Re: How to get system localtime/wall time from eBPF?

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Vincent Li <vincent.mc.li@gmail.com> writes:

> On Mon, Mar 20, 2023 at 2:57 PM Vincent Li <vincent.mc.li@gmail.com> wrote:
>>
>> On Mon, Mar 20, 2023 at 10:15 AM Jesper Dangaard Brouer
>> <jbrouer@redhat.com> wrote:
>> >
>> >
>> > On 20/03/2023 16.33, Vincent Li wrote:
>> > >
>> > > if I have a XDP based firewall to block ip  access based on system
>> > > localtime/wall time, is it still impossible like what mentioned here
>> > > https://github.com/xdp-project/xdp-tutorial/issues/204#issuecomment-819419800?
>> > > If so, is there any way to workaround this?
>> >
>> > You could use the BPF-helper named: bpf_ktime_get_tai_ns()
>> >
>> > See man clock_gettime(2). It is the same as CLOCK_TAI, which is
>> > (currently) offset with 37 sec to CLOCK_REALTIME which is wall-clock.
>> >
>
> sorry a follow-up question, the bpf_ktime_get_tai_ns will return time
> as nano seconds, right?
> I do not need the nanoseconds precision, I only need HH:MM like hours
> and minutes of the day, for example 10:30 = 10 * 60 + 30 is good
> enough,
>  any trick get that from bpf_ktime_get_tai_ns()

Populate the time at latest midnight from userspace when installing the
program and calculate offsets from that?