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Date:   Wed, 10 Apr 2019 19:57:09 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@amacapital.net>,
        "H. Peter Anvin" <hpa@zytor.com>, Andi Kleen <andi@firstfloor.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        libc-alpha <libc-alpha@sourceware.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        carlos <carlos@redhat.com>, x86 <x86@kernel.org>
Subject: Re: rseq/x86: choosing rseq code signature
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On Wed, Apr 10, 2019 at 11:47:40AM -0400, Mathieu Desnoyers wrote:
> ----- On Apr 10, 2019, at 2:54 AM, Peter Zijlstra peterz@infradead.org wrote:
> 
> > On Tue, Apr 09, 2019 at 04:43:42PM -0400, Mathieu Desnoyers wrote:
> >> +/*
> >> + * RSEQ_SIG is used with the following privileged instructions, which trap in
> >> user-space:
> >> + * x86-32:    0f 01 3d 53 30 05 53      invlpg 0x53053053
> >> + * x86-64:    0f 01 3d 53 30 05 53      invlpg 0x53053053(%rip)
> >> + */
> > 
> > Right, and the alternative is: 0f b9 3d $SIG, which decodes to:
> > 
> >  UD1 $SIG(%rip),%edi
> > 
> > which will trap unconditionally. The only problem is that gas will not
> > actually assemble it, but since we're .byte coding it, it doesn't
> > matter.
> > 
> > UD1 is specified by both AMD and Intel to take a ModR/M, unlike UD0
> > where they disagree on the ModR/M.
> 
> UD1 is even better from a code emulator perspective. It won't have to
> try to emulate invlpg if it sees it.

Some emulators terminate on UD2, not aware of any special UD1 behaviour.

> Byte coding UD1 as your example above gives the following objdump output,
> is it expected ?
> 
> objdump --version
> GNU objdump (GNU Binutils for Debian) 2.28
> 
> x86-32:
> 
>   14:	0f b9                	ud1    
>   16:	3d 53 30 05 53       	cmp    $0x53053053,%eax
> 
> x86-64:
> 
>    b:	0f b9                	ud1    
>    d:	3d 53 30 05 53       	cmp    $0x53053053,%eax

GNU objdump (GNU Binutils for Debian) 2.31.1

     0f b9 3d 78 56 34 12    ud1    0x12345678(%rip),%edi

So I suppose your objdump is too old :/