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

[Andy Lutomirski]

On Tue, Jun 2, 2015 at 2:46 PM, Masami Hiramatsu
<masami.hiramatsu.pt@xxxxxxxxxxx> wrote:
> On 2015/06/02 14:44, Ingo Molnar wrote:
>>
>> * Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx> wrote:
>>
>>> On 2015/06/02 2:04, Andy Lutomirski wrote:
>>>> On Mon, Jun 1, 2015 at 9:32 AM, Eugene Shatokhin
>>>> <eugene.shatokhin@xxxxxxxxxx> 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.

--Andy
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