Date: Fri, 11 Feb 2022 10:47:12 -0500 From: Mark Johnston <markj@freebsd.org> To: "Bjoern A. Zeeb" <bzeeb-lists@lists.zabbadoz.net> Cc: dtrace@freebsd.org Subject: Re: DTrace, kernel loader, unknown probes, enable on load? Message-ID: <YgaFAJ7XAmKxB3w8@nuc> In-Reply-To: <YgZxcxon9xJxEpbR@nuc> References: <alpine.BSF.2.00.2202111403340.68830@ai.fobar.qr> <YgZxcxon9xJxEpbR@nuc>
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On Fri, Feb 11, 2022 at 09:23:47AM -0500, Mark Johnston wrote:
> On Fri, Feb 11, 2022 at 02:10:30PM +0000, Bjoern A. Zeeb wrote:
> > Hi,
> >
> > one of the drawbacks of Dtrace (and other tracing frameworks out there
> > on various OSes) is that they do need a list of probes upfront before
> > they can enable.
>
> The probes don't have to exist, add -Z to the dtrace(1) parameters.
>
> > Say I want to trace a kernel module from the moment it is loaded, that
> > is currently not possible.
>
> So something like:
>
> # dtrace -n 'fbt::coretemp_identify:entry {stack();}' -Z
> # kldload coretemp
>
> ought to work, I think, but it doesn't. It's been a while since I
> looked at this code but I think it might be related to the unimplemented
> (on FreeBSD) portion of dtrace_probe_provide(). IIRC that's due to a
> lock order reversal...
I see now: the problem is that FBT registers probes after KLD SYSINITs
and module hooks are invoked. See dtrace_module_loaded(), which
(asynchronously) calls dtrace_enabling_matchall() to see if any newly
registered probes match pending "retained enablings", in this case,
enablings created by dtrace -Z. We could perhaps add a new eventhandler
that gets called before anything in the KLD is executed, and register
probes at that point. Or, if the code you're interested runs after
SYSINITs are finished, then -Z might be sufficient today.
> > What I am wondering is how hard it would be to "ignore" unknown probes
> > but enable them the moment on loading matching ones. I assume that
> > will probably need a callback through user space to complicate things
> > but it should not be impossible.
> >
> > In case anyone has some spare time to draft this up, I'll be happily a
> > first tester; otherwise the idea is out at least for someone to find
> > it one day ...
> >
> > Lots of health,
> > /bz
> >
> > --
> > Bjoern A. Zeeb r15:7
> >
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