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Subject: Re: [PATCH v2 1/3] syscalls,x86 Expose arch_prctl on x86-32.
To: Kyle Huey <me@kylehuey.com>
References: <1473886902-17902-1-git-send-email-khuey@kylehuey.com>
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 <57D9C139.1000601@linux.intel.com>
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Cc: "Robert O'Callahan" <robert@ocallahan.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Anna Schumaker <Anna.Schumaker@netapp.com>,
        David Howells <dhowells@redhat.com>,
        "Dmitry V. Levin" <ldv@altlinux.org>,
        Eric B Munson <emunson@akamai.com>, Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Michael S. Tsirkin" <mst@redhat.com>, Jiri Slaby <jslaby@suse.cz>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Borislav Petkov <bp@alien8.de>, Dmitry Vyukov <dvyukov@google.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@vger.kernel.org>
From: Dave Hansen <dave.hansen@linux.intel.com>
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Date: Wed, 14 Sep 2016 14:46:51 -0700
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On 09/14/2016 02:35 PM, Kyle Huey wrote:
> It's not quite a plain move.  To leave the existing arch_prctls only
> accessible to 64 bit callers, I added the is_32 bit and the four early
> returns for each existing ARCH_BLAH.  These cases are now
> conditionally compiled out in a 32 bit kernel, so we only have to
> handle the 32 bit process on a 64 bit kernel case at runtime.

I think it would make a lot of sense to do the move and the modification
in two patches.

Oh, and arch_prctl() really *is* 64-bit only.  I didn't realize that.
That would have been nice to call out in the changelog, too.  It's
totally non-obvious.

You're going to owe some manpage updates after this too, I guess.  It
says: "arch_prctl() is supported only on Linux/x86-64 for 64-bit
programs currently."

FWIW, I don't think it would be horrible to leave the existing
do_arch_prctl() code in process_64.h and call it
do_64_bit_only_something_arch_prctl(), and only call in to it from the
generic do_arch_prctl().  You really have one reason for all the "if
(is_32)"'s and it would be nice to document why in one single place.