Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()")

[Steven Rostedt]

On Wed, 3 Feb 2021 22:33:28 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> 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