util/coroutine-sigaltstack.c

   1 /*
   2  * sigaltstack coroutine initialization code
   3  *
   4  * Copyright (C) 2006  Anthony Liguori <anthony@codemonkey.ws>
   5  * Copyright (C) 2011  Kevin Wolf <kwolf@redhat.com>
   6  * Copyright (C) 2012  Alex Barcelo <abarcelo@ac.upc.edu>
   7 ** This file is partly based on pth_mctx.c, from the GNU Portable Threads
   8 **  Copyright (c) 1999-2006 Ralf S. Engelschall <rse@engelschall.com>
   9  *
  10  * This library is free software; you can redistribute it and/or
  11  * modify it under the terms of the GNU Lesser General Public
  12  * License as published by the Free Software Foundation; either
  13  * version 2.1 of the License, or (at your option) any later version.
  14  *
  15  * This library is distributed in the hope that it will be useful,
  16  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  17  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  18  * Lesser General Public License for more details.
  19  *
  20  * You should have received a copy of the GNU Lesser General Public
  21  * License along with this library; if not, see <http://www.gnu.org/licenses/>.
  22  */
  23
  24 /* XXX Is there a nicer way to disable glibc's stack check for longjmp? */
  25 #undef _FORTIFY_SOURCE
  26 #define _FORTIFY_SOURCE 0
  27
  28 #include "qemu/osdep.h"
  29 #include <pthread.h>
  30 #include "qemu/coroutine_int.h"
  31
  32 #ifdef CONFIG_SAFESTACK
  33 #error "SafeStack is not compatible with code run in alternate signal stacks"
  34 #endif
  35
  36 typedef struct {
  37     Coroutine base;
  38     void *stack;
  39     size_t stack_size;
  40     sigjmp_buf env;
  41 } CoroutineSigAltStack;
  42
  43 /**
  44  * Per-thread coroutine bookkeeping
  45  */
  46 typedef struct {
  47     /** Currently executing coroutine */
  48     Coroutine *current;
  49
  50     /** The default coroutine */
  51     CoroutineSigAltStack leader;
  52
  53     /** Information for the signal handler (trampoline) */
  54     sigjmp_buf tr_reenter;
  55     volatile sig_atomic_t tr_called;
  56     void *tr_handler;
  57 } CoroutineThreadState;
  58
  59 static pthread_key_t thread_state_key;
  60
  61 static CoroutineThreadState *coroutine_get_thread_state(void)
  62 {
  63     CoroutineThreadState *s = pthread_getspecific(thread_state_key);
  64
  65     if (!s) {
  66         s = g_malloc0(sizeof(*s));
  67         s->current = &s->leader.base;
  68         pthread_setspecific(thread_state_key, s);
  69     }
  70     return s;
  71 }
  72
  73 static void qemu_coroutine_thread_cleanup(void *opaque)
  74 {
  75     CoroutineThreadState *s = opaque;
  76
  77     g_free(s);
  78 }
  79
  80 static void __attribute__((constructor)) coroutine_init(void)
  81 {
  82     int ret;
  83
  84     ret = pthread_key_create(&thread_state_key, qemu_coroutine_thread_cleanup);
  85     if (ret != 0) {
  86         fprintf(stderr, "unable to create leader key: %s\n", strerror(errno));
  87         abort();
  88     }
  89 }
  90
  91 /* "boot" function
  92  * This is what starts the coroutine, is called from the trampoline
  93  * (from the signal handler when it is not signal handling, read ahead
  94  * for more information).
  95  */
  96 static void coroutine_bootstrap(CoroutineSigAltStack *self, Coroutine *co)
  97 {
  98     /* Initialize longjmp environment and switch back the caller */
  99     if (!sigsetjmp(self->env, 0)) {
 100         siglongjmp(*(sigjmp_buf *)co->entry_arg, 1);
 101     }
 102
 103     while (true) {
 104         co->entry(co->entry_arg);
 105         qemu_coroutine_switch(co, co->caller, COROUTINE_TERMINATE);
 106     }
 107 }
 108
 109 /*
 110  * This is used as the signal handler. This is called with the brand new stack
 111  * (thanks to sigaltstack). We have to return, given that this is a signal
 112  * handler and the sigmask and some other things are changed.
 113  */
 114 static void coroutine_trampoline(int signal)
 115 {
 116     CoroutineSigAltStack *self;
 117     Coroutine *co;
 118     CoroutineThreadState *coTS;
 119
 120     /* Get the thread specific information */
 121     coTS = coroutine_get_thread_state();
 122     self = coTS->tr_handler;
 123     coTS->tr_called = 1;
 124     co = &self->base;
 125
 126     /*
 127      * Here we have to do a bit of a ping pong between the caller, given that
 128      * this is a signal handler and we have to do a return "soon". Then the
 129      * caller can reestablish everything and do a siglongjmp here again.
 130      */
 131     if (!sigsetjmp(coTS->tr_reenter, 0)) {
 132         return;
 133     }
 134
 135     /*
 136      * Ok, the caller has siglongjmp'ed back to us, so now prepare
 137      * us for the real machine state switching. We have to jump
 138      * into another function here to get a new stack context for
 139      * the auto variables (which have to be auto-variables
 140      * because the start of the thread happens later). Else with
 141      * PIC (i.e. Position Independent Code which is used when PTH
 142      * is built as a shared library) most platforms would
 143      * horrible core dump as experience showed.
