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

From: Denys Vlasenko <dvlasenk@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Ingo Molnar <mingo@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Oleg Nesterov <oleg@redhat.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Alexei Starovoitov <ast@plumgrid.com>,
	Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
	X86 ML <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 5/5] x86/asm/entry/32: Simplify ptrace register shuffling
Date: Tue, 09 Jun 2015 21:14:35 +0200	[thread overview]
Message-ID: <55773B1B.7040901@redhat.com> (raw)
In-Reply-To: <CALCETrXXwRv5U6umHTPdoAn3=pexKO6_C+=0A7BRJQqDXwGXkw@mail.gmail.com>

On 06/09/2015 08:59 PM, Andy Lutomirski wrote:
> On Tue, Jun 9, 2015 at 11:54 AM, Denys Vlasenko <dvlasenk@redhat.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).