linux-kernel - Re: [PATCH 1/5] x86/vm86: Move userspace accesses to do_sys

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Date:	Mon, 13 Jul 2015 11:36:47 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Brian Gerst <brgerst@...il.com>
Cc:	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/5] x86/vm86: Move userspace accesses to do_sys_vm86()

On Mon, Jul 13, 2015 at 9:45 AM, Brian Gerst <brgerst@...il.com> wrote:
> On Sat, Jul 11, 2015 at 12:37 PM, Andy Lutomirski <luto@...capital.net> wrote:
>> On Fri, Jul 10, 2015 at 10:09 PM, Brian Gerst <brgerst@...il.com> wrote:
>>> Move the userspace accesses down into the common function in
>>> preparation for the next set of patches.
>>>
>>
>> One thing I don't like about the current code that makes these patches
>> harder to review is the bizarre approach to copying.  If you changed
>> this:
>>
>>> -       tmp = copy_vm86_regs_from_user(&info.regs, &v86->regs,
>>> -                                      offsetof(struct kernel_vm86_struct, vm86plus) -
>>> -                                      sizeof(info.regs));
>>
>> into a normal field-by-field get_user / copy_from_user (the latter for
>> the big regs struct) then it would be clear what the ABI is and it
>> would be much easier to read the patches and confirm that you aren't
>> accidentally changing the ABI.
>>
>> You could also get rid of the constraint that certain fields in
>> apparently kernel-internal structs had to be in a certain order.
>>
>> Other than that, patches 1-4 look good on cursory inspection.  I'll
>> look more carefully later.  I need to think about patch 5 more.
>>
>> --Andy
>
> Any other comments before I start working on v2?
>

Nothing major.  I'm a bit nervous about leaving ds, es, fs, and gs in
pt_regs more or less undefined until save_v86_state happens, but it's
unlikely that there's any ABI to break there.  The results from perf
might be a bit odd with your patches applied.  Of course, they're
probably useless without your patch.

It might also be worth renaming save_v86_state in patch 5.

Do your patches pass my upgraded entry_from_vm86 test?  You're
changing handle_vm86_trap so it always returns, which may have
unexpected side effects (or I missed something in your patch).

--Andy
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