linux-kernel - Re: Why do kprobes and uprobes singlestep?

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Message-ID: <20210301165130.GA5351@redhat.com>
Date:   Mon, 1 Mar 2021 17:51:31 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>, X86 ML <x86@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: Why do kprobes and uprobes singlestep?

Hi Andy,

sorry for delay.

On 02/23, Andy Lutomirski 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.

Not sure I understand you correctly, I know almost nothing about low-level
x86  magic.

But I guess this has nothing to do with uprobes, they do not single-step
in kernel mode, right?

> 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.)

Uprobes use single-step from the very beginning, probably because this
is the most simple and "standard" way to implement xol.

And please note that CALL/JMP/etc emulation was added much later to fix the
problems with non-canonical addresses, and this emulation it still incomplete.

Oleg.