LD_PRELOAD problems with pthread_create

Question

I was just playing around with the concept of LD_PRELOAD. Seems like it works fine until I start to use the pthread library functions in my code. When I do that, I get segmentation fault. Does LD_PRELOAD has some kind of aversion to the pthread library or what?

Shown here are both program and LD_PRELOADed .so code.

Program Code

// main.c
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/types.h>

#define USE_PTHREADS

#define NUM_THREADS     8

pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;

int sum;
float total = 1;
extern int __did_libc_start_main;

void *PrintHello(void *threadid)
{
   long tid;
   tid = (long)threadid;
   pthread_mutex_lock( &m );
   sum++;
   total *= total + tid * 0.097891313423578;
   printf( "p%d, tid%d, total = %g, start = %d!\n", getpid(), tid, 
      total, 0 );
   pthread_mutex_unlock( &m );
   printf("Hello World! It's me, thread #%ld!\n", tid);
   pthread_exit(NULL);
}

int main (int argc, char *argv[])
{
   pthread_t threads[NUM_THREADS];
   int rc;
   long t;
   char * p;
   char * m;

   fork();

   p = (char*)mmap(0, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, 
     -1, 0);
   p[0] = 78;
   printf( "p = %p, p[0] = %d, pid = %d!\n", p, p[0], getpid() );
   m = (char*)malloc( 80 );
   printf( "m = %p!\n", m );
#ifdef USE_PTHREADS // If we disable this part of code, LD_PRELOAD works fine
   for(t=0; t<NUM_THREADS; t++)
   {
      printf("In main: creating thread %ld\n", t);
      rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
      if (rc){
         printf("ERROR; return code from pthread_create() is %d\n", rc);
         exit(-1);
      }
   }
   for(t=0; t<NUM_THREADS; t++)
    pthread_join(threads[t], NULL);

   printf( "\n\nTotal = %g\n\n", total );

   /* Last thing that main() should do */
   pthread_exit(NULL);
#endif
   printf( "\n\n%d: Done without major problems\n\n", getpid() );
   return 0;
}

LD_PRELOADed .so code

// prelib.c
#define _GNU_SOURCE

//#include <sys/syscall.h>
//#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <dlfcn.h>

typedef pid_t (*getpidType)(void);
typedef void* (*mmapType)(void *addr, size_t len, int prot, int flags,
       int fildes, off_t off);

pid_t getpid(void)
{
    static int first_time = 1;
    static getpidType f;

    printf("Hello, getpid!\n");

    if ( first_time )
    {
        f = (getpidType)dlsym(RTLD_NEXT, "getpid");
        first_time = 0;
    }

    return f();
}
void *mmap(void *addr, size_t len, int prot, int flags,
       int fildes, off_t off)
{
    static int first_time = 1;
    static mmapType f;

    printf( "My mmap!\n" );
    if ( first_time )
    {
        f = (mmapType)dlsym(RTLD_NEXT, "mmap" );
        first_time = 0;
    }

    return f( addr, len, prot, flags, fildes, off );
}

I get an error like this when USE_PTHREADS is defined.

My mmap!
<< mutex_trylock >>
<< mutex_unlock >>
Hello, getpid!
p = 0x2ab9bc969000, p[0] = 78, pid = 9763!
m = 0x179d4010!
In main: creating thread 0
My mmap!
<< mutex_trylock >>
My mmap!
<< mutex_trylock >>
<< mutex_unlock >>
Hello, getpid!
p = 0x2ab9bc969000, p[0] = 78, pid = 9762!
m = 0x179d4010!
In main: creating thread 0
My mmap!
<< mutex_trylock >>
Segmentation fault

Answer 1

I doubt this is your only problem, but your "first_time" hack is not synchronized and thus not thread-safe. You should be using the pthread_once function for this purpose, or roll your own version with a mutex or semaphore.

Answer 2

Only thing I can imagine is:

printf( "p%d, tid%d, total = %g, start = %d!\n", getpid(), tid, total, 0 );

should be tid%ld

Maybe you are running on 64 bit and longs are not int sized. So then when it goes to print the float, it sees crap and you are crashing printf.