On 2023-05-17 12:37, Brian Hutchinson wrote:
On Wed, May 17, 2023 at 12:08 PM Mathieu Desnoyers
<mathieu.desnoy...@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
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.
Hmm, was really hoping to be able to do dynamic tracing without having
to modify code.
uprobes with the proper validations about instruction boundaries would
eventually provide this. Another approach we want to invest time in is
to integrate libpatch from Olivier Dion into lttng-ust. This would
provide dynamic instrumentation with the performance of a purely
userspace tracer.
But those are all things that were never prioritized by any of our
customers, so they progress at a "back burner" pace.
I guess if I add a function call to a debug statement or something at
the point I want to probe then I could use the elf example.
Yes.
So we only support inserting uprobe on functions and SDT probes at
the moment.
I've heard of system tap but never used it. Will have to look into that.
I really want to get lttng-ust working but I'm getting pushback on the
time I'm spending trying to get it to work ... and would really like
to demonstrate something (was hoping kernel events and uprobes)
quickly to an audience that knows nothing about lttng or full stack
tracing to gain "buy in" for the effort.
Understood.
The main thing we are missing to help you on the UST front is a console
log of the _application_ with LTTNG_UST_DEBUG=1. I suspect it is not
collected in your tests.
Thanks,
Mathieu
You know, those pesky things called schedules.
Thanks,
Brian
Thanks,
Mathieu
Thanks,
Brian
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--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
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