From: Dmitrii Banshchikov <me@ubique.spb.ru>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
syzbot <syzbot+43fd005b5a1b4d10781e@syzkaller.appspotmail.com>,
John Stultz <john.stultz@linaro.org>,
LKML <linux-kernel@vger.kernel.org>,
sboyd@kernel.org,
syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
Peter Zijlstra <peterz@infradead.org>,
Mark Rutland <mark.rutland@arm.com>,
Steven Rostedt <rosted@goodmis.org>,
Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>, bpf <bpf@vger.kernel.org>
Subject: Re: [syzbot] possible deadlock in ktime_get_coarse_ts64
Date: Sun, 7 Nov 2021 00:07:33 +0400 [thread overview]
Message-ID: <20211106200733.meank7oonwvsdjy4@amnesia> (raw)
In-Reply-To: <875yt64isx.ffs@tglx>
On Fri, Nov 05, 2021 at 06:24:30PM +0100, Thomas Gleixner wrote:
> On Fri, Nov 05 2021 at 21:03, Dmitrii Banshchikov wrote:
> > On Fri, Nov 05, 2021 at 08:53:06AM -0700, Alexei Starovoitov wrote:
> >> > Timestamps from within a tracepoint can only be taken with:
> >> >
> >> > 1) jiffies
> >> > 2) sched_clock()
> >> > 3) ktime_get_*_fast_ns()
> >> >
> >> > Those are NMI safe and can be invoked from anywhere.
> >> >
> >> > All other time getters which have to use the timekeeping seqcount
> >> > protection are prone to live locks and _cannot_ be used from
> >> > tracepoints ever.
> >>
> >> Obviously.
> >> That helper was added for networking use cases and accidentally
> >> enabled for tracing.
> >
> > Sorry for that.
> > I'm preparing a patch that will forbid using bpf_ktime_get_coarse_ns()
> > helper in BPF_LINK_TYPE_RAW_TRACEPOINT.
>
> It cannot be used in TRACING and PERF_EVENT either. But those contexts
> have to exclude other functions as well:
>
> bpf_ktime_get_ns
> bpf_ktime_get_boot_ns
>
> along with
>
> bpf_spin_lock/unlock
> bpf_timer_*
1) bpf_ktime_get_ns and bpf_ktime_get_boot_ns use
ktime_get_{mono,boot}_fast_ns.
2) bpf_spin_lock/unlock have notrace attribute set.
3) bpf_timer_* helpers fail early if they are in NMI.
Why they have to be excluded?
--
Dmitrii Banshchikov
next prev parent reply other threads:[~2021-11-06 20:07 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-04 16:18 [syzbot] possible deadlock in ktime_get_coarse_ts64 syzbot
2021-11-05 13:10 ` Thomas Gleixner
2021-11-05 13:21 ` Steven Rostedt
2021-11-05 15:53 ` Alexei Starovoitov
2021-11-05 17:03 ` Dmitrii Banshchikov
2021-11-05 17:24 ` Thomas Gleixner
2021-11-06 20:07 ` Dmitrii Banshchikov [this message]
2021-11-07 10:32 ` Thomas Gleixner
2021-11-07 13:51 ` Dmitrii Banshchikov
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