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From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Nikolay Borisov <nborisov@suse.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>, bpf <bpf@vger.kernel.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()")
Date: Tue, 2 Feb 2021 19:30:34 +0100
Message-ID: <YBmaStZn9XEU0QE+@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210202115623.08e8164d@gandalf.local.home>

On Tue, Feb 02, 2021 at 11:56:23AM -0500, Steven Rostedt wrote:

> NMIs are special, and they always have been. They shouldn't be doing much
> anyway. If they are, then that's a problem.

There is a fair amount of NMI level code these days, and it seems to be
ever increasing...

> My question wasn't to have them do it, I was simply asking if they do. I
> was assuming that they do not.

per nmi_enter() we do:

  __preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);       \

> > But it doesn't help with:
> > 
> > 	spin_lock_irq(&foo); // task context
> > 	#DB
> > 	  spin_lock_irq(&foo); // interrupt context per your above
> 
> The statement above said:
> 
>  "If #DB and #BP do not change the in_interrupt() context"
> 
> Which would make the above be in the same context and the handler would
> not be called for the #DB case.

But then replace the above #DB with __fentry__ and explain how it is
fundamentally different? And consider that direct call into god knows
what code option you have. That doesn't go past any recursion checks
IIRC.

> I'm fine with #DB and #BP being a "in_nmi()", as they are probably even
> more special than NMIs.

That does mean that kprobes are then fundamentally running from
in_nmi(), which is what started all this.

Sure, the opt-probes and ftrace-probes don't actually have in_nmi() set
today (because they don't trigger an exception), but given that that is
all optional, any kprobe handler had better be in_nmi() clean.

  reply index

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <25cd2608-03c2-94b8-7760-9de9935fde64@suse.com>
     [not found] ` <20210128001353.66e7171b395473ef992d6991@kernel.org>
     [not found]   ` <20210128002452.a79714c236b69ab9acfa986c@kernel.org>
     [not found]     ` <a35a6f15-9ab1-917c-d443-23d3e78f2d73@suse.com>
     [not found]       ` <20210128103415.d90be51ec607bb6123b2843c@kernel.org>
2021-01-28  3:38         ` Masami Hiramatsu
2021-01-28  7:11           ` Nikolay Borisov
2021-01-28 16:12           ` Nikolay Borisov
2021-01-28 16:45             ` Nikolay Borisov
2021-01-28 16:50               ` Josh Poimboeuf
2021-01-28 21:52                 ` [PATCH] x86: Disable CET instrumentation in the kernel Josh Poimboeuf
2021-01-29  6:23                   ` Nikolay Borisov
2021-01-29 10:21                   ` Borislav Petkov
     [not found]                     ` <20210129151034.iba4eaa2fuxsipqa@treble>
2021-01-29 16:30                       ` Borislav Petkov
2021-01-29 16:49                         ` Josh Poimboeuf
2021-01-29 16:54                           ` Nikolay Borisov
2021-01-29 17:03                             ` Josh Poimboeuf
2021-01-29 17:07                               ` Borislav Petkov
2021-01-29 17:58                                 ` Seth Forshee
2021-01-28 18:24               ` kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()") Peter Zijlstra
2021-01-29  1:34                 ` Alexei Starovoitov
2021-01-29  6:36                   ` Nikolay Borisov
     [not found]                   ` <YBPNyRyrkzw2echi@hirez.programming.kicks-ass.net>
     [not found]                     ` <20210129224011.81bcdb3eba1227c414e69e1f@kernel.org>
     [not found]                       ` <20210129105952.74dc8464@gandalf.local.home>
2021-01-29 16:24                         ` Peter Zijlstra
2021-01-29 17:45                           ` Alexei Starovoitov
2021-01-29 17:59                             ` Peter Zijlstra
2021-01-29 19:01                               ` Steven Rostedt
2021-01-29 21:05                                 ` Alexei Starovoitov
2021-01-30  1:41                                   ` Masami Hiramatsu
2021-01-29 21:24                                 ` Steven Rostedt
2021-01-30  8:28                                   ` Peter Zijlstra
2021-01-30 12:44                                     ` Steven Rostedt
2021-02-02 10:45                                       ` Peter Zijlstra
2021-02-02 14:52                                         ` Steven Rostedt
2021-02-02 16:45                                           ` Peter Zijlstra
2021-02-02 16:56                                             ` Steven Rostedt
2021-02-02 18:30                                               ` Peter Zijlstra [this message]
2021-02-02 21:05                                                 ` Steven Rostedt
2021-02-03 13:33                                                   ` Masami Hiramatsu
2021-02-03 13:52                                                     ` Steven Rostedt
2021-01-30  2:02                               ` Masami Hiramatsu
2021-01-30  3:08                                 ` Alexei Starovoitov
2021-01-30 12:10                                   ` Masami Hiramatsu

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