----- On Apr 10, 2019, at 1:57 PM, Peter Zijlstra pet...@infradead.org wrote:
> On Wed, Apr 10, 2019 at 11:47:40AM -0400, Mathieu Desnoyers wrote: >> ----- On Apr 10, 2019, at 2:54 AM, Peter Zijlstra pet...@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 :/ Well at least it decodes _something_ which matches the overall instruction length of 7 bytes, which I think should be OK. So let's use ud1 unless anyone objects. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com