override.c

/*****************************************************************************
 * override.c: overridden function calls for VLC media player
 *****************************************************************************
 * Copyright (C) 2010 Rémi Denis-Courmont
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License as published by
 * the Free Software Foundation; either version 2.1 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
 *****************************************************************************/

#ifdef HAVE_CONFIG_H
# include "config.h"
#endif

#include <stdbool.h>
#define MAX_ERRORS 5

void vlc_enable_override (void);

#if defined (__GNUC__) \
 && (defined (__ELF__) && !defined (__sun__))
/* Solaris crashes on printf("%s", NULL); which is legal, but annoying. */

#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dlfcn.h>
#include <pthread.h>
#ifdef HAVE_EXECINFO_H
# include <execinfo.h>
# include <unistd.h>
#endif
#ifdef NDEBUG
# undef HAVE_BACKTRACE
#endif

static bool override = false;

static void vlc_reset_override (void)
{
    override = false;
}

void vlc_enable_override (void)
{
    override = true;
    pthread_atfork (NULL, NULL, vlc_reset_override);
}

static void vlogbug (unsigned *pc, const char *level, const char *func,
                     const char *fmt, va_list ap)
{
#ifdef HAVE_BACKTRACE
    const size_t framec = 5;
    void *framev[framec];

    backtrace (framev, framec);
#endif
    flockfile (stderr);
    if (*pc < MAX_ERRORS)
    {
        (*pc)++;
        fprintf (stderr, "%s: call to %s(", level, func);
        vfprintf (stderr, fmt, ap);
        fputs (")\n", stderr);
        fflush (stderr);
#ifdef HAVE_BACKTRACE
        backtrace_symbols_fd (framev + 2, framec - 2, STDERR_FILENO);
#endif
    }
    funlockfile (stderr);
}

static void logbug (unsigned *pc, const char *level, const char *func,
                    const char *fmt, ...)
{
    va_list ap;

    va_start (ap, fmt);
    vlogbug (pc, level, func, fmt, ap);
    va_end (ap);
}

static void *getsym (const char *name)
{
    void *sym = dlsym (RTLD_NEXT, name);
    if (sym == NULL)
    {
        fprintf (stderr, "Cannot resolve symbol %s: %s\n", name,
                 dlerror ());
        abort ();
    }
    return sym;
}

#define LOG(level, ...) \
    do { \
        static unsigned counter = 0; \
        logbug(&counter, level, __func__, __VA_ARGS__); \
    } while (0)

/* Evil non-standard GNU C macro ;)
 *  typeof keyword,
 *  statement-expression
 */
#define CALL(func, ...) \
({ typeof (func) *sym = getsym ( # func); sym (__VA_ARGS__); })

/*** Environment ***
 *
 * "Conforming multi-threaded applications shall not use the environ variable
 *  to access or modify any environment variable while any other thread is
 *  concurrently modifying any environment variable." -- POSIX.
 *
 * Some evil libraries modify the environment. We currently ignore the calls as
 * they could crash the process. This may cause funny behaviour though. */
int putenv (char *str)
{
    if (override)
    {
        LOG("Blocked", "\"%s\"", str);
        return 0;
    }
    return CALL(putenv, str);
}

int setenv (const char *name, const char *value, int overwrite)
{
    if (override)
    {
        LOG("Blocked", "\"%s\", \"%s\", %d", name, value, overwrite);
        return 0;
    }
    return CALL(setenv, name, value, overwrite);
}

int unsetenv (const char *name)
{
    if (override)
    {
        LOG("Blocked", "\"%s\"", name);
        return 0;
    }
    return CALL(unsetenv, name);
}


