pthread_cond_broadcast broken with dlsym? - c

Question

I am trying to interpose calls to pthread_cond_broadcast using LD_PRELOAD mechanism. My interposed pthread_cond_broadcast function just calls the original pthread_cond_broadcast. However, for a very simple pthread code where both pthread_cond_wait and pthread_cond_broadcast get invoked, I either end up with a segfault in glibc (for glibc 2.11.1) or the program hangs (for glibc 2.15). Any clues on that is going on?
The interposition code (that gets compiled as a shared library):
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <dlfcn.h>
static int (*orig_pthread_cond_broadcast)(pthread_cond_t *cond) = NULL;
__attribute__((constructor))
static void start() {
orig_pthread_cond_broadcast =
(int (*)()) dlsym(RTLD_NEXT, "pthread_cond_broadcast");
if (orig_pthread_cond_broadcast == NULL) {
printf("pthread_cond_broadcast not found!!!\n");
exit(1);
}
}
__attribute__((__visibility__("default")))
int pthread_cond_broadcast(pthread_cond_t *cond) {
return orig_pthread_cond_broadcast(cond);
}
The simple pthread program:
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
pthread_mutex_t cond_mutex;
pthread_cond_t cond_var;
int condition;
void *thread0_work(void *arg) {
pthread_mutex_lock(&cond_mutex);
printf("Signal\n");
condition = 1;
pthread_cond_broadcast(&cond_var);
pthread_mutex_unlock(&cond_mutex);
return NULL;
}
void *thread1_work(void *arg) {
pthread_mutex_lock(&cond_mutex);
while (condition == 0) {
printf("Wait\n");
pthread_cond_wait(&cond_var, &cond_mutex);
printf("Done waiting\n");
}
pthread_mutex_unlock(&cond_mutex);
return NULL;
}
int main() {
pthread_t thread1;
pthread_mutex_init(&cond_mutex, NULL);
pthread_cond_init(&cond_var, NULL);
pthread_create(&thread1, NULL, thread1_work, NULL);
// Slowdown this thread, so the thread 1 does pthread_cond_wait.
usleep(1000);
thread0_work(NULL);
pthread_join(thread1, NULL);
return 0;
}
EDIT:
For glibc 2.11.1, gdb bt gives:
(gdb) set environment LD_PRELOAD=./libintercept.so
(gdb) run
Starting program: /home/seguljac/intercept/main
[Thread debugging using libthread_db enabled]
[New Thread 0x7ffff7436700 (LWP 19165)]
Wait
Signal
Before pthread_cond_broadcast
Program received signal SIGSEGV, Segmentation fault.
0x00007ffff79ca0e7 in pthread_cond_broadcast##GLIBC_2.3.2 () from /lib/libpthread.so.0
(gdb) bt
#0 0x00007ffff79ca0e7 in pthread_cond_broadcast##GLIBC_2.3.2 () from /lib/libpthread.so.0
#1 0x00007ffff7bdb769 in pthread_cond_broadcast () from ./libintercept.so
#2 0x00000000004008e8 in thread0_work ()
#3 0x00000000004009a4 in main ()
EDIT 2:
(Solved)
As suggested by R.. (thanks!), the issue is that on my platform pthread_cond_broadcast is a versioned symbol, and dlsym gives the wrong version. This blog explains this situation in great detail: http://blog.fesnel.com/blog/2009/08/25/preloading-with-multiple-symbol-versions/

Answer 1

The call through your function seems to end up in a different version of the function:
With LD_PRELOAD: __pthread_cond_broadcast_2_0 (cond=0x804a060) at old_pthread_cond_broadcast.c:37
Without LD_PRELOAD: pthread_cond_broadcast##GLIBC_2.3.2 () at ../nptl/sysdeps/unix/sysv/linux/i386/i686/../i486/pthread_cond_broadcast.S:39
So your situation is similar to this question, i.e. you are getting incompatible versions of pthread functions: symbol versioning and dlsym
This page gives one way to solve the problem, though a bit complex: http://blog.fesnel.com/blog/2009/08/25/preloading-with-multiple-symbol-versions/