Re: Copying TLS/user register data per perf-sample?

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 4 Apr 2024 12:26:41 -0700
Beau Belgrave <beaub@linux.microsoft.com> wrote:

> Hello,
> 
> I'm looking into the possibility of capturing user data that is pointed
> to by a user register (IE: fs/gs for TLS on x86/64) for each sample via
> perf_events.
> 
> I was hoping to find a way to do this similar to PERF_SAMPLE_STACK_USER.
> I think it could even use roughly the same ABI in the perf ring buffer.
> Or it may be possible by some kprobe linked to the perf sample function.
> 
> This would allow a profiler to collect TLS (or other values) on x64. In
> the Open Telemetry profiling SIG [1], we are trying to find a fast way
> to grab a tracing association quickly on a per-thread basis. The team
> at Elastic has a bespoke way to do this [2], however, I'd like to see a
> more general way to achieve this. The folks I've been talking with seem
> open to the idea of just having a TLS value for this we could capture
> upon each sample. We could then just state, Open Telemetry SDKs should
> have a TLS value for span correlation. However, we need a way to sample
> the TLS value(s) when a sampling event is generated.
> 
> Is this already possible via some other means? It'd be great to be able
> to do this directly at the perf_event sample via the ABI or a probe.
> 

Have you tried to use uprobes? It should be able to access user-space
registers including fs/gs.

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>