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Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     "Yu, Yu-cheng" <yu-cheng.yu@intel.com>,
        Andy Lutomirski <luto@kernel.org>
Cc:     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>,
        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>
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Message-ID: <086c73d8-9b06-f074-e315-9964eb666db9@intel.com>
Date:   Tue, 25 Aug 2020 12:19:39 -0700
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On 8/25/20 11:43 AM, Yu, Yu-cheng wrote:
>>> arch_prctl(ARCH_X86_CET_MMAP_SHSTK, u64 *args)
>>>      Allocate a new shadow stack.
>>>
>>>      The parameter 'args' is a pointer to a user buffer.
>>>
>>>      *args = desired size
>>>      *(args + 1) = MAP_32BIT or MAP_POPULATE
>>>
>>>      On returning, *args is the allocated shadow stack address.
>>
>> This is hideous.  Would this be better as a new syscall?
> 
> Could you point out why this is hideous, so that I can modify the
> arch_prctl?

Passing values in memory is hideous when we don't have to.  A syscall
would let you have separate arguments for size and flags and would also
let you have a nice return value instead of needing to do that in memory
too.

> I think this is more arch-specific.  Even if it becomes a new syscall,
> we still need to pass the same parameters.

Right, but without the copying in and out of memory.