Record kernel samples

[DaniPopes]

Kernel samples are not recorded nor decoded during samply record. This PR is mainly two commits:

• 18611d3: enables PERF_SAMPLE_CALLCHAIN and decodes kernel stack frames from the callchain in the sample events
• 4ef1638: register kernel and modules during profiler setup, just like in perf, so that Converter is notified of the kernel symbols

Based on #734.

[ishitatsuyuki]

A surprising coincidence, but I have been working on something similar locally. I've found an issue with using the extents from /proc/modules. Details below:

When CONFIG_ARCH_WANTS_MODULES_DATA_IN_VMALLOC is enabled, .text and .data goes into separate memory regions. However /proc/modules reports start of .text as the basis but the module_total_size (code). So combining these two values will get you overlapping module ranges, and in samply overlapping module ranges will evict the previous one, causing symbolization to fail.

This probably needs to be taken upstream but until then I think we might need to clamp the module extents so that they don't overlap.

[ishitatsuyuki]

Sorry, I got confused. On x64, CONFIG_ARCH_WANTS_MODULES_DATA_IN_VMALLOC is not enabled, so it's not due to it. The correct explanation seems to be that data symbols precedes .text so using total size is still wrong.

[DaniPopes]

I pushed b8822c6, is this what you had in mind?

[ishitatsuyuki]

Thanks, I'll test when I'm back at desk.

[ishitatsuyuki]

Okay, the kernel module bases seem to resolve correctly, but symbolization for modules does not work. You probably need to parse module entries from kallsyms as well. Do you plan to do it? Otherwise I can see if I can pick up some of my local changes on top.

https://share.firefox.dev/4pPtAId

[ishitatsuyuki]

I quickly created a module symbol parser in 9eaf3fa. It seems to symbolize correctly on my end.

[DaniPopes]

Nice, thanks, cherry-picked it

[mstange]

I'm curious how you're running samply in a way that gives you useful kernel symbols here. Did you change your /proc/sys/kernel/kptr_restrict setting? Or are you running samply as root?

[ishitatsuyuki]

On my end, I have both kptr_restrict and perf_event_paranoid relaxed. That allows reading kallsyms without root.

But in any case root should work.

https://github.com/torvalds/linux/blob/master/kernel/ksyms_common.c