[lttng-dev] Feature request: dynamically load from prefix

[Norbert Lange]

Hello,

The production rootfs should be untouched, ideally read-only,
for development/tests a subdirectory can be mounted (eg. /usr/local).
Idea is that the contents of that directory alone (and at most some
env variables)
should allow enabling development features.

For lttng I would have wanted to add a library
'/usr/local/lib/libmyservice-tracepoints.so' with runpath
'/usr/local/lib' that would activate lttng tracing,
pulling in lttng libraries (ust, ust-tracepoint) from /usr/local/lib.

There is a caveat though, unless 'libmyservice-tracepoints.so'' is
preloaded, the code in lttng/tracepoint.h will run constructor functions
to register the tracepoint probes, trying to dlopen the lttng-ust-tracepoint
library and fail at that because this is not in the library search paths.

At a later time,  'libmyservice-tracepoints.so'' will be loaded, and
lttng-ust-tracepoint (along with lttng-ust) can be resolved. but the
tracepoints are not registered.

So I guess what I would need is to either retrigger the registration
of tracepoints
(likely complicated with static and weak symbols potentially causing a mess), or
redirect the dlopen function.
Useful would be either try to find the library in /usr/local/lib or
use '/usr/local/lib/libmyservice-tracepoints.so''
instead of  lttng-ust-tracepoint (both have (dis)advantages).

At any rate, I would welcome some customization macro.

For illustration the current hack-around is following

Norbert Lange

#define TRACEPOINT_DEFINE
#define TRACEPOINT_PROBE_DYNAMIC_LINKAGE

#include <dlfcn.h>

static inline void *s_remap_dlopen(const char *localfilename, int
flags, unsigned prelen) {
    void *md = (dlopen)(localfilename + prelen, flags);
    return md ? md : (dlopen)(localfilename, flags);
}

# ideally this would be LTTNG_TRACEPOINT_PROBE_DLOPEN instead of the dlopen mess
#define dlopen(x, y) s_remap_dlopen("/usr/local/lib/" x, y,
(unsigned)sizeof("/usr/local/lib/") - 1U)

#include "trace_mytracepoints.h"