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Date: Thu, 26 Mar 2015 11:25:51 +0100
From: Borislav Petkov <bp@alien8.de>
To: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Andy Lutomirski <luto@amacapital.net>, 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>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns
Message-ID: <20150326102551.GC27751@pd.tnic>
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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.

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