Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 2 Jun 2015 14:55:23 -0700 | Subject | Re: [PATCH 2/2] kprobes/x86: Use 16 bytes for each instruction slot again |
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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.
--Andy
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