Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 24 Feb 2021 22:03:12 -0800 | Subject | Re: Why do kprobes and uprobes singlestep? |
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On Wed, Feb 24, 2021 at 6:22 PM Masami Hiramatsu <mhiramat@kernel.org> wrote: > > On Wed, 24 Feb 2021 11:45:10 -0800 > Andy Lutomirski <luto@kernel.org> wrote: > > > On Tue, Feb 23, 2021 at 5:18 PM Masami Hiramatsu <mhiramat@kernel.org> wrote: > > > > > > On Tue, 23 Feb 2021 15:24:19 -0800 > > > Andy Lutomirski <luto@kernel.org> wrote: > > > > > > > A while back, I let myself be convinced that kprobes genuinely need to > > > > single-step the kernel on occasion, and I decided that this sucked but > > > > I could live with it. it would, however, be Really Really Nice (tm) > > > > if we could have a rule that anyone running x86 Linux who single-steps > > > > the kernel (e.g. kgdb and nothing else) gets to keep all the pieces > > > > when the system falls apart around them. Specifically, if we don't > > > > allow kernel single-stepping and if we suitably limit kernel > > > > instruction breakpoints (the latter isn't actually a major problem), > > > > then we don't really really need to use IRET to return to the kernel, > > > > and that means we can avoid some massive NMI nastiness. > > > > > > Would you mean using "pop regs + popf + ret" instead of IRET after > > > int3 handled for avoiding IRET releasing the NMI mask? Yeah, it is > > > possible. I don't complain about that. > > > > Yes, more or less. > > > > > > > > However, what is the relationship between the IRET and single-stepping? > > > I think we can do same thing in do_debug... > > > > Because there is no way to single-step without using IRET. POPF; RET > > will trap after RET and you won't make forward progress. > > Ah, indeed. "POPF; RET" is not atomically exceute. > > > > > But I was contemplating the code, and I'm no longer convinced. > > > > Uprobes seem to single-step user code for no discernable reason. > > > > (They want to trap after executing an out of line instruction, AFAICT. > > > > Surely INT3 or even CALL after the out-of-line insn would work as well > > > > or better.) Why does kprobe single-step? I spend a while staring at > > > > the code, and it was entirely unclear to me what the purpose of the > > > > single-step is. > > > > > > For kprobes, there are 2 major reasons for (still relaying on) single stepping. > > > One is to provide post_handler, another is executing the original code, > > > which is replaced by int3, without modifying code nor emulation. > > > > I don't follow. Suppose we execute out of line. If we originally have: > > > > INSN > > > > we replace it with: > > > > INT3 > > > > and we have, out of line: > > > > INSN [but with displacement modified if it's RIP-relative] > > > > right now, we single-step the out of line copy. But couldn't we instead do: > > > > INSN [but with displacement modified if it's RIP-relative] > > INT3 > > If the INSN is "jmp +127", it will skip the INT3. So those instructions > must be identified and emulated. We did it already in the arm64 (see commit > 7ee31a3aa8f4 ("arm64: kprobes: Use BRK instead of single-step when executing > instructions out-of-line")), because arm64 already emulated the branch > instructions. I have to check x86 insns can be emulated without side-effects.
Off the top of my head:
JMP changes RIP but has no other side effects. Jcc is the same except that the condition needs checking, which would be a bit tedious.
CALL changes RIP and does a push but has no other side effects. We have special infrastructure to emulate CALL from int3 context: int3_emulate_call().
RET pops and changes RIP. No other side effects.
RET imm is rare. I don't think it occurs in the kernel at all.
LRET is rare. I don't think kprobe needs to support it.
IRET is rare, and trying to kprobe it seems likely to cause a disaster, although it's within the realm of possibility that the IRET in sync_core() could work.
JMP FAR and CALL FAR are rare. I see no reason to support them.
> > > > > or even > > > > INSN [but with displacement modified if it's RIP-relative] > > JMP kprobe_post_handler > > This needs a sequence of push-regs etc. ;) > > > > > and avoid single-stepping? > > > > I guess I see the point for CALL, JMP and RET, but it seems like we > > could emulate those cases instead fairly easily. > > OK, let's try to do it. I think it should be possible because even in the > current code, resume fixup code (adjust IP register) works only for a few > groups of instructions.
I suspect that emulating them would give a nice performance boost, too. Single-stepping is very slow on x86.
I should let you know, though, that I might have found a sneaky alternative solution to handling NMIs, so this is a bit lower priority from my perspective than I thought it was. I'm not quite 100% convinced my idea works, but I'll play with it.
--Andy
> > Thank you, > > -- > Masami Hiramatsu <mhiramat@kernel.org>
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