linux-kernel - Re: [PATCH 2/2] kprobes/x86: Use 16 bytes for each instruction slot again

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Date:	Tue, 2 Jun 2015 07:44:31 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Eugene Shatokhin <eugene.shatokhin@...alab.ru>,
	Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] kprobes/x86: Use 16 bytes for each instruction slot
 again


* Masami Hiramatsu <masami.hiramatsu.pt@...achi.com> wrote:

> On 2015/06/02 2:04, Andy Lutomirski wrote:
> > On Mon, Jun 1, 2015 at 9:32 AM, Eugene Shatokhin
> > <eugene.shatokhin@...alab.ru> wrote:
> >> Commit 91e5ed49fca0 ("x86/asm/decoder: Fix and enforce max instruction
> >> size in the insn decoder") has changed MAX_INSN_SIZE from 16 to 15 bytes
> >> on x86.
> >>
> >> As a side effect, the slots Kprobes use to store the instructions became
> >> 1 byte shorter. This is unfortunate because, for example, the Kprobes'
> >> "boost" feature can not be used now for the instructions of length 11,
> >> like a quite common kind of MOV:
> >> * movq $0xffffffffffffffff,-0x3fe8(%rax) (48 c7 80 18 c0 ff ff ff ff ff ff)
> >> * movq $0x0,0x88(%rdi)                   (48 c7 87 88 00 00 00 00 00 00 00)
> >> and so on.
> >>
> >> This patch makes the insn slots 16 bytes long, like they were before while
> >> keeping MAX_INSN_SIZE intact.
> >>
> >> Other tools may benefit from this change as well.
> > 
> > What is a "slot" and why does this patch make sense?  Naively, I'd
> > expect that the check you're patching is entirely unnecessary -- I
> > don't see what the size of the instruction being probed has to do with
> > the safety of executing it out of line and then jumping back.
> > 
> > Is there another magic 16 somewhere that this is enforcing that we
> > don't overrun?
> 
> The kprobe-"booster" adds a jump back code (jmp <probed address + insn length>) 
> right after the instruction in the out-of-code buffer(slot). So we need at least 
> the insn-length + 5 bytes for the slot, it's the trick of the magic :)

Please at minimum rename it to 'dynamic code buffer' or some other sensible name - 
the name 'slot' is pretty meaningless at best and misleading at worst.

Thanks,

	Ingo
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