[lttng-dev] dlopen, gcc -l and LD_PRELOAD

[Jérémie Galarneau]

On Fri, Mar 15, 2013 at 12:41 PM, Thibault, Daniel
<Daniel.Thibault at drdc-rddc.gc.ca> wrote:
>    In the lttng-ust man pages, there is a paragraph that reads:
>
> Note about dlopen() usage: Due to locking side-effects caused by the way libc lazily resolves Thread-Local Storage (TLS) symbols when a library is dlopened, linking the tracepoint probe or liblttng-ust with dlopen() is discouraged.  They should be linked with the application using "-llibname" or loaded with LD_PRELOAD.
>
>    I'm confused by the LD_PRELOAD bit: as far as I know, LD_PRELOAD serves only to substitute a dynamic library (the LD_PRELOAD argument) for another (the one the executable was linked against).  I don't see how one could compile an instrumented application and leave its tracepoint provider unlinked, which is what the stated -llibname <-> LD_PRELOAD opposition implies.
>

You may want to have a look at the comment at
lttng-ust/include/lttng/tracepoint.h:229 which explains how this is
accomplished. Basically, the tracepoint provider symbols are
initialised and the probe is registered when the provider's shared
object constructor is invoked. See ust-tracepoint-event.h:671 if you
are interested in the actual implementation.

>    (Also, shouldn't that paragraph drop 'lazily', assuming the locking problems also arise if dlopen is used with RTLD_NOW?  Because if RTLD_NOW fixed the locking problem, that's what would be recommended here, no?)

Good question. I think this problem has been fixed as part of the 2.1
release. However, I do know there are other issues related to
"dlopen()"-ing probe providers; details on the bug tracker[1].

Jérémie
[1] https://bugs.lttng.org/issues/447

>
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