Messages in this thread | | | Date | Thu, 25 Feb 2021 11:22:45 +0900 | From | Masami Hiramatsu <> | Subject | Re: Why do kprobes and uprobes singlestep? |
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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.
> > 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.
Thank you,
-- Masami Hiramatsu <mhiramat@kernel.org>
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