OpenSSL and multi-threads - windows

Question

I've been reading about the requirement that if OpenSSL is used in a multi-threaded application, you have to register a thread identification function (and also a mutex creation function) with OpenSSL.
On Linux, according to the example provided by OpenSSL, a thread is normally identified by registering a function like this:
static unsigned long id_function(void){
return (unsigned long)pthread_self();
}
pthread_self() returns a pthread_t, and this works on Linux since pthread_t is just a typedef of unsigned long.
On Windows pthreads, FreeBSD, and other operating systems, pthread_t is a struct, with the following structure:
struct {
void * p; /* Pointer to actual object */
unsigned int x; /* Extra information - reuse count etc */
}
This can't be simply cast to an unsigned long, and when I try to do so, it throws a compile error. I tried taking the void *p and casting that to an unsigned long, on the theory that the memory pointer should be consistent and unique across threads, but this just causes my program to crash a lot.
What can I register with OpenSSL as the thread identification function when using Windows pthreads or FreeBSD or any of the other operating systems like this?
Also, as an additional question:
Does anyone know if this also needs to be done if OpenSSL is compiled into and used with QT, and if so how to register QThreads with OpenSSL? Surprisingly, I can't seem to find the answer in QT's documentation.

Answer 1

I will just put this code here. It is not panacea, as it doesn't deal with FreeBSD, but it is helpful in most cases when all you need is to support Windows and and say Debian. Of course, the clean solution assumes usage of CRYPTO_THREADID_* family introduced recently. (to give an idea, it has a CRYPTO_THREADID_cmp callback, which can be mapped to pthread_equal)
#include <pthread.h>
#include <openssl/err.h>
#if defined(WIN32)
#define MUTEX_TYPE HANDLE
#define MUTEX_SETUP(x) (x) = CreateMutex(NULL, FALSE, NULL)
#define MUTEX_CLEANUP(x) CloseHandle(x)
#define MUTEX_LOCK(x) WaitForSingleObject((x), INFINITE)
#define MUTEX_UNLOCK(x) ReleaseMutex(x)
#define THREAD_ID GetCurrentThreadId()
#else
#define MUTEX_TYPE pthread_mutex_t
#define MUTEX_SETUP(x) pthread_mutex_init(&(x), NULL)
#define MUTEX_CLEANUP(x) pthread_mutex_destroy(&(x))
#define MUTEX_LOCK(x) pthread_mutex_lock(&(x))
#define MUTEX_UNLOCK(x) pthread_mutex_unlock(&(x))
#define THREAD_ID pthread_self()
#endif
/* This array will store all of the mutexes available to OpenSSL. */
static MUTEX_TYPE *mutex_buf=NULL;
static void locking_function(int mode, int n, const char * file, int line)
{
if (mode & CRYPTO_LOCK)
MUTEX_LOCK(mutex_buf[n]);
else
MUTEX_UNLOCK(mutex_buf[n]);
}
static unsigned long id_function(void)
{
return ((unsigned long)THREAD_ID);
}
int thread_setup(void)
{
int i;
mutex_buf = malloc(CRYPTO_num_locks() * sizeof(MUTEX_TYPE));
if (!mutex_buf)
return 0;
for (i = 0; i < CRYPTO_num_locks( ); i++)
MUTEX_SETUP(mutex_buf[i]);
CRYPTO_set_id_callback(id_function);
CRYPTO_set_locking_callback(locking_function);
return 1;
}
int thread_cleanup(void)
{
int i;
if (!mutex_buf)
return 0;
CRYPTO_set_id_callback(NULL);
CRYPTO_set_locking_callback(NULL);
for (i = 0; i < CRYPTO_num_locks( ); i++)
MUTEX_CLEANUP(mutex_buf[i]);
free(mutex_buf);
mutex_buf = NULL;
return 1;
}

Answer 2

I only can answer the Qt part. Use QThread::currentThreadId(), or even QThread::currentThread() as the pointer value should be unique.

Answer 3

From the OpenSSL doc you linked:
threadid_func(CRYPTO_THREADID *id) is needed to record the currently-executing thread's identifier into id. The implementation of this callback should not fill in id directly, but should use CRYPTO_THREADID_set_numeric() if thread IDs are numeric, or CRYPTO_THREADID_set_pointer() if they are pointer-based. If the application does not register such a callback using CRYPTO_THREADID_set_callback(), then a default implementation is used - on Windows and BeOS this uses the system's default thread identifying APIs, and on all other platforms it uses the address of errno. The latter is satisfactory for thread-safety if and only if the platform has a thread-local error number facility.
As shown providing your own ID is really only useful if you can provide a better ID than OpenSSL's default implementation.
The only fail-safe way to provide IDs, when you don't know whether pthread_t is a pointer or an integer, is to maintain your own per-thread IDs stored as a thread-local value.