linux-kernel - Re: [PATCH 5/5] x86/asm/entry/32: Simplify ptrace register shuffling

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Date:	Tue, 09 Jun 2015 21:14:35 +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>,
	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 5/5] x86/asm/entry/32: Simplify ptrace register shuffling

On 06/09/2015 08:59 PM, Andy Lutomirski wrote:
> On Tue, Jun 9, 2015 at 11:54 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>> Before this patch, we were clearing pt_regs->r8..r11 on stack.
>> We can as well just store actual r8..r11 registers there:
>> they came from userspace, we leak no information by showing them to ptrace.
>> This allows to get rid of one insn ("xor %eax,%eax").
>> Not a big deal, but still...
>>
>> After call to syscall_trace_enter(), before this patch we were restoring
>> clobbered registers and jump to code which converts 32-bit syscall
>> ABI to 64-bit C ABI. This is unnecessary work, we can combine both
>> steps into one (similar to what audit code does already).
> 
> I think like zeroing it better.  There's nothing wrong with zeroing
> it,

Yes, there is nothing wrong with zeroing. It just requires a bit
more code.

> and it makes testing (if we ever started testing this stuff)
> easier, I think.

Currently, we don't zero *all* high regs, only r8..10. (r11 is
nonzero due to SYSRET ABI; r12..15 are preserved by virtue of being
callee-saved regs).



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