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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>,
        Florian Weimer <fweimer@redhat.com>
Cc:     Dave Martin <Dave.Martin@arm.com>, 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>
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Message-ID: <4f2dfefc-b55e-bf73-f254-7d95f9c67e5c@intel.com>
Date:   Wed, 26 Aug 2020 12:57:31 -0700
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On 8/26/20 11:49 AM, Yu, Yu-cheng wrote:
>> I would expect things like Go and various JITs to call it directly.
>>
>> If we wanted to be fancy and add a potentially more widely useful
>> syscall, how about:
>>
>> mmap_special(void *addr, size_t length, int prot, int flags, int type);
>>
>> Where type is something like MMAP_SPECIAL_X86_SHSTK.  Fundamentally,
>> this is really just mmap() except that we want to map something a bit
>> magical, and we don't want to require opening a device node to do it.
> 
> One benefit of MMAP_SPECIAL_* is there are more free bits than MAP_*.
> Does ARM have similar needs for memory mapping, Dave?

No idea.

But, mmap_special() is *basically* mmap2() with extra-big flags space.
I suspect it will grow some more uses on top of shadow stacks.  It could
have, for instance, been used to allocate MPX bounds tables.