Re: rseq/x86: choosing rseq code signature

[Peter Zijlstra]

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.