sysdeps/unix/sysv/linux/mq_notify.c

   1 /* Copyright (C) 2004-2020 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contribute by Ulrich Drepper <drepper@redhat.com>, 2004.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <assert.h>
  20 #include <errno.h>
  21 #include <fcntl.h>
  22 #include <mqueue.h>
  23 #include <pthread.h>
  24 #include <signal.h>
  25 #include <stdlib.h>
  26 #include <string.h>
  27 #include <sysdep.h>
  28 #include <unistd.h>
  29 #include <sys/socket.h>
  30 #include <not-cancel.h>
  31 #include <nptl/pthreadP.h>
  32
  33
  34 /* Defined in the kernel headers: */
  35 #define NOTIFY_COOKIE_LEN       32      /* Length of the cookie used.  */
  36 #define NOTIFY_WOKENUP          1       /* Code for notifcation.  */
  37 #define NOTIFY_REMOVED          2       /* Code for closed message queue
  38                                            of de-notifcation.  */
  39
  40
  41 /* Data structure for the queued notification requests.  */
  42 union notify_data
  43 {
  44   struct
  45   {
  46     void (*fct) (union sigval); /* The function to run.  */
  47     union sigval param;         /* The parameter to pass.  */
  48     pthread_attr_t *attr;       /* Attributes to create the thread with.  */
  49     /* NB: on 64-bit machines the struct as a size of 24 bytes.  Which means
  50        byte 31 can still be used for returning the status.  */
  51   };
  52   char raw[NOTIFY_COOKIE_LEN];
  53 };
  54
  55
  56 /* Keep track of the initialization.  */
  57 static pthread_once_t once = PTHREAD_ONCE_INIT;
  58
  59
  60 /* The netlink socket.  */
  61 static int netlink_socket = -1;
  62
  63
  64 /* Barrier used to make sure data passed to the new thread is not
  65    resused by the parent.  */
  66 static pthread_barrier_t notify_barrier;
  67
  68
  69 /* Modify the signal mask.  We move this into a separate function so
  70    that the stack space needed for sigset_t is not deducted from what
  71    the thread can use.  */
  72 static int
  73 __attribute__ ((noinline))
  74 change_sigmask (int how, sigset_t *oss)
  75 {
  76   sigset_t ss;
  77   sigfillset (&ss);
  78   return pthread_sigmask (how, &ss, oss);
  79 }
  80
  81
  82 /* The function used for the notification.  */
  83 static void *
  84 notification_function (void *arg)
  85 {
  86   /* Copy the function and parameter so that the parent thread can go
  87      on with its life.  */
  88   volatile union notify_data *data = (volatile union notify_data *) arg;
  89   void (*fct) (union sigval) = data->fct;
  90   union sigval param = data->param;
  91
  92   /* Let the parent go.  */
  93   (void) __pthread_barrier_wait (&notify_barrier);
  94
  95   /* Make the thread detached.  */
  96   (void) pthread_detach (pthread_self ());
  97
  98   /* The parent thread has all signals blocked.  This is probably a
  99      bit surprising for this thread.  So we unblock all of them.  */
 100   (void) change_sigmask (SIG_UNBLOCK, NULL);
 101
 102   /* Now run the user code.  */
 103   fct (param);
 104
 105   /* And we are done.  */
 106   return NULL;
 107 }
 108
 109
 110 /* Helper thread.  */
 111 static void *
 112 helper_thread (void *arg)
 113 {
 114   while (1)
 115     {
 116       union notify_data data;
 117
 118       ssize_t n = __recv (netlink_socket, &data, sizeof (data),
 119                           MSG_NOSIGNAL | MSG_WAITALL);
 120       if (n < NOTIFY_COOKIE_LEN)
 121         continue;
 122
 123       if (data.raw[NOTIFY_COOKIE_LEN - 1] == NOTIFY_WOKENUP)
 124         {
 125           /* Just create the thread as instructed.  There is no way to
 126              report a problem with creating a thread.  */
 127           pthread_t th;
 128           if (__builtin_expect (pthread_create (&th, data.attr,
 129                                                 notification_function, &data)
 130                                 == 0, 0))
 131             /* Since we passed a pointer to DATA to the new thread we have
 132                to wait until it is done with it.  */
 133             (void) __pthread_barrier_wait (&notify_barrier);
 134         }
 135       else if (data.raw[NOTIFY_COOKIE_LEN - 1] == NOTIFY_REMOVED)
 136         /* The only state we keep is the copy of the thread attributes.  */
 137         free (data.attr);
 138     }
 139   return NULL;
 140 }
 141
 142
 143 static void
 144 reset_once (void)
 145 {
 146   once = PTHREAD_ONCE_INIT;
 147 }
 148
 149
