linux-kernel - Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi

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Date:   Wed, 3 Feb 2021 08:52:32 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Nikolay Borisov <nborisov@...e.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter()
 with nmi_enter()")

On Wed, 3 Feb 2021 22:33:28 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> Ah, that is what I worried about. ftrace and kprobes handler usually want to
> know "what is the actual status of the system where the probe hits".
> 
> If the new kernel_exception_enter() for ftrace/kprobes or any other kernel
> instrumention does
> 
>   __preempt_count_add(KEX_OFFSET + NMI_OFFSET + HARDIRQ_OFFSET);
> 
> And we can distinguish the KEX from NMI, and get the original status of the context.
> What would you think about?

Oh, that reminds me about the obvious difference between an NMI and a
ftrace handler. A ftrace handler doesn't disable interrupts nor
preemption. Thus, if you set "in_nmi" to a ftrace handler, and an
interrupt (or NMI) comes in, then any ftrace handlers called by the
interrupt / NMI will be ignored, since it will think it is recursing
from NMI context.

-- Steve