Messages in this thread | | | Date | Wed, 3 Jun 2015 09:28:26 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 2/2] kprobes/x86: Use 16 bytes for each instruction slot again |
| |
* 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?
Yeah, 'code buffer' sounds good to me.
Thanks,
Ingo
| |