[lttng-dev] [PATCH lttng-ust v2 4/6] Manually dlopen() liblttng-ust.so to prevent unloading

[Mathieu Desnoyers]

Cleaned up this changelog, which has tabs (should be spaces), and has
lines above ~74 columns (which is odd in a git log).

Thanks,

Mathieu

----- On Feb 9, 2018, at 3:15 PM, Francis Deslauriers francis.deslauriers at efficios.com wrote:

> 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 at efficios.com>
> ---
> 
> v2:
> Rephrase commit message and comment
> ---
> 
> configure.ac                  |  1 +
> liblttng-ust/Makefile.am      |  2 ++
> liblttng-ust/lttng-ust-comm.c | 25 +++++++++++++++++++++++++
> 3 files changed, 28 insertions(+)
> 
> diff --git a/configure.ac b/configure.ac
> index b0b4157..4fc6f9c 100644
> --- 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 982be69..a7edfd5 100644
> --- 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 511b9cf..57e5ce6 100644
> --- 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 ?
>  */
> @@ -1648,6 +1652,7 @@ 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;
> @@ -1662,6 +1667,26 @@ 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
> 	 * sessiond (otherwise leading to errors when trying to create
> --
> 2.7.4

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com