linux-kernel - Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

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Date:	Tue, 24 Feb 2015 21:13:03 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	"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 Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] x86: entry.S: tidy up several suboptimal insns

On Tue, Feb 24, 2015 at 8:58 PM, Borislav Petkov <bp@...en8.de> wrote:
> On Tue, Feb 24, 2015 at 07:51:32PM +0100, Denys Vlasenko wrote:
>> In all three 32-bit entry points, %eax is zero-extended to %rax.
>> It is safe to do 32-bit compare when checking that syscall#
>> is not too large.
>>
>> The last instance of "mysterious" SS+8 constant is replaced by SIZEOF_PTREGS.
>>
>> The $AUDIT_ARCH_X86_64 parameter to syscall_trace_enter_phase1/2
>> is a 32-bit constant, loading it with 64-bit MOV produces 10-byte insn
>> instead of 5-byte one.
>>
>> After TEST insn, JE anctually means "jump of zero",
>> let's use JZ mnemonic instead.
>
> Actually, JE == LZ as that's the same opcode for testing ZF=1.

Yes, I know that :)

> And I have to object:
>
>         testl $3,CS(%rsp)
>         je retint_kernel
>
> is much more understandable than
>
>         testl $3,CS(%rsp)
>         jz retint_kernel
>
> It basically says are $3 and CS(%rsp) equal.

They aren't equal.  $1 and $2 in two lowest bits will also
be interpreted as "userspace" here. "Equal to $3" sends
a wrong message here to a human reading the code,
the code doesn't test for CPL=3, it tests for any nonzero CPL.

> JZ, on the other hand, not so clear: the TEST ANDed the two operands and
> set flags accordingly, so JZ is testing the ZF. This means, you actually
> know what TEST does and you haven't forgotten.

JZ says "jump if zero", in this case, "jump if CPL is zero".
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