Licensing issues (round 1)
The COPYING file at the top-level of the source tree indicates the whole thing is "LGPL v2.1 or later". Some parts (like most of the stuff in snprintf/) are under a BSD license, which can be relicensed to LGPL so no problems there.
However there are some other inconsistencies:
./include/lttng/ust-ctl.h: GPL (with incorrect FSF address)
./include/lttng/bitfield.h: MIT/X11 (BSD like)
./include/ust_snprintf.h: LGPL (v2.1 or later)
Most of these files have something custom that is similar to the MIT/X11 license, so it might be worth using the exact MIT/X11 text (the one in bitfield.h).
Some of the files also mention "LGPL v2.1 only", which is incompatible with "LGPL v2.1 or later". Either the whole thing has to be "2.1 or later" or the project can only be used under 2.1.
ust-ctl.h is (by mistake?) pure GPL, which is clearly incompatible.