linux-kernel - Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns

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Message-ID: <20150326102551.GC27751@pd.tnic>
Date:	Thu, 26 Mar 2015 11:25:51 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...capital.net>,
	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>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns

On Thu, Mar 26, 2015 at 11:07:42AM +0100, Denys Vlasenko wrote:
> In my experiment, GAS uses 10-byte insn only for constants which
> won't work with 7-byte encoding; or if I explicitly ask for "movabs":
> 
> _start:         .globl  _start
>                 mov $0x12345678,%edi  # 5 bytes
>                 mov $0x12345678,%rdi  # 7 bytes
>                 movq $0x12345678,%rdi  # 7 bytes
>                 mov $0x80000000,%rdi  # 10 bytes

Right, and since they're signed immediates, the AUDIT_ARCH_X86_64 thing
is 0xc000003e and does not fit in an s32, thus the 64-bit immediate with
bf opcode:

	mov $0x80000000-1,%rdi
	mov $0x80000000,%rdi
	mov $0xc000003e,%rdi
	mov $0xc000003e,%edi

...

  21:   48 c7 c7 ff ff ff 7f    mov    $0x7fffffff,%rdi
  28:   48 bf 00 00 00 80 00    movabs $0x80000000,%rdi
  2f:   00 00 00 
  32:   48 bf 3e 00 00 c0 00    movabs $0xc000003e,%rdi
  39:   00 00 00
  3c:   bf 3e 00 00 c0          mov    $0xc000003e,%edi

Makes sense to me.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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