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Date: Thu, 18 Jun 2015 12:11:10 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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        Denys Vlasenko <vda.linux@googlemail.com>,
        Borislav Petkov <bp@alien8.de>, Kees Cook <keescook@chromium.org>,
        Brian Gerst <brgerst@gmail.com>
Subject: Re: [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code
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* 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.)

The most deadly complexity in our asm code are IMHO the intertwined threads of 
control flow - all of that should go into C, where it's much easier to see what's 
going on, because we have named variables, established code patterns and a 
compiler checking for common mistakes and such.

The other big area of complexity are our partial save/restore tricks, which makes 
tracking of what is saved (and what is not) tricky and fragile.

Thanks,

	Ingo