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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAK1hOcNQpFr9ykWZWrzf3OD3hocFtZ7pvce-6RSV2SRouQXe=g@mail.gmail.com>
Date:	Thu, 26 Mar 2015 11:53:12 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Borislav Petkov <bp@...en8.de>,
	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 11:25 AM, Ingo Molnar <mingo@...nel.org> wrote:
> * Denys Vlasenko <vda.linux@...glemail.com> 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
>>                 mov $0x123456789,%rdi  # 10 bytes
>>                 movabs $0x12345678,%rdi # 10 bytes
>>
>>
>> $ gcc -nostartfiles -nostdlib -c z.S && objdump -dr z.o
>> z.o:     file format elf64-x86-64
>> Disassembly of section .text:
>> 0000000000000000 <_start>:
>>    0:    bf 78 56 34 12           mov    $0x12345678,%edi
>>    5:    48 c7 c7 78 56 34 12     mov    $0x12345678,%rdi
>>    c:    48 c7 c7 78 56 34 12     mov    $0x12345678,%rdi
>>   13:    48 bf 00 00 00 80 00     movabs $0x80000000,%rdi
>>   1a:    00 00 00
>>   1d:    48 bf 89 67 45 23 01     movabs $0x123456789,%rdi
>>   24:    00 00 00
>>   27:    48 bf 78 56 34 12 00     movabs $0x12345678,%rdi
>>   2e:    00 00 00
>
> I see, so:
>
>        movq $AUDIT_ARCH_X86_64, %rsi
>
> generated a 10-byte MOVABS opcode, while moving into %esi generates
> the 5-byte 32-bit MOV opcode?

Exactly.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/