[lttng-dev] Bug in LTTng-UST

[Philippe Proulx]

On Fri, Dec 12, 2014 at 11:21 PM, Simon Marchi <simon.marchi at polymtl.ca> wrote:
> Actually, I think the problem comes from UST not handling when a
> program dlcloses a lib. There is a simpler way to show the problem.
> Just have an app dlopen a .so that has a tracepoint and dlclose it.
> When the app quits, UST will try to unregister the tracepoint and
> read/write in the memory previously mapped to the library, causing a
> segfault.
>
> I have made a small example. You just need to click on "Download gist"
> in [1] to download a tarball, and a then simple make should be enough.
> Start a UST session with -u -a and run ./app. Segfault should ensue.
>
> [1] https://gist.github.com/simark/9fc0e2395899fcd8d10d

Don't you have to _preload_ the tracepoint provider in this context? Either
by dlopen()ing the tracepoint provider shared object manually, or by using
LD_PRELOAD?

Try this instead <https://gist.github.com/eepp/d280a27f5e7d0a64abaa>.
Look at the changes to Makefile, tracepoints.c and truc.c. Make, then
launch the app while tracing is started:

    ./app

Nothing is recorded: the tracepoint provider shared object is not loaded.
Now try preloading the tracepoint provider and launching the app:

    LD_PRELOAD=./tracepoints.so ./app

I get this:

    lttng stop
    lttng view
    ...
    [23:55:36.393480541] (+0.000074994) eeppdesk
sample_tracepoint:message: { cpu_id = 0 }, { intfield = 4 }

and no segfault.

See: <http://lttng.org/docs/#doc-building-tracepoint-providers-and-user-application>,
"Dynamic linking" section.

Phil

>
>
>
> On 12 December 2014 at 19:54, Matthew Khouzam
> <matthew.khouzam at ericsson.com> wrote:
>> Hi, it appears as though if a tracepoint is in a so, loading it,
>> unloading it and reloading it makes it no longer visible.
>>
>> To confirm this you can try out this repo :
>> https://github.com/TheMatthew/yquake2
>> (I know, it's not a serious program... but the bug is serious, I think)
>> You can get the demo files here
>> http://www.fileplanet.com/6584/0/fileinfo/Quake-2-Demo-v3.14
>> make the program (make from the git dir)
>> copy the contents of the baseq2 in the demo into the release/baseq2
>> directory
>>
>> start a ust session
>> run the program.
>> Ignore the baddr deadlocks :)
>> The tracepoint is not visible, it is located in the game.so file.
>>
>> Chances are though that the problem is on my side, but the evidence
>> (single registration ) seems to indicate that this is an actual problem.
>>
>>
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