 150 static void
 151 init_mq_netlink (void)
 152 {
 153   /* This code might be called a second time after fork().  The file
 154      descriptor is inherited from the parent.  */
 155   if (netlink_socket == -1)
 156     {
 157       /* Just a normal netlink socket, not bound.  */
 158       netlink_socket = __socket (AF_NETLINK, SOCK_RAW | SOCK_CLOEXEC, 0);
 159       /* No need to do more if we have no socket.  */
 160       if (netlink_socket == -1)
 161         return;
 162     }
 163
 164   int err = 1;
 165
 166   /* Initialize the barrier.  */
 167   if (__builtin_expect (__pthread_barrier_init (&notify_barrier, NULL, 2) == 0,
 168                         0))
 169     {
 170       /* Create the helper thread.  */
 171       pthread_attr_t attr;
 172       (void) pthread_attr_init (&attr);
 173       (void) pthread_attr_setdetachstate (&attr, PTHREAD_CREATE_DETACHED);
 174       /* We do not need much stack space, the bare minimum will be enough.  */
 175       (void) pthread_attr_setstacksize (&attr, __pthread_get_minstack (&attr));
 176
 177       /* Temporarily block all signals so that the newly created
 178          thread inherits the mask.  */
 179       sigset_t oss;
 180       int have_no_oss = change_sigmask (SIG_BLOCK, &oss);
 181
 182       pthread_t th;
 183       err = pthread_create (&th, &attr, helper_thread, NULL);
 184
 185       /* Reset the signal mask.  */
 186       if (!have_no_oss)
 187         pthread_sigmask (SIG_SETMASK, &oss, NULL);
 188
 189       (void) pthread_attr_destroy (&attr);
 190
 191       if (err == 0)
 192         {
 193           static int added_atfork;
 194
 195           if (added_atfork == 0
 196               && pthread_atfork (NULL, NULL, reset_once) != 0)
 197             {
 198               /* The child thread will call recv() which is a
 199                  cancellation point.  */
 200               (void) pthread_cancel (th);
 201               err = 1;
 202             }
 203           else
 204             added_atfork = 1;
 205         }
 206     }
 207
 208   if (err != 0)
 209     {
 210       __close_nocancel_nostatus (netlink_socket);
 211       netlink_socket = -1;
 212     }
 213 }
 214
 215
 216 /* Register notification upon message arrival to an empty message queue
 217    MQDES.  */
 218 int
 219 mq_notify (mqd_t mqdes, const struct sigevent *notification)
 220 {
 221   /* Make sure the type is correctly defined.  */
 222   assert (sizeof (union notify_data) == NOTIFY_COOKIE_LEN);
 223
 224   /* Special treatment needed for SIGEV_THREAD.  */
 225   if (notification == NULL || notification->sigev_notify != SIGEV_THREAD)
 226     return INLINE_SYSCALL (mq_notify, 2, mqdes, notification);
 227
 228   /* The kernel cannot directly start threads.  This will have to be
 229      done at userlevel.  Since we cannot start threads from signal
 230      handlers we have to create a dedicated thread which waits for
 231      notifications for arriving messages and creates threads in
 232      response.  */
 233
 234   /* Initialize only once.  */
 235   pthread_once (&once, init_mq_netlink);
 236
 237   /* If we cannot create the netlink socket we cannot provide
 238      SIGEV_THREAD support.  */
 239   if (__glibc_unlikely (netlink_socket == -1))
 240     {
 241       __set_errno (ENOSYS);
 242       return -1;
 243     }
 244
 245   /* Create the cookie.  It will hold almost all the state.  */
 246   union notify_data data;
 247   memset (&data, '\0', sizeof (data));
 248   data.fct = notification->sigev_notify_function;
 249   data.param = notification->sigev_value;
 250
 251   if (notification->sigev_notify_attributes != NULL)
 252     {
 253       /* The thread attribute has to be allocated separately.  */
 254       data.attr = (pthread_attr_t *) malloc (sizeof (pthread_attr_t));
 255       if (data.attr == NULL)
 256         return -1;
 257
 258       memcpy (data.attr, notification->sigev_notify_attributes,
 259               sizeof (pthread_attr_t));
 260     }
 261
 262   /* Construct the new request.  */
 263   struct sigevent se;
 264   se.sigev_notify = SIGEV_THREAD;
 265   se.sigev_signo = netlink_socket;
 266   se.sigev_value.sival_ptr = &data;
 267
 268   /* Tell the kernel.  */
 269   int retval = INLINE_SYSCALL (mq_notify, 2, mqdes, &se);
 270
 271   /* If it failed, free the allocated memory.  */
 272   if (__glibc_unlikely (retval != 0))
 273     free (data.attr);
 274
 275   return retval;
 276 }