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From:   Florian Weimer <fweimer@redhat.com>
To:     Dave Martin <Dave.Martin@arm.com>
Cc:     "Yu\, Yu-cheng" <yu-cheng.yu@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "open list\:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        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>,
        "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>,
        Weijiang Yang <weijiang.yang@intel.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for shadow stack
References: <20200825002540.3351-1-yu-cheng.yu@intel.com>
        <20200825002540.3351-26-yu-cheng.yu@intel.com>
        <CALCETrVpLnZGfWWLpJO+aZ9aBbx5KGaCskejXiCXF1GtsFFoPg@mail.gmail.com>
        <2d253891-9393-44d0-35e0-4b9a2da23cec@intel.com>
        <086c73d8-9b06-f074-e315-9964eb666db9@intel.com>
        <73c2211f-8811-2d9f-1930-1c5035e6129c@intel.com>
        <af258a0e-56e9-3747-f765-dfe45ce76bba@intel.com>
        <ef7f9e24-f952-d78c-373e-85435f742688@intel.com>
        <20200826164604.GW6642@arm.com>
Date:   Wed, 26 Aug 2020 18:51:48 +0200
In-Reply-To: <20200826164604.GW6642@arm.com> (Dave Martin's message of "Wed,
        26 Aug 2020 17:46:05 +0100")
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* Dave Martin:

> On Tue, Aug 25, 2020 at 04:34:27PM -0700, Yu, Yu-cheng wrote:
>> On 8/25/2020 4:20 PM, Dave Hansen wrote:
>> >On 8/25/20 2:04 PM, Yu, Yu-cheng wrote:
>> >>>>I think this is more arch-specific.=C2=A0 Even if it becomes a new s=
yscall,
>> >>>>we still need to pass the same parameters.
>> >>>
>> >>>Right, but without the copying in and out of memory.
>> >>>
>> >>Linux-api is already on the Cc list.=C2=A0 Do we need to add more peop=
le to
>> >>get some agreements for the syscall?
>> >What kind of agreement are you looking for?  I'd suggest just coding it
>> >up and posting the patches.  Adding syscalls really is really pretty
>> >straightforward and isn't much code at all.
>> >
>>=20
>> Sure, I will do that.
>
> Alternatively, would a regular prctl() work here?

Is this something appliation code has to call, or just the dynamic
loader?

prctl in glibc is a variadic function, so if there's a mismatch between
the kernel/userspace syscall convention and the userspace calling
convention (for variadic functions) for specific types, it can't be made
to work in a generic way.

The loader can use inline assembly for system calls and does not have
this issue, but applications would be implcated by it.

Thanks,
Florian