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From: Kyle Huey <me@kylehuey.com>
Date: Wed, 14 Sep 2016 14:35:38 -0700
Message-ID: <CAP045Aoy0dHAFMQgeYanowef8Ye1UgzRp6_tGbr80ONSfKyvbw@mail.gmail.com>
Subject: Re: [PATCH v2 1/3] syscalls,x86 Expose arch_prctl on x86-32.
To: Dave Hansen <dave.hansen@linux.intel.com>
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>,
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        "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)" 
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On Wed, Sep 14, 2016 at 2:29 PM, Dave Hansen
<dave.hansen@linux.intel.com> wrote:
> On 09/14/2016 02:01 PM, Kyle Huey wrote:
>> Signed-off-by: Kyle Huey <khuey@kylehuey.com>
>> ---
>>  arch/x86/entry/syscalls/syscall_32.tbl |  1 +
>>  arch/x86/kernel/process.c              | 80 ++++++++++++++++++++++++++++++++++
>>  arch/x86/kernel/process_64.c           | 66 ----------------------------
>>  3 files changed, 81 insertions(+), 66 deletions(-)
>
> Could you explain a bit about what is going on here?  Is it just a plain
> old code move, _why_ you had to do it this way, etc...?

Sure.  In the subsequent patches in this series I add an arch_prctl
that is available for both 64 and 32 bit programs/kernels.  Since
process_64.c is only built for 64 bit kernels, this syscall can't stay
there anymore.

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 considered doing this instead with a compat wrapper for the syscall
on 32 bit systems that would filter these arch_prctls before getting
to do_arch_prctl. I didn't see any prior art for it, so decided not to
proceed that way.

- Kyle