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Message-ID: <676f710b9747b091783aed38fb07259af3ca5b43.camel@intel.com>
Subject: Re: [PATCH v10 26/26] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     Eugene Syromiatnikov <esyr@redhat.com>
Cc:     Kees Cook <keescook@chromium.org>, 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>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        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>, mtk.manpages@gmail.com
Date:   Fri, 22 May 2020 11:13:24 -0700
In-Reply-To: <20200522172934.GI12341@asgard.redhat.com>
References: <20200429220732.31602-1-yu-cheng.yu@intel.com>
         <20200429220732.31602-27-yu-cheng.yu@intel.com>
         <202005211528.A12B4AD@keescook>
         <c9c9314374c7db0bf9b6e39670855afe5b0d4014.camel@intel.com>
         <20200522172934.GI12341@asgard.redhat.com>
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On Fri, 2020-05-22 at 19:29 +0200, Eugene Syromiatnikov wrote:
> 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.

Ok, use EOPNOTSUPP and add a comment why.

Yu-cheng