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

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

On 2015/06/03 6:55, Andy Lutomirski wrote:
> On Tue, Jun 2, 2015 at 2:46 PM, Masami Hiramatsu
> <masami.hiramatsu.pt@hitachi.com> wrote:
>> On 2015/06/02 14:44, Ingo Molnar wrote:
>>>
>>> * 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.
>>
>> OK, would 'exec_buffer' is sensible? or just a 'code_buffer' is better?
> 
> redirected_code_buffer_size?
> 
> Anyway, regardless of the exact name, I also think it should be
> measured in bytes instead of weird per-arch units.

OK, will try to use u8 and void *.

Thanks!

-- 
Masami HIRAMATSU
Linux Technology Research Center, System Productivity Research Dept.
Center for Technology Innovation - Systems Engineering
Hitachi, Ltd., Research & Development Group
E-mail: masami.hiramatsu.pt@hitachi.com