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From: Andy Lutomirski <luto@amacapital.net>
Date: Mon, 1 Jun 2015 14:57:55 -0700
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Subject: Re: [PATCH 2/2] kprobes/x86: Use 16 bytes for each instruction slot again
To: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Eugene Shatokhin <eugene.shatokhin@rosalab.ru>,
        Ingo Molnar <mingo@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Jun 1, 2015 at 2:49 PM, 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 :)

This still doesn't explain what a "slot" is.

I broke (?) something because I didn't see anything that looked
relevant that I was changing.  But now I see it:

-       .insn_size = MAX_INSN_SIZE,
+       .insn_size = KPROBE_INSN_SLOT_SIZE,

Would it make sense to clean this up?  insn_size isn't the size of an
instruction at all -- it's the size of a kprobe jump target in units
of sizeof(kprobe_opcode_t).

How about renaming insn_size to something sensible (and maybe
specifying the size in *bytes*)?

--Andy