Date: Mon, 23 Jan 2023 11:52:12 -0500 From: Mark Johnston <markj@freebsd.org> To: Martin Cracauer <cracauer@cons.org> Cc: freebsd-dtrace@freebsd.org Subject: Re: DTrace - capturing two userspace strack frames on top of system call Message-ID: <Y867PBRdCiuyE%2BOX@nuc> In-Reply-To: <Y835UomAxqh3EwRY@cons.org> References: <Y83o15sbizWURbsr@cons.org> <Y835UomAxqh3EwRY@cons.org>
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On Sun, Jan 22, 2023 at 10:04:50PM -0500, Martin Cracauer wrote:
> Actually the error only appears on arm64 for me. I moved the script
> to amd64 and it works as I thought.
Support for userspace tracing on arm64 is definitely less mature than on
amd64, so this isn't too surprising, unfortunately.
Is the problem reproducible with a trivial program compiled with
-fno-omit-frame-pointer?
> Martin
>
> Martin Cracauer wrote on Sun, Jan 22, 2023 at 08:54:31PM -0500:
> > I want to capture the (userland) stack trace on top of the write(2)
> > system call. I seem to have some difficulty switching from kernel to
> > user mode here. For every write system call I want to print the
> > calling userlevel frames. I can't care whether they are individually
> > printed or counted.
> >
> > Here is what I think should do it:
> > syscall::write*:entry /arg1/ { @traces[ustack()] = count(); }
> >
> > However, I get one error each for each write call:
> > dtrace: error on enabled probe ID 2 (ID 56902:
> > syscall:freebsd:write:entry): invalid address (0x0) in action #2
> >
> > This gives the same error:
> > syscall::write*:entry /arg1/ { ustack(); }
> >
> >
> > %%
> >
> > If I use system stackframes it works, but of course it doesn't print
> > the calling frames:
> >
> > syscall::write*:entry /arg1/ { @traces[stack()] = count(); }
> >
> > dtrace: script 'stack-to-write.dtrace' matched 3 probes
> > dtrace: buffer size lowered to 2m
> > dtrace: aggregation size lowered to 2m
> > dtrace: pid 11790 has exited
> >
> >
> > kernel`handle_el0_sync+0x40
> > 136
> >
> > %%
> >
> > Is what I am trying to do even possible? Can I mix kernel and
> > userlevel space like this?
> >
> > Any other ideas? I could brute-force it with LD_PRELOAD overloading
> > of write(2), but dtrace would be more elegant.
> >
> > Thanks in advance
> > Martin
> > --
> > %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> > Martin Cracauer <cracauer@cons.org> http://www.cons.org/cracauer/
>
> --
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> Martin Cracauer <cracauer@cons.org> http://www.cons.org/cracauer/
>
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