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

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Message-ID: <YBMBTsY1uuQb9wCP@hirez.programming.kicks-ass.net>
Date:   Thu, 28 Jan 2021 19:24:14 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nikolay Borisov <nborisov@...e.com>
Cc:     Masami Hiramatsu <masami.hiramatsu@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf@...r.kernel.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter()
 with nmi_enter()")

On Thu, Jan 28, 2021 at 06:45:56PM +0200, Nikolay Borisov wrote:
> it would be placed on the __fentry__ (and not endbr64) hence it works.
> So perhaps a workaround outside of bpf could essentially detect this
> scenario and adjust the probe to be on the __fentry__ and not preceding
> instruction if it's detected to be endbr64 ?

Arguably the fentry handler should also set the nmi context, it can,
after all, interrupt pretty much any other context by construction.