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Date:   Fri, 22 May 2020 19:29:34 +0200
From:   Eugene Syromiatnikov <esyr@...hat.com>
To:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:     Kees Cook <keescook@...omium.org>, 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>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        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>, mtk.manpages@...il.com
Subject: Re: [PATCH v10 26/26] x86/cet/shstk: Add arch_prctl functions for
 shadow stack

On Fri, May 22, 2020 at 10:17:43AM -0700, Yu-cheng Yu wrote:
> On Thu, 2020-05-21 at 15:42 -0700, Kees Cook wrote:
> > On Wed, Apr 29, 2020 at 03:07:32PM -0700, Yu-cheng Yu wrote:
> [...]
> > > +
> > > +int prctl_cet(int option, u64 arg2)
> > > +{
> > > +	struct cet_status *cet;
> > > +
> > > +	if (!IS_ENABLED(CONFIG_X86_INTEL_CET))
> > > +		return -EINVAL;
> > 
> > Using -EINVAL here means userspace can't tell the difference between an
> > old kernel and a kernel not built with CONFIG_X86_INTEL_CET. Perhaps
> > -ENOTSUPP?
> 
> Looked into this.  The kernel and GLIBC are not in sync.  So maybe we still use
> EINVAL here?
> 
> Yu-cheng
> 
> 
> 
> In kernel:
> ----------
> 
> #define EOPNOTSUPP	95
> #define ENOTSUPP 	524
> 
> In GLIBC:
> ---------
> 
> printf("ENOTSUP=%d\n", ENOTSUP);
> printf("EOPNOTSUPP=%d\n", EOPNOTSUPP);
> printf("%s=524\n", strerror(524));
>  
> ENOTSUP=95
> EOPNOTSUPP=95
> Unknown error 524=524

EOPNOTSUPP/ENOTSUP/ENOTSUPP is actually a mess, it's summarized recently
by Michael Kerrisk[1].  From the kernel's point of view, I think it
would be reasonable to return EOPNOTSUPP, and expect that the userspace
would use ENOTSUP to match against it.

[1] https://lore.kernel.org/linux-man/cb4c685b-6c5d-9c16-aade-0c95e57de4b9@gmail.com/

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