jsig.c

/*
 * Copyright (c) 2001, 2015, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code 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 General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 *
 */

/* CopyrightVersion 1.2 */

/* This is a special library that should be loaded before libc &
 * libthread to interpose the signal handler installation functions:
 * sigaction(), signal(), sigset().
 * Used for signal-chaining. See RFE 4381843.
 */

#include <signal.h>
#include <dlfcn.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>

#define MAXSIGNUM 32
#define MASK(sig) ((unsigned int)1 << sig)

static struct sigaction sact[MAXSIGNUM]; /* saved signal handlers */
static unsigned int jvmsigs = 0; /* signals used by jvm */

static pthread_key_t reentry_flag_key;
static pthread_once_t reentry_key_init_once = PTHREAD_ONCE_INIT;

/* used to synchronize the installation of signal handlers */
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
static pthread_t tid = 0;

typedef void (*sa_handler_t)(int);
typedef void (*sa_sigaction_t)(int, siginfo_t *, void *);
typedef sa_handler_t (*signal_t)(int, sa_handler_t);
typedef int (*sigaction_t)(int, const struct sigaction *, struct sigaction *);

static signal_t os_signal = 0; /* os's version of signal()/sigset() */
static sigaction_t os_sigaction = 0; /* os's version of sigaction() */

static bool jvm_signal_installing = false;
static bool jvm_signal_installed = false;

#define check_status(cmd) \
  do { \
    int status = (cmd); \
    if (status != 0) { \
      printf("error %s (%d) in " #cmd "\n", strerror(status), status); \
      exit(1); \
    } \
  } while (0)

static void signal_lock() {
  pthread_mutex_lock(&mutex);
  /* When the jvm is installing its set of signal handlers, threads
   * other than the jvm thread should wait */
  if (jvm_signal_installing) {
    if (tid != pthread_self()) {
      pthread_cond_wait(&cond, &mutex);
    }
  }
}

static void signal_unlock() {
  pthread_mutex_unlock(&mutex);
}

static void reentry_tls_init() {
    // value for reentry_flag_key will default to NULL (false)
    check_status(pthread_key_create(&reentry_flag_key, NULL));
}

static sa_handler_t call_os_signal(int sig, sa_handler_t disp,
                                   bool is_sigset) {
  sa_handler_t res;

  if (os_signal == NULL) {
    if (!is_sigset) {
      os_signal = (signal_t)dlsym(RTLD_NEXT, "signal");
    } else {
      os_signal = (signal_t)dlsym(RTLD_NEXT, "sigset");
    }
    if (os_signal == NULL) {
      printf("%s\n", dlerror());
      exit(0);
    }
  }
  check_status(pthread_once(&reentry_key_init_once, reentry_tls_init));
  // set reentry_flag_key to non-NULL to show reentry
  check_status(pthread_setspecific(reentry_flag_key, &res));
  res = (*os_signal)(sig, disp);
  check_status(pthread_setspecific(reentry_flag_key, NULL));
  return res;
}

static void save_signal_handler(int sig, sa_handler_t disp) {
  sigset_t set;
  sact[sig].sa_handler = disp;
  sigemptyset(&set);
  sact[sig].sa_mask = set;
  sact[sig].sa_flags = 0;
}

static sa_handler_t set_signal(int sig, sa_handler_t disp, bool is_sigset) {
  sa_handler_t oldhandler;
  bool sigused;

  signal_lock();

  sigused = (MASK(sig) & jvmsigs) != 0;
  if (jvm_signal_installed && sigused) {
    /* jvm has installed its signal handler for this signal. */
    /* Save the handler. Don't really install it. */
    oldhandler = sact[sig].sa_handler;
    save_signal_handler(sig, disp);

    signal_unlock();
    return oldhandler;
  } else if (jvm_signal_installing) {
    /* jvm is installing its signal handlers. Install the new
     * handlers and save the old ones. jvm uses sigaction().
     * Leave the piece here just in case. */
    oldhandler = call_os_signal(sig, disp, is_sigset);
    save_signal_handler(sig, oldhandler);

    /* Record the signals used by jvm */
    jvmsigs |= MASK(sig);

    signal_unlock();
    return oldhandler;
  } else {
    /* jvm has no relation with this signal (yet). Install the
     * the handler. */
    oldhandler = call_os_signal(sig, disp, is_sigset);

    signal_unlock();
    return oldhandler;
  }
}

sa_handler_t signal(int sig, sa_handler_t disp) {
  return set_signal(sig, disp, false);
}

sa_handler_t sigset(int sig, sa_handler_t disp) {
  printf("sigset() is not supported by BSD");
  exit(0);
 }

static int call_os_sigaction(int sig, const struct sigaction  *act,
                             struct sigaction *oact) {
  if (os_sigaction == NULL) {
    os_sigaction = (sigaction_t)dlsym(RTLD_NEXT, "sigaction");
    if (os_sigaction == NULL) {
      printf("%s\n", dlerror());
      exit(0);
    }
  }
  return (*os_sigaction)(sig, act, oact);
}

int sigaction(int sig, const struct sigaction *act, struct sigaction *oact) {
  int res;
  bool sigused;
  struct sigaction oldAct;

  check_status(pthread_once(&reentry_key_init_once, reentry_tls_init));
  if (pthread_getspecific(reentry_flag_key) != NULL) {
    return call_os_sigaction(sig, act, oact);
  }

  signal_lock();

  sigused = (MASK(sig) & jvmsigs) != 0;
  if (jvm_signal_installed && sigused) {
    /* jvm has installed its signal handler for this signal. */
    /* Save the handler. Don't really install it. */
    if (oact != NULL) {
      *oact = sact[sig];
    }
    if (act != NULL) {
      sact[sig] = *act;
    }

    signal_unlock();
    return 0;
  } else if (jvm_signal_installing) {
    /* jvm is installing its signal handlers. Install the new
     * handlers and save the old ones. */
    res = call_os_sigaction(sig, act, &oldAct);
    sact[sig] = oldAct;
    if (oact != NULL) {
      *oact = oldAct;
    }

    /* Record the signals used by jvm */
    jvmsigs |= MASK(sig);

    signal_unlock();
    return res;
  } else {
    /* jvm has no relation with this signal (yet). Install the
     * the handler. */
    res = call_os_sigaction(sig, act, oact);

    signal_unlock();
    return res;
  }
}

/* The three functions for the jvm to call into */
void JVM_begin_signal_setting() {
  signal_lock();
  jvm_signal_installing = true;
  tid = pthread_self();
  signal_unlock();
}

void JVM_end_signal_setting() {
  signal_lock();
  jvm_signal_installed = true;
  jvm_signal_installing = false;
  pthread_cond_broadcast(&cond);
  signal_unlock();
}

struct sigaction *JVM_get_signal_action(int sig) {
  /* Does race condition make sense here? */
  if ((MASK(sig) & jvmsigs) != 0) {
    return &sact[sig];
  }
  return NULL;
}