/*** Pseudo random numbers ***
 *
 * The C PRNG is not thread-safe (and generally sucks, the POSIX 48-bits PRNG
 * is much better as a reproducible non-secure PRNG). To work around this, we
 * force evil callers to serialize. This makes the call safe, but fails to
 * preserve reproducibility of the number sequence (which usually does not
 * matter).
 **/
static struct
{
    pthread_mutex_t lock;
    unsigned int seed;
} prng = { PTHREAD_MUTEX_INITIALIZER, 0, };

void srand (unsigned int seed)
{
    pthread_mutex_lock (&prng.lock);
    LOG("Warning", "%d", seed);
    prng.seed = seed;
    pthread_mutex_unlock (&prng.lock);
}

int rand (void)
{
    int ret;

    pthread_mutex_lock (&prng.lock);
    LOG("Warning", "");
    ret = rand_r (&prng.seed);
    pthread_mutex_unlock (&prng.lock);
    return ret;
}


#if 0
/** Signals **/
#include <signal.h>

static bool blocked_signal (int num)
{
    switch (num)
    {
        case SIGINT:
        case SIGHUP:
        case SIGQUIT:
        case SIGTERM:
        case SIGPIPE:
        case SIGCHLD:
            return true;
    }
    return false;
}

void (*signal (int signum, void (*handler) (int))) (int)
{
    if (override)
    {
        if (handler != SIG_IGN && handler != SIG_DFL)
            goto error;
        if (!blocked_signal (signum))
            goto error;
        /* For our blocked signals, the handler won't matter much... */
        if (handler == SIG_DFL)
            LOG("Warning", "%d, SIG_DFL", signum, handler);
    }
    return CALL(signal, signum, handler);
error:
    LOG("Blocked", "%d, %p", signum, handler);
    return SIG_DFL;
}

int sigaction (int signum, const struct sigaction *act, struct sigaction *old)
{
    if (override && act != NULL)
    {
        if ((act->sa_flags & SA_SIGINFO)
         || (act->sa_handler != SIG_IGN && act->sa_handler != SIG_DFL))
            goto error;
        if (act->sa_handler == SIG_DFL)
            LOG("Warning", "%d, %p, SIG_DFL", signum, act);
    }
    return CALL(sigaction, signum, act, old);
error:
    LOG("Blocked", "%d, %p, %p", signum, act, old);
    return -1;
}
#endif


/*** Locales ***
 * setlocale() is not thread-safe and has a tendency to crash other threads as
 * quite many libc and libintl calls depend on the locale.
 * Use uselocale() instead for thread-safety.
 */
#include <locale.h>

char *setlocale (int cat, const char *locale)
{
    if (override && locale != NULL)
    {
        LOG("Blocked", "%d, \"%s\"", cat, locale);
        return NULL;
    }
    return CALL(setlocale, cat, locale);
}


/* strerror() is not thread-safe in theory (POSIX), nor in practice (glibc).
 * This caused quite nasty crashes in the history of VLC/Linux. */
char *strerror (int val)
{
    if (override)
    {
        static const char msg[] =
            "Error message unavailable (use strerror_r instead of strerror)!";
        LOG("Blocked", "%d", val);
        return (char *)msg;
    }
    return CALL(strerror, val);
}

/*** Xlib ****/
#ifdef HAVE_X11_XLIB_H
# include <X11/Xlib.h>

static pthread_mutex_t xlib_lock = PTHREAD_MUTEX_INITIALIZER;

int (*XSetErrorHandler (int (*handler) (Display *, XErrorEvent *)))
     (Display *, XErrorEvent *)
{
    if (override)
    {
        int (*ret) (Display *, XErrorEvent *);

        pthread_mutex_lock (&xlib_lock);
        LOG("Error", "%p", handler);
        ret = CALL(XSetErrorHandler, handler);
        pthread_mutex_unlock (&xlib_lock);
        return ret;
    }
    return CALL(XSetErrorHandler, handler);
}

int (*XSetIOErrorHandler (int (*handler) (Display *))) (Display *)
{
    if (override)
    {
        int (*ret) (Display *);

        pthread_mutex_lock (&xlib_lock);
        LOG("Error", "%p", handler);
        ret = CALL(XSetIOErrorHandler, handler);
        pthread_mutex_unlock (&xlib_lock);
        return ret;
    }
    return CALL(XSetIOErrorHandler, handler);
}
#endif
#else
void vlc_enable_override (void)
{
}
#endif