[lttng-dev] [PATCH lttng-ust 6/8] Manually dlopen() liblttng-ust.so to prevent unloading

[Mathieu Desnoyers]

----- On Feb 8, 2018, at 3:24 PM, Francis Deslauriers francis.deslauriers at efficios.com wrote:

> 2018-02-07 15:59 GMT-05:00 Mathieu Desnoyers <mathieu.desnoyers at efficios.com>:
>> ----- On Feb 2, 2018, at 2:48 PM, Francis Deslauriers
>> francis.deslauriers at efficios.com wrote:
>>
>>> dlopen() increments the refcount of the library thus preventing the
>>> refcount to reach zero in the case of dlclose;
>>
>> The changelog and comment do not explain _why_ this is needed.
>>
>> The scenario is:
>>
>> - an application is _not_ linked against liblttng-ust.so
>> - the application dlopen() a tracepoint probe,
>> - the tracepoint probe .so is linked against liblttng-ust, and this is what ends
>> up
>>   getting lttng-ust loaded,
>>
>> Given that our goal is to allow dlclose() of tracepoint probes (new features),
>> we don't
>> want the side effect of this dlclose() to also try to unload liblttng-ust.
>> Because
>> liblttng-ust does _not_ support being dlclose'd, we need to "pin" it in memory
>> by
>> grabbing an extra reference on the library, with a NODELETE RTLD flag.
>>
> I agree, here is a revised version of the commit message and comment:
> 
> Manually dlopen() liblttng-ust.so to prevent unloading
> 
> The support of 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 the probe provider needs the
> liblttng-ust.so library to complete its own unloading. As we can't control the
> order in which libraries are unloaded, we must ensure that liblttng-ust.so is
> not unloaded before the probe provider. 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.

I don't think this was the reason *why* we could not dlclose() liblttng-ust.
But I do not recall how that fails exactly. Thoughts ?

Thanks,

Mathieu


> 
> 
> Thank you,
> Francis
> 
>> Thanks,
>>
>> Mathieu
>>
>>
>>>
>>> Signed-off-by: Francis Deslauriers <francis.deslauriers at efficios.com>
>>> ---
>>> configure.ac                  |  1 +
>>> liblttng-ust/Makefile.am      |  2 ++
>>> liblttng-ust/lttng-ust-comm.c | 22 ++++++++++++++++++++++
>>> 3 files changed, 25 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..ed912b8 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 +1667,23 @@ void __attribute__((constructor)) lttng_ust_init(void)
>>>       lttng_ust_loaded = 1;
>>>
>>>       /*
>>> +      * 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
> 
> 
> 
> --
> Francis Deslauriers
> Software developer
> EfficiOS inc.

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