From: Steven Rostedt <rostedt@goodmis.org>
To: Peter Zijlstra <peterz@infradead.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 11:56:23 -0500 [thread overview]
Message-ID: <20210202115623.08e8164d@gandalf.local.home> (raw)
In-Reply-To: <YBmBu0c24RjNYFet@hirez.programming.kicks-ass.net>
On Tue, 2 Feb 2021 17:45:47 +0100
Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Feb 02, 2021 at 09:52:49AM -0500, Steven Rostedt wrote:
>
> > But from a handler, you could do:
> >
> > if (in_nmi())
> > return;
> > local_irq_save(flags);
> > /* Now you are safe from being re-entrant. */
>
> But that's an utter crap thing to do. That's like saying I don't care
> about my events, at which point you might as well not bother at all.
>
> And you can still do that, you just get less coverage today than you
> used to. You used to throw things under the bus, now you throw more
> under the bus. If you didn't care, I can't seem to find myself caring
> either.
NMIs are special, and they always have been. They shouldn't be doing much
anyway. If they are, then that's a problem.
But if you want to make the stack tracer work on all contexts, I'm happy to
take patches. I don't have time to work on it today.
>
> > Where as there's no equivalent in a NMI handler. That's what makes
> > kprobe/ftrace handlers different than NMI handlers.
>
> I don't see how.
>
> > > Also, given how everything can nest, it had better all be lockless
> > > anyway. You can get your regular function trace interrupted, which can
> > > hit a #DB, which can function trace, which can #BP which can function
> > > trace again which can get #NMI etc.. Many wonderfun nestings possible.
> >
> > I would call #DB an #BP handlers very special.
>
> They are, just like NMI is special, which is why they're classed
> together.
>
> > Question: Do #DB and #BP set "in_interrupt()"? Because the function tracer
> > has infrastructure to prevent recursion in the same context.
>
> Sure we _could_ do that, but then we get into the 'fun' problem of
> getting a breakpoint/int3 at random places and calling random code and
> having deadlocks because they take the same lock.
My question wasn't to have them do it, I was simply asking if they do. I
was assuming that they do not.
>
> There was very little that stopped that from happening.
>
> > That is, a
> > ftrace handler calls something that gets traced, the recursion protection
> > will detect that and prevent the handler from being called again. But the
> > recursion protection is interrupt context aware and lets the handler get
> > called again if the recursion happens from a different context:
>
> > If #DB and #BP do not change the in_interrupt() context, then the above
> > still will protect the ftrace handlers from recursion due to them.
>
> 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.
>
> All you need to do is put a breakpoint on a piece of code that holds a
> spinlock and a handler that takes the same spinlock.
>
> There was very little from stopping that.
>
> > That would require refactoring all the code that's been around since 2008.
>
> Because I couldn't tell why/if any of that was correct at all. #DB/#BP
> don't play by the normal rules. They're _far_ more NMI-like than they're
> IRQ-like due to ignoring IF.
I'm fine with #DB and #BP being a "in_nmi()", as they are probably even
more special than NMIs.
-- Steve
next prev parent reply other threads:[~2021-02-02 16:59 UTC|newest]
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 ` kprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()") 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 [this message]
2021-02-02 18:30 ` Peter Zijlstra
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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