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

[Mathieu Desnoyers]

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.

So we only support inserting uprobe on functions and SDT probes at
the moment.

Thanks,

Mathieu


> Thanks,
> 
> Brian
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-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com