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

[lttng-ust.git] / liblttng-ust / lttng-ust-comm.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