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From:   "H.J. Lu" <hjl.tools@gmail.com>
Date:   Thu, 27 Aug 2020 06:26:11 -0700
Message-ID: <CAMe9rOqt9kbqERC8U1+K-LiDyNYuuuz3TX++DChrRJwr5ajt6Q@mail.gmail.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     "Yu, Yu-cheng" <yu-cheng.yu@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
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On Wed, Aug 26, 2020 at 12:57 PM Dave Hansen <dave.hansen@intel.com> wrote:
>
> 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.

There is no reason we can't use

long arch_prctl (int, unsigned long, unsigned long, unsigned long, ..);

for ARCH_X86_CET_MMAP_SHSTK.   We just need to use

syscall (SYS_arch_prctl, ARCH_X86_CET_MMAP_SHSTK, ...);

-- 
H.J.