From: Masami Hiramatsu <mhiramat@kernel.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
LKML <linux-kernel@vger.kernel.org>,
Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
"David S. Miller" <davem@davemloft.net>, X86 ML <x86@kernel.org>,
Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: Why do kprobes and uprobes singlestep?
Date: Thu, 25 Feb 2021 11:22:45 +0900 [thread overview]
Message-ID: <20210225112245.607c70ec13cf8d279390e89e@kernel.org> (raw)
In-Reply-To: <CALCETrWoKh0aemrvTGZ13bUzN27s3WGW3CyvTptvayWLQEk91Q@mail.gmail.com>
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>
next prev parent reply other threads:[~2021-02-25 2:23 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-23 23:24 Why do kprobes and uprobes singlestep? Andy Lutomirski
2021-02-24 1:17 ` Masami Hiramatsu
2021-02-24 19:45 ` Andy Lutomirski
2021-02-25 2:22 ` Masami Hiramatsu [this message]
2021-02-25 6:03 ` Andy Lutomirski
2021-02-25 9:11 ` Masami Hiramatsu
2021-03-01 14:08 ` [RFC PATCH 0/1] x86/kprobes: Remoev single-step trap from x86 kprobes Masami Hiramatsu
2021-03-01 14:08 ` [RFC PATCH 1/1] x86/kprobes: Use int3 instead of debug trap for single-step Masami Hiramatsu
2021-03-02 8:06 ` Peter Zijlstra
2021-03-02 8:38 ` Peter Zijlstra
2021-03-02 8:41 ` Peter Zijlstra
2021-03-02 8:54 ` Peter Zijlstra
2021-03-02 12:51 ` Masami Hiramatsu
2021-03-02 13:58 ` Peter Zijlstra
2021-03-02 15:25 ` [PATCH -tip 0/3] x86/kprobes: Remoev single-step trap from x86 kprobes Masami Hiramatsu
2021-03-02 15:25 ` [PATCH -tip 1/3] x86/kprobes: Retrieve correct opcode for group instruction Masami Hiramatsu
2021-03-23 15:15 ` [tip: x86/core] " tip-bot2 for Masami Hiramatsu
2021-03-02 15:25 ` [PATCH -tip 2/3] x86/kprobes: Identify far indirect JMP correctly Masami Hiramatsu
2021-03-23 15:15 ` [tip: x86/core] " tip-bot2 for Masami Hiramatsu
2021-03-02 15:25 ` [PATCH -tip 3/3] x86/kprobes: Use int3 instead of debug trap for single-step Masami Hiramatsu
2021-03-23 15:15 ` [tip: x86/core] " tip-bot2 for Masami Hiramatsu
2021-03-17 14:55 ` [PATCH -tip 0/3] x86/kprobes: Remoev single-step trap from x86 kprobes Masami Hiramatsu
2021-03-17 16:26 ` Peter Zijlstra
2021-03-17 17:45 ` Andy Lutomirski
2021-02-25 9:59 ` Why do kprobes and uprobes singlestep? Peter Zijlstra
2021-03-01 16:51 ` Oleg Nesterov
2021-03-02 1:36 ` Andy Lutomirski
2021-03-02 20:24 ` Alexei Starovoitov
2021-03-02 21:02 ` Andy Lutomirski
2021-03-03 1:22 ` Alexei Starovoitov
2021-03-03 1:46 ` Andy Lutomirski
2021-03-03 2:18 ` Alexei Starovoitov
2021-03-03 13:27 ` Oleg Nesterov
2021-03-03 18:11 ` Daniel Xu
2021-03-03 19:14 ` Andy Lutomirski
2021-03-02 20:25 ` Oleg Nesterov
2021-03-02 20:35 ` Andy Lutomirski
2021-03-02 20:28 ` Oleg Nesterov
2021-03-02 2:22 ` Masami Hiramatsu
2021-03-02 2:48 ` Andy Lutomirski
2021-03-02 20:31 ` Oleg Nesterov
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