From: Ingo Molnar <mingo@kernel.org>
To: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
Eugene Shatokhin <eugene.shatokhin@rosalab.ru>,
Ingo Molnar <mingo@redhat.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] kprobes/x86: Use 16 bytes for each instruction slot again
Date: Tue, 2 Jun 2015 07:44:31 +0200 [thread overview]
Message-ID: <20150602054431.GA21382@gmail.com> (raw)
In-Reply-To: <556CD37E.9000501@hitachi.com>
* Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote:
> On 2015/06/02 2:04, Andy Lutomirski wrote:
> > On Mon, Jun 1, 2015 at 9:32 AM, Eugene Shatokhin
> > <eugene.shatokhin@rosalab.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
next prev parent reply other threads:[~2015-06-02 5:44 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-01 16:32 [PATCH 0/2] kprobes/x86: Allow "boost" for 10- and 11-byte instructions Eugene Shatokhin
2015-06-01 16:32 ` [PATCH 1/2] kprobes/x86: boost: Fix checking if there is enough room for a jump Eugene Shatokhin
2015-06-01 21:51 ` Masami Hiramatsu
2015-06-01 16:32 ` [PATCH 2/2] kprobes/x86: Use 16 bytes for each instruction slot again Eugene Shatokhin
2015-06-01 17:04 ` Andy Lutomirski
2015-06-01 21:49 ` Masami Hiramatsu
2015-06-01 21:57 ` Andy Lutomirski
2015-06-02 21:42 ` Masami Hiramatsu
2015-06-02 5:44 ` Ingo Molnar [this message]
2015-06-02 21:46 ` Masami Hiramatsu
2015-06-02 21:55 ` Andy Lutomirski
2015-06-04 21:59 ` Masami Hiramatsu
2015-06-03 7:28 ` Ingo Molnar
2015-06-01 21:58 ` Masami Hiramatsu
2015-06-02 5:47 ` Ingo Molnar
2015-06-01 21:44 ` [PATCH 0/2] kprobes/x86: Allow "boost" for 10- and 11-byte instructions Masami Hiramatsu
2015-06-03 7:54 ` [PATCH v2 2/2] kprobes/x86: Use 16 bytes for each instruction slot again Eugene Shatokhin
2015-06-04 14:42 ` Jeff Epler
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