linux-kernel - Re: [PATCH 2/3] x86/asm/entry/32: Remove most of SYSCALL32 code, part 1

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Date:	Sat, 25 Jul 2015 20:36:05 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Krzysztof A. Sobiecki" <sobkas@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] x86/asm/entry/32: Remove most of SYSCALL32 code,
 part 1

On 07/24/2015 07:50 PM, Andy Lutomirski wrote:
> On Fri, Jul 24, 2015 at 6:47 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>> SYSCALL32 code is nearly identical to SYSCALL32, except for initial
>> section. Merge them.
>>
>> The removal is split into two parts, to make review eaiser. This is part 1.
>>
>> auditsys_entry_common and auditsys_exit macros are indented one more tab without
>> any changes. This prevents diff from becoming unreadable.
>> They will be removed in part 2.
> 
> I need to read these more closely, which is, at present, exceeding my
> ability to look at asm.  (See the big NMI thread.)  I'll look soon.

The "sysenter_fix_flags" thingy prevented the diff from being
a pure delete, so it is not as clear as I hoped.

What patch is doing is actually very simple. It "amputates"
entire SYSENTER code path after it finished creating partially
filled pt_regs, loaded arg6 and dealt with EFLAGS sanitization -
after this is done, the state is identical to the similar
state in SYSCALL code, so we can just use SYSCALL code from that moment
onward! :)


> Meanwhile, this code is incredibly fragile wrt syscall restart.
> (Syscall restart on compat is really weird.)

Weird in what way?

> Do we have a decent test for it?

I don't know of any such test.

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