From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Peter Zijlstra <peterz@infradead.org>
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
Date: Wed, 10 Apr 2019 14:05:50 -0400 (EDT) [thread overview]
Message-ID: <1829916645.3219.1554919550686.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <20190410175709.GW4038@hirez.programming.kicks-ass.net>
----- On Apr 10, 2019, at 1:57 PM, Peter Zijlstra peterz@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 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 :/
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
prev parent reply other threads:[~2019-04-10 18:05 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-09 19:32 rseq/x86: choosing rseq code signature Mathieu Desnoyers
2019-04-09 20:43 ` Mathieu Desnoyers
2019-04-10 0:50 ` Zack Weinberg
2019-04-10 1:57 ` Andy Lutomirski
2019-04-10 4:19 ` Mathieu Desnoyers
2019-04-10 6:54 ` Peter Zijlstra
2019-04-10 15:47 ` Mathieu Desnoyers
2019-04-10 17:57 ` Peter Zijlstra
2019-04-10 18:05 ` Mathieu Desnoyers [this message]
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