nptl/pthread_cancel.c

   1 /* Copyright (C) 2002-2022 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #include <errno.h>
  19 #include <signal.h>
  20 #include <stdlib.h>
  21 #include "pthreadP.h"
  22 #include <atomic.h>
  23 #include <sysdep.h>
  24 #include <unistd.h>
  25 #include <unwind-link.h>
  26 #include <stdio.h>
  27 #include <gnu/lib-names.h>
  28 #include <sys/single_threaded.h>
  29
  30 /* For asynchronous cancellation we use a signal.  */
  31 static void
  32 sigcancel_handler (int sig, siginfo_t *si, void *ctx)
  33 {
  34   /* Safety check.  It would be possible to call this function for
  35      other signals and send a signal from another process.  This is not
  36      correct and might even be a security problem.  Try to catch as
  37      many incorrect invocations as possible.  */
  38   if (sig != SIGCANCEL
  39       || si->si_pid != __getpid()
  40       || si->si_code != SI_TKILL)
  41     return;
  42
  43   struct pthread *self = THREAD_SELF;
  44
  45   int oldval = atomic_load_relaxed (&self->cancelhandling);
  46   while (1)
  47     {
  48       /* We are canceled now.  When canceled by another thread this flag
  49          is already set but if the signal is directly send (internally or
  50          from another process) is has to be done here.  */
  51       int newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
  52
  53       if (oldval == newval || (oldval & EXITING_BITMASK) != 0)
  54         /* Already canceled or exiting.  */
  55         break;
  56
  57       if (atomic_compare_exchange_weak_acquire (&self->cancelhandling,
  58                                                 &oldval, newval))
  59         {
  60           self->result = PTHREAD_CANCELED;
  61
  62           /* Make sure asynchronous cancellation is still enabled.  */
  63           if ((oldval & CANCELTYPE_BITMASK) != 0)
  64             /* Run the registered destructors and terminate the thread.  */
  65             __do_cancel ();
  66         }
  67     }
  68 }
  69
  70 int
  71 __pthread_cancel (pthread_t th)
  72 {
  73   volatile struct pthread *pd = (volatile struct pthread *) th;
  74
  75   if (pd->tid == 0)
  76     /* The thread has already exited on the kernel side.  Its outcome
  77        (regular exit, other cancelation) has already been
  78        determined.  */
  79     return 0;
  80
  81   static int init_sigcancel = 0;
  82   if (atomic_load_relaxed (&init_sigcancel) == 0)
  83     {
  84       struct sigaction sa;
  85       sa.sa_sigaction = sigcancel_handler;
  86       /* The signal handle should be non-interruptible to avoid the risk of
  87          spurious EINTR caused by SIGCANCEL sent to process or if
  88          pthread_cancel() is called while cancellation is disabled in the
  89          target thread.  */
  90       sa.sa_flags = SA_SIGINFO | SA_RESTART;
  91       __sigemptyset (&sa.sa_mask);
  92       __libc_sigaction (SIGCANCEL, &sa, NULL);
  93       atomic_store_relaxed (&init_sigcancel, 1);
  94     }
  95
  96 #ifdef SHARED
  97   /* Trigger an error if libgcc_s cannot be loaded.  */
  98   {
  99     struct unwind_link *unwind_link = __libc_unwind_link_get ();
 100     if (unwind_link == NULL)
 101       __libc_fatal (LIBGCC_S_SO
 102                     " must be installed for pthread_cancel to work\n");
 103   }
 104 #endif
 105
 106   /* Some syscalls are never restarted after being interrupted by a signal
 107      handler, regardless of the use of SA_RESTART (they always fail with
 108      EINTR).  So pthread_cancel cannot send SIGCANCEL unless the cancellation
 109      is enabled and set as asynchronous (in this case the cancellation will
 110      be acted in the cancellation handler instead by the syscall wrapper).
 111      Otherwise the target thread is set as 'cancelling' (CANCELING_BITMASK)
 112      by atomically setting 'cancelhandling' and the cancelation will be acted
 113      upon on next cancellation entrypoing in the target thread.
 114
 115      It also requires to atomically check if cancellation is enabled and
 116      asynchronous, so both cancellation state and type are tracked on
 117      'cancelhandling'.  */
 118
 119   int result = 0;
 120   int oldval = atomic_load_relaxed (&pd->cancelhandling);
 121   int newval;
 122   do
 123     {
 124     again:
 125       newval = oldval | CANCELING_BITMASK | CANCELED_BITMASK;
 126       if (oldval == newval)
 127         break;
 128
 129       /* If the cancellation is handled asynchronously just send a
 130          signal.  We avoid this if possible since it's more
 131          expensive.  */
 132       if (cancel_enabled_and_canceled_and_async (newval))
 133         {
 134           /* Mark the cancellation as "in progress".  */
 135           int newval2 = oldval | CANCELING_BITMASK;
 136           if (!atomic_compare_exchange_weak_acquire (&pd->cancelhandling,
 137                                                      &oldval, newval2))
 138             goto again;
 139
 140           if (pd == THREAD_SELF)
 141             /* This is not merely an optimization: An application may
 142                call pthread_cancel (pthread_self ()) without calling
 143                pthread_create, so the signal handler may not have been
 144                set up for a self-cancel.  */
 145             {
 146               pd->result = PTHREAD_CANCELED;
 147               if ((newval & CANCELTYPE_BITMASK) != 0)
 148                 __do_cancel ();
 149             }
 150           else
 151             /* The cancellation handler will take care of marking the
 152                thread as canceled.  */
 153             result = __pthread_kill_internal (th, SIGCANCEL);
 154
 155           break;
 156         }
 157
 158         /* A single-threaded process should be able to kill itself, since
 159            there is nothing in the POSIX specification that says that it
 160            cannot.  So we set multiple_threads to true so that cancellation
 161            points get executed.  */
 162         THREAD_SETMEM (THREAD_SELF, header.multiple_threads, 1);
 163 #ifndef TLS_MULTIPLE_THREADS_IN_TCB
 164       __libc_multiple_threads = 1;
 165 #endif
 166     }
 167   while (!atomic_compare_exchange_weak_acquire (&pd->cancelhandling, &oldval,
 168                                                 newval));
 169
 170   return result;
 171 }
 172 versioned_symbol (libc, __pthread_cancel, pthread_cancel, GLIBC_2_34);
 173
 174 #if OTHER_SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_34)
 175 compat_symbol (libpthread, __pthread_cancel, pthread_cancel, GLIBC_2_0);
 176 #endif
 177
 178 /* Ensure that the unwinder is always linked in (the __pthread_unwind
 179    reference from __do_cancel is weak).  Use ___pthread_unwind_next
 180    (three underscores) to produce a strong reference to the same
 181    file.  */
 182 PTHREAD_STATIC_FN_REQUIRE (___pthread_unwind_next)