dlopen() liblttng-ust.so from constructor to prevent unloading

The support of probe provider dlclose() allows for the following
problematic scenario:
- Application is not linked against the liblttng-ust.so
- Application dlopen() a probe provider library that is linked against
  liblttng-ust.so
- Application dlclose() the probe provider

In this scenario, the probe provider has a dependency on
liblttng-ust.so, so when it's loaded by the application, liblttng-ust.so
is loaded too. The probe provider library now has the only reference to
the liblttng-ust.so library.  When the application calls dlclose() on
it, all its references are dropped, thus triggering the unloading of
both the probe provider library and liblttng-ust.so.

This scenario is problematic because lttng ust_listener_threads are in
DETACHED state. We cannot join them and therefore we cannot unload the
library containing the code they run. Only the operating system can free
those resources.

The reason why those threads are in DETACHED state is to quickly
teardown applications on process exit.

A possible solution to investigate: if we can determine whether
liblttng-ust.so is being dlopen (directly or undirectly) or it's linked
against the application, we could set the detached state accordingly.

To prevent that unloading, we pin it in memory by grabbing an extra
reference on the library, with a RTLD_NODELETE flag. This will prevent
the dynamic loader from ever removing the liblttng-ust.so library from
the process' address space.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/configure.ac b/configure.ac
index b0b415729e23a6bef5c4ee48310c4b69c08c3d5e..4fc6f9ca62d9fe39752d59004cfa94d3c9842696 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -25,6 +25,7 @@ m4_define([UST_LIB_V_MINOR], [0])
 m4_define([UST_LIB_V_PATCH], [0])
 
 AC_SUBST([LTTNG_UST_LIBRARY_VERSION], [UST_LIB_V_MAJOR:UST_LIB_V_MINOR:UST_LIB_V_PATCH])
+AC_SUBST([LTTNG_UST_LIBRARY_VERSION_MAJOR], [UST_LIB_V_MAJOR])
 # note: remember to update tracepoint.h dlopen() to match this version
 # number. TODO: eventually automate by exporting the major number.
 

diff --git a/liblttng-ust/Makefile.am b/liblttng-ust/Makefile.am
index 982be69b7dd45157458b78a758c54d4168349c74..a7edfd5c30291562d4ebb39a56674d5cfb78e385 100644 (file)
--- a/liblttng-ust/Makefile.am
+++ b/liblttng-ust/Makefile.am
@@ -60,6 +60,8 @@ liblttng_ust_runtime_la_SOURCES = \
        string-utils.c \
        string-utils.h
 
+liblttng_ust_runtime_la_CFLAGS = -DLTTNG_UST_LIBRARY_VERSION_MAJOR=\"$(LTTNG_UST_LIBRARY_VERSION_MAJOR)\"
+
 if HAVE_PERF_EVENT
 liblttng_ust_runtime_la_SOURCES += \
        lttng-context-perf-counters.c \

diff --git a/liblttng-ust/lttng-ust-comm.c b/liblttng-ust/lttng-ust-comm.c
index 0cf5ecd9687536be5fc7794c4bfeb28ed135a11f..43914465d30a64045c448574290866f577715f30 100644 (file)
--- a/liblttng-ust/lttng-ust-comm.c
+++ b/liblttng-ust/lttng-ust-comm.c
@@ -27,6 +27,7 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/wait.h>
+#include <dlfcn.h>
 #include <fcntl.h>
 #include <unistd.h>
 #include <errno.h>
@@ -59,6 +60,9 @@
 #include "../libringbuffer/getcpu.h"
 #include "getenv.h"
 
+/* Concatenate lttng ust shared library name with its major version number. */
+#define LTTNG_UST_LIB_SO_NAME "liblttng-ust.so." LTTNG_UST_LIBRARY_VERSION_MAJOR
+
 /*
  * Has lttng ust comm constructor been called ?
  */
@@ -1685,6 +1689,7 @@ void __attribute__((constructor)) lttng_ust_init(void)
        pthread_attr_t thread_attr;
        int timeout_mode;
        int ret;
+       void *handle;
 
        if (uatomic_xchg(&initialized, 1) == 1)
                return;
@@ -1698,6 +1703,26 @@ void __attribute__((constructor)) lttng_ust_init(void)
 
        lttng_ust_loaded = 1;
 
+       /*
+        * We need to ensure that the liblttng-ust library is not unloaded to avoid
+        * the unloading of code used by the ust_listener_threads as we can not
+        * reliably know when they exited. To do that, manually load
+        * liblttng-ust.so to increment the dynamic loader's internal refcount for
+        * this library so it never becomes zero, thus never gets unloaded from the
+        * address space of the process. Since we are already running in the
+        * constructor of the LTTNG_UST_LIB_SO_NAME library, calling dlopen will
+        * simply increment the refcount and no additionnal work is needed by the
+        * dynamic loader as the shared library is already loaded in the address
+        * space. As a safe guard, we use the RTLD_NODELETE flag to prevent
+        * unloading of the UST library if its refcount becomes zero (which should
+        * never happen). Do the return value check but discard the handle at the
+        * end of the function as it's not needed.
+        */
+       handle = dlopen(LTTNG_UST_LIB_SO_NAME, RTLD_LAZY | RTLD_NODELETE);
+       if (!handle) {
+               ERR("dlopen of liblttng-ust shared library (%s).", LTTNG_UST_LIB_SO_NAME);
+       }
+
        /*
         * We want precise control over the order in which we construct
         * our sub-libraries vs starting to receive commands from