kprobe: fall back to tracefs if kernel doesn't support dots in PMU symbol

[chenhengqi]

Centos 8 (kernel 4.18) has kprobe PMU, but don't allow . in symbol name,
add a new option to workaround this.

[lmb]

@chenhengqi how does the centos kernel fail when . is in the name? In the best case we could add an if errors.Is(err, EFOO) case that enables the already existing tracefs fallback.

[chenhengqi]

@chenhengqi how does the centos kernel fail when . is in the name? In the best case we could add an if errors.Is(err, EFOO) case that enables the already existing tracefs fallback.

The version of kernel shipped in CentOS 8 Linux is 4.18.

In 4.18, the kernel don't allow dot in symbol name in kprobe PMU case:

https://github.com/torvalds/linux/blob/94710cac0ef4ee177a63b5227664b38c95bbf703/kernel/trace/trace_kprobe.c#L340-L343

Can we always fallback to tracefs interface ? Or check the error against EINVAL ?

[ti-mo]

No need for feature detection IMO, falling back under a specific error like Lorenz suggested sounds like the best option to me. Is this possible?

[chenhengqi]

OK, just updated the code as you suggest.

[ti-mo]

I've added a missing call to sanitizedSymbol() in closeTraceFSProbeEvent(). Closing a tracefs event created with a sanitized name would of course fail otherwise.

I've also converted TestK(ret)probe to be table-driven tests and found some kernel symbols with local visibility that were present in kernel versions ranging from 4.4 to 5.15, so this fallback behaviour can actually be exercised.

[chenhengqi]

Thanks.