Add jiffies builtin with bpf_jiffies64

[hyptrap]

Add jiffies builtin, one of the use case is to calculate the sched_clock indirectly within some old kernels.

Result:

bpftrace --unsafe -e 'BEGIN { system("grep jiffies: /proc/timer_list | head -1"); print(jiffies); exit() }'
Attaching 1 probe...
jiffies: 4325204698
4325204697

Checklist

• Language changes are updated in man/adoc/bpftrace.adoc and if needed in docs/reference_guide.md
• User-visible and non-trivial changes updated in CHANGELOG.md
• The new behaviour is covered by tests

[viktormalik]

Thanks a lot for implementing this!

Looks good, a couple of things are missing, though:

• detection if the helper is present in the kernel, it should go to BPFFeature and then be called from semantic analyser with a proper error message if the helper is missing,
• semantic analyser tests,
• runtime tests, although I'm not sure if we can come up with something useful here.

See e.g. my recent PR #2692 for inspiration how to implement the above points.

[hyptrap]

Thanks a lot for implementing this!

Looks good, a couple of things are missing, though:

* detection if the helper is present in the kernel, it should go to `BPFFeature` and then be called from semantic analyser with a proper error message if the helper is missing,

* semantic analyser tests,

* runtime tests, although I'm not sure if we can come up with something useful here.

See e.g. my recent PR #2692 for inspiration how to implement the above points.

I'm quite unfamiliar here, thanks for the advising! Pushed a new commit, fixed those things.

As for the runtime tests, I want to compare the jiffies between bpftrace and /proc/timer_list, but neither the test engine nor the bpftrace can achieve that easily (the output of system() call in bpftrace cannot be extracted, and putting the comparison outside bpftrace will cause a huge jiffies delay between them), so yes, nothing useful idea has came up 😶‍🌫️.

[viktormalik]

Please fix the problems reported by clang-format.

[viktormalik]

Thanks!

[danobi]

lgtm, thanks!

[danobi]

Thanks for the cleanup!

[viktormalik]

Could you squash the first two commits together? Thanks!

[hyptrap]

Could you squash the first two commits together? Thanks!

Done!