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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Dmitrii Banshchikov <me@ubique.spb.ru>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     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
In-Reply-To: <20211105170328.fjnzr6bnbca7mdfq@amnesia>
References: <00000000000013aebd05cff8e064@google.com> <87lf224uki.ffs@tglx>
 <CAADnVQLcuMAr3XMTD1Lys5S5ybME4h=NL3=adEwib2UT6b-E9w@mail.gmail.com>
 <20211105170328.fjnzr6bnbca7mdfq@amnesia>
Date:   Fri, 05 Nov 2021 18:24:30 +0100
Message-ID: <875yt64isx.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
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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_*

Thanks,

        tglx