.10188-25-yu-cheng.yu@intel.com>
 <ba39586d-25b6-6ea5-19c3-adf17b59f910@intel.com>
 <761ae8ce-0560-24cc-e6f7-684475cb3708@intel.com>
 <6720b1a9-f785-dbbd-1f0e-8c9090be2069@intel.com>
 <a7ef2df2-815f-9994-11b7-026afe45094f@intel.com>
From:   "Yu, Yu-cheng" <yu-cheng.yu@intel.com>
Message-ID: <c68f0f9b-2e56-6ca6-511d-58acd3d96ff2@intel.com>
Date:   Wed, 3 Feb 2021 14:28:45 -0800
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On 2/3/2021 2:11 PM, Dave Hansen wrote:
> 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.
> 

Maybe I will keep it ENOTSUPP for now.  If any logical reason should 
come up, I will be happy to change it again.  Thanks!

--
Yu-cheng