[lttng-dev] Trying to understand use of lttng enable-event --kernel --userspace-probe=

[Brian Hutchinson]

On Wed, May 17, 2023 at 12:08 PM Mathieu Desnoyers
<mathieu.desnoyers at efficios.com> wrote:
>
> On 2023-05-16 22:11, Brian Hutchinson via lttng-dev wrote:
> > Hi,
> >
> > I'm trying to figure out how to use uprobes with lttng.
> >
> > I can't use a normal uprobe for a line number just using the address I
> > want to probe obtained from objdump?  As in:
> >
> > echo 'p /usr/local/bin/my_app:0x2c3a8' >>
> > /sys/kernel/debug/tracing/uprobe_events
> >
> > ... which isn't a function entry, it's just a line of code I want to probe on.
> >
> > This link says it has to be elf or sdt:
> > https://lttng.org/man/1/lttng-enable-event/v2.11/#doc-opt--userspace-probe
> >
> > So can I not probe on just a line of code by specifying an address???
> >
> > It doesn't look like these methods above will do what I'm wanting to
> > do.  I've tried to find examples of using enable-event --kernel
> > --userspace-probe= but there doesn't appear to be many.
> >
>
> There are examples here:
>
> https://lttng.org/docs/v2.13/#doc-enabling-disabling-events

I added a function that's called when the event I want to probe occurs.

I tried to enable the event with:
root at localhost:/usr/local/bin# lttng enable-event --kernel
--userspace-probe=/usr/local/bin/my_app:my_probe_event
Error: Missing event name(s).

... and:

root at localhost:/usr/local/bin# lttng enable-event --kernel
--userspace-probe=elf:/usr/local/bin/my_app:my_probe_event
Error: Missing event name(s).

I can do ${CROSS_COMPILE}objdump -d and verify that the symbol above
for the function I added is indeed there.

What am I doing wrong?

>
> Indeed inserting a lttng-modules uprobe within functions is not
> supported at the moment, mainly because we prefer to err towards safety
> and don't have the validation in place to prevent corrupting the
> program's instructions if an end user would try to insert a uprobe at an
> address which is not an instruction boundary.
>
> So we only support inserting uprobe on functions and SDT probes at
> the moment.
>
> Thanks,
>
> Mathieu
>
>
> > Thanks,
> >
> > Brian
> > _______________________________________________
> > lttng-dev mailing list
> > lttng-dev at lists.lttng.org
> > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>