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 #ifdef _FORTIFY_SOURCE
  26 #undef _FORTIFY_SOURCE
  27 #endif
  28 #include "qemu/osdep.h"
  29 #include <pthread.h>
  30 #include "qemu-common.h"
  31 #include "qemu/coroutine_int.h"
  32
  33 typedef struct {
  34     Coroutine base;
  35     void *stack;
  36     size_t stack_size;
  37     sigjmp_buf env;
  38 } CoroutineSigAltStack;
  39
  40 /**
  41  * Per-thread coroutine bookkeeping
  42  */
  43 typedef struct {
  44     /** Currently executing coroutine */
  45     Coroutine *current;
  46
  47     /** The default coroutine */
  48     CoroutineSigAltStack leader;
  49
  50     /** Information for the signal handler (trampoline) */
  51     sigjmp_buf tr_reenter;
  52     volatile sig_atomic_t tr_called;
  53     void *tr_handler;
  54 } CoroutineThreadState;
  55
  56 static pthread_key_t thread_state_key;
  57
  58 static CoroutineThreadState *coroutine_get_thread_state(void)
  59 {
  60     CoroutineThreadState *s = pthread_getspecific(thread_state_key);
  61
  62     if (!s) {
  63         s = g_malloc0(sizeof(*s));
  64         s->current = &s->leader.base;
  65         pthread_setspecific(thread_state_key, s);
  66     }
  67     return s;
  68 }
  69
  70 static void qemu_coroutine_thread_cleanup(void *opaque)
  71 {
  72     CoroutineThreadState *s = opaque;
  73
  74     g_free(s);
  75 }
  76
  77 static void __attribute__((constructor)) coroutine_init(void)
  78 {
  79     int ret;
  80
  81     ret = pthread_key_create(&thread_state_key, qemu_coroutine_thread_cleanup);
  82     if (ret != 0) {
  83         fprintf(stderr, "unable to create leader key: %s\n", strerror(errno));
  84         abort();
  85     }
  86 }
  87
  88 /* "boot" function
  89  * This is what starts the coroutine, is called from the trampoline
  90  * (from the signal handler when it is not signal handling, read ahead
  91  * for more information).
  92  */
  93 static void coroutine_bootstrap(CoroutineSigAltStack *self, Coroutine *co)
  94 {
  95     /* Initialize longjmp environment and switch back the caller */
  96     if (!sigsetjmp(self->env, 0)) {
  97         siglongjmp(*(sigjmp_buf *)co->entry_arg, 1);
  98     }
  99
 100     while (true) {
 101         co->entry(co->entry_arg);
 102         qemu_coroutine_switch(co, co->caller, COROUTINE_TERMINATE);
 103     }
 104 }
 105
 106 /*
 107  * This is used as the signal handler. This is called with the brand new stack
 108  * (thanks to sigaltstack). We have to return, given that this is a signal
 109  * handler and the sigmask and some other things are changed.
 110  */
 111 static void coroutine_trampoline(int signal)
 112 {
 113     CoroutineSigAltStack *self;
 114     Coroutine *co;
 115     CoroutineThreadState *coTS;
 116
 117     /* Get the thread specific information */
 118     coTS = coroutine_get_thread_state();
 119     self = coTS->tr_handler;
 120     coTS->tr_called = 1;
 121     co = &self->base;
 122
 123     /*
 124      * Here we have to do a bit of a ping pong between the caller, given that
 125      * this is a signal handler and we have to do a return "soon". Then the
 126      * caller can reestablish everything and do a siglongjmp here again.
 127      */
 128     if (!sigsetjmp(coTS->tr_reenter, 0)) {
 129         return;
 130     }
 131
 132     /*
 133      * Ok, the caller has siglongjmp'ed back to us, so now prepare
 134      * us for the real machine state switching. We have to jump
 135      * into another function here to get a new stack context for
 136      * the auto variables (which have to be auto-variables
 137      * because the start of the thread happens later). Else with
 138      * PIC (i.e. Position Independent Code which is used when PTH
 139      * is built as a shared library) most platforms would
 140      * horrible core dump as experience showed.
