Re: [syzbot] possible deadlock in ktime_get_coarse_ts64

Thomas Gleixner
2021-11-05T17:24:51Z

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