 144      */
 145     coroutine_bootstrap(self, co);
 146 }
 147
 148 Coroutine *qemu_coroutine_new(void)
 149 {
 150     CoroutineSigAltStack *co;
 151     CoroutineThreadState *coTS;
 152     struct sigaction sa;
 153     struct sigaction osa;
 154     stack_t ss;
 155     stack_t oss;
 156     sigset_t sigs;
 157     sigset_t osigs;
 158     sigjmp_buf old_env;
 159     static pthread_mutex_t sigusr2_mutex = PTHREAD_MUTEX_INITIALIZER;
 160
 161     /* The way to manipulate stack is with the sigaltstack function. We
 162      * prepare a stack, with it delivering a signal to ourselves and then
 163      * put sigsetjmp/siglongjmp where needed.
 164      * This has been done keeping coroutine-ucontext as a model and with the
 165      * pth ideas (GNU Portable Threads). See coroutine-ucontext for the basics
 166      * of the coroutines and see pth_mctx.c (from the pth project) for the
 167      * sigaltstack way of manipulating stacks.
 168      */
 169
 170     co = g_malloc0(sizeof(*co));
 171     co->stack_size = COROUTINE_STACK_SIZE;
 172     co->stack = qemu_alloc_stack(&co->stack_size);
 173     co->base.entry_arg = &old_env; /* stash away our jmp_buf */
 174
 175     coTS = coroutine_get_thread_state();
 176     coTS->tr_handler = co;
 177
 178     /*
 179      * Preserve the SIGUSR2 signal state, block SIGUSR2,
 180      * and establish our signal handler. The signal will
 181      * later transfer control onto the signal stack.
 182      */
 183     sigemptyset(&sigs);
 184     sigaddset(&sigs, SIGUSR2);
 185     pthread_sigmask(SIG_BLOCK, &sigs, &osigs);
 186     sa.sa_handler = coroutine_trampoline;
 187     sigfillset(&sa.sa_mask);
 188     sa.sa_flags = SA_ONSTACK;
 189
 190     /*
 191      * sigaction() is a process-global operation.  We must not run
 192      * this code in multiple threads at once.
 193      */
 194     pthread_mutex_lock(&sigusr2_mutex);
 195     if (sigaction(SIGUSR2, &sa, &osa) != 0) {
 196         abort();
 197     }
 198
 199     /*
 200      * Set the new stack.
 201      */
 202     ss.ss_sp = co->stack;
 203     ss.ss_size = co->stack_size;
 204     ss.ss_flags = 0;
 205     if (sigaltstack(&ss, &oss) < 0) {
 206         abort();
 207     }
 208
 209     /*
 210      * Now transfer control onto the signal stack and set it up.
 211      * It will return immediately via "return" after the sigsetjmp()
 212      * was performed. Be careful here with race conditions.  The
 213      * signal can be delivered the first time sigsuspend() is
 214      * called.
 215      */
 216     coTS->tr_called = 0;
 217     pthread_kill(pthread_self(), SIGUSR2);
 218     sigfillset(&sigs);
 219     sigdelset(&sigs, SIGUSR2);
 220     while (!coTS->tr_called) {
 221         sigsuspend(&sigs);
 222     }
 223
 224     /*
 225      * Inform the system that we are back off the signal stack by
 226      * removing the alternative signal stack. Be careful here: It
 227      * first has to be disabled, before it can be removed.
 228      */
 229     sigaltstack(NULL, &ss);
 230     ss.ss_flags = SS_DISABLE;
 231     if (sigaltstack(&ss, NULL) < 0) {
 232         abort();
 233     }
 234     sigaltstack(NULL, &ss);
 235     if (!(oss.ss_flags & SS_DISABLE)) {
 236         sigaltstack(&oss, NULL);
 237     }
 238
 239     /*
 240      * Restore the old SIGUSR2 signal handler and mask
 241      */
 242     sigaction(SIGUSR2, &osa, NULL);
 243     pthread_mutex_unlock(&sigusr2_mutex);
 244
 245     pthread_sigmask(SIG_SETMASK, &osigs, NULL);
 246
 247     /*
 248      * Now enter the trampoline again, but this time not as a signal
 249      * handler. Instead we jump into it directly. The functionally
 250      * redundant ping-pong pointer arithmetic is necessary to avoid
 251      * type-conversion warnings related to the `volatile' qualifier and
 252      * the fact that `jmp_buf' usually is an array type.
 253      */
 254     if (!sigsetjmp(old_env, 0)) {
 255         siglongjmp(coTS->tr_reenter, 1);
 256     }
 257
 258     /*
 259      * Ok, we returned again, so now we're finished
 260      */
 261
 262     return &co->base;
 263 }
 264
 265 void qemu_coroutine_delete(Coroutine *co_)
 266 {
 267     CoroutineSigAltStack *co = DO_UPCAST(CoroutineSigAltStack, base, co_);
 268
 269     qemu_free_stack(co->stack, co->stack_size);
 270     g_free(co);
 271 }
 272
 273 CoroutineAction qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
 274                                       CoroutineAction action)
 275 {
 276     CoroutineSigAltStack *from = DO_UPCAST(CoroutineSigAltStack, base, from_);
 277     CoroutineSigAltStack *to = DO_UPCAST(CoroutineSigAltStack, base, to_);
 278     CoroutineThreadState *s = coroutine_get_thread_state();
 279     int ret;
 280
 281     s->current = to_;
 282
 283     ret = sigsetjmp(from->env, 0);
 284     if (ret == 0) {
 285         siglongjmp(to->env, action);
 286     }
 287     return ret;
 288 }
 289
 290 Coroutine *qemu_coroutine_self(void)
 291 {
 292     CoroutineThreadState *s = coroutine_get_thread_state();
 293
 294     return s->current;
 295 }
 296
 297 bool qemu_in_coroutine(void)
 298 {
 299     CoroutineThreadState *s = pthread_getspecific(thread_state_key);
 300
 301     return s && s->current->caller;
 302 }
 303