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

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

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 :)

Thank you,


> 
> --Andy
> 


-- 
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