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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 18 Jun 2015 04:06:22 -0700
Message-ID: <CALCETrVRF34yFtujk+gOUiiPG53bNu+WQH9MgvOa6pdUejb+Kw@mail.gmail.com>
Subject: Re: [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code
To: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        =?UTF-8?B?RnLDqWTDqXJpYyBXZWlzYmVja2Vy?= <fweisbec@gmail.com>,
        Rik van Riel <riel@redhat.com>, Oleg Nesterov <oleg@redhat.com>,
        Denys Vlasenko <vda.linux@googlemail.com>,
        Borislav Petkov <bp@alien8.de>, Kees Cook <keescook@chromium.org>,
        Brian Gerst <brgerst@gmail.com>
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On Thu, Jun 18, 2015 at 3:11 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Andy Lutomirski <luto@amacapital.net> wrote:
>
>> > The only low level bits remaining in assembly will be low level hardware ABI
>> > details: saving registers and restoring registers to the expected format - no
>> > 'active' code whatsoever.
>>
>> I think this is true for syscalls.  Getting the weird special cases (IRET and GS
>> fault) for error_entry to work correctly in C could be tricky.
>
> Correct, and I double checked the IRET fault path yesterday (fixup_bad_iret), and
> it looks like a straightforward exception handler with limited control flow. It
> can stay in asm just fine, it seems mostly orthogonal to the rest.
>
> I didn't check the GS fault path, but that only affects 32-bit, as we use SWAPGS
> on 64-bit, right? In any case, that code too (32-bit RESTORE_REGS) belongs into
> the natural 'hardware ABI preparation code' that should stay in assembly. (Unless
> I missed some other code that might cause trouble.)

Look for "gs_change".  To change the gs selector, we do swapgs, then
load gs, then swapgs again.  If the gs load fails, then we trigger a
special fixup.

--Andy