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Date: Tue, 09 Jun 2015 21:18:04 +0200
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Andy Lutomirski <luto@amacapital.net>
CC: Ingo Molnar <mingo@kernel.org>,
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        Kees Cook <keescook@chromium.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/5] x86/asm/entry/32: Replace RESTORE_RSI_RDI[_RDX] with
 open-coded 32-bit reads
References: <1433876051-26604-1-git-send-email-dvlasenk@redhat.com> <1433876051-26604-4-git-send-email-dvlasenk@redhat.com> <CALCETrUasq4cFacbHhzK-TVBsHD3An3YDSW93kONvaxwnvPv1w@mail.gmail.com> <5577389F.4090301@redhat.com> <CALCETrXc5biurf1ZFQP0_67RewnWcxP0iQG=Df=oy_LTyf_esw@mail.gmail.com>
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On 06/09/2015 09:11 PM, Andy Lutomirski wrote:
> On Tue, Jun 9, 2015 at 12:03 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>> On 06/09/2015 09:01 PM, Andy Lutomirski wrote:
>>> On Tue, Jun 9, 2015 at 11:54 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>>>> This doesn't change much, but this uses shorter 32-bit insns:
>>>>
>>>>         -48 8b 74 24 68         mov    0x68(%rsp),%rsi
>>>>         -48 8b 7c 24 70         mov    0x70(%rsp),%rdi
>>>>         -48 8b 54 24 60         mov    0x60(%rsp),%rdx
>>>>         +8b 74 24 68            mov    0x68(%rsp),%esi
>>>>         +8b 7c 24 70            mov    0x70(%rsp),%edi
>>>>         +8b 54 24 60            mov    0x60(%rsp),%edx
>>>>
>>>> Since these are the only uses of RESTORE_RSI_RDI[_RDX], drop these macros.
>>>>
>>>
>>> It probably doesn't matter for these fast paths, but, for the full
>>> slow path return, we really do need to restore the full pt_regs.
>>> After all, the syscall we're returning from might be sigreturn.
>>
>> This is compat 32-bit syscall handling code.
>> IIUC we do not restore high half of any registers for 32-bit tasks.
>>
>> Am I missing something?
> 
> Yes -- 64-bit tasks can call 32-bit compat syscalls.

Not via SYSCALL and SYSENTER code paths. This patch touches only those
code paths.