 141      */
 142     coroutine_bootstrap(self, co);
 143 }
 144
 145 Coroutine *qemu_coroutine_new(void)
 146 {
 147     CoroutineSigAltStack *co;
 148     CoroutineThreadState *coTS;
 149     struct sigaction sa;
 150     struct sigaction osa;
 151     stack_t ss;
 152     stack_t oss;
 153     sigset_t sigs;
 154     sigset_t osigs;
 155     sigjmp_buf old_env;
 156
 157     /* The way to manipulate stack is with the sigaltstack function. We
 158      * prepare a stack, with it delivering a signal to ourselves and then
 159      * put sigsetjmp/siglongjmp where needed.
 160      * This has been done keeping coroutine-ucontext as a model and with the
 161      * pth ideas (GNU Portable Threads). See coroutine-ucontext for the basics
 162      * of the coroutines and see pth_mctx.c (from the pth project) for the
 163      * sigaltstack way of manipulating stacks.
 164      */
 165
 166     co = g_malloc0(sizeof(*co));
 167     co->stack_size = COROUTINE_STACK_SIZE;
 168     co->stack = qemu_alloc_stack(&co->stack_size);
 169     co->base.entry_arg = &old_env; /* stash away our jmp_buf */
 170
 171     coTS = coroutine_get_thread_state();
 172     coTS->tr_handler = co;
 173
 174     /*
 175      * Preserve the SIGUSR2 signal state, block SIGUSR2,
 176      * and establish our signal handler. The signal will
 177      * later transfer control onto the signal stack.
 178      */
 179     sigemptyset(&sigs);
 180     sigaddset(&sigs, SIGUSR2);
 181     pthread_sigmask(SIG_BLOCK, &sigs, &osigs);
 182     sa.sa_handler = coroutine_trampoline;
 183     sigfillset(&sa.sa_mask);
 184     sa.sa_flags = SA_ONSTACK;
 185     if (sigaction(SIGUSR2, &sa, &osa) != 0) {
 186         abort();
 187     }
 188
 189     /*
 190      * Set the new stack.
 191      */
 192     ss.ss_sp = co->stack;
 193     ss.ss_size = co->stack_size;
 194     ss.ss_flags = 0;
 195     if (sigaltstack(&ss, &oss) < 0) {
 196         abort();
 197     }
 198
 199     /*
 200      * Now transfer control onto the signal stack and set it up.
 201      * It will return immediately via "return" after the sigsetjmp()
 202      * was performed. Be careful here with race conditions.  The
 203      * signal can be delivered the first time sigsuspend() is
 204      * called.
 205      */
 206     coTS->tr_called = 0;
 207     pthread_kill(pthread_self(), SIGUSR2);
 208     sigfillset(&sigs);
 209     sigdelset(&sigs, SIGUSR2);
 210     while (!coTS->tr_called) {
 211         sigsuspend(&sigs);
 212     }
 213
 214     /*
 215      * Inform the system that we are back off the signal stack by
 216      * removing the alternative signal stack. Be careful here: It
 217      * first has to be disabled, before it can be removed.
 218      */
 219     sigaltstack(NULL, &ss);
 220     ss.ss_flags = SS_DISABLE;
 221     if (sigaltstack(&ss, NULL) < 0) {
 222         abort();
 223     }
 224     sigaltstack(NULL, &ss);
 225     if (!(oss.ss_flags & SS_DISABLE)) {
 226         sigaltstack(&oss, NULL);
 227     }
 228
 229     /*
 230      * Restore the old SIGUSR2 signal handler and mask
 231      */
 232     sigaction(SIGUSR2, &osa, NULL);
 233     pthread_sigmask(SIG_SETMASK, &osigs, NULL);
 234
 235     /*
 236      * Now enter the trampoline again, but this time not as a signal
 237      * handler. Instead we jump into it directly. The functionally
 238      * redundant ping-pong pointer arithmetic is necessary to avoid
 239      * type-conversion warnings related to the `volatile' qualifier and
 240      * the fact that `jmp_buf' usually is an array type.
 241      */
 242     if (!sigsetjmp(old_env, 0)) {
 243         siglongjmp(coTS->tr_reenter, 1);
 244     }
 245
 246     /*
 247      * Ok, we returned again, so now we're finished
 248      */
 249
 250     return &co->base;
 251 }
 252
 253 void qemu_coroutine_delete(Coroutine *co_)
 254 {
 255     CoroutineSigAltStack *co = DO_UPCAST(CoroutineSigAltStack, base, co_);
 256
 257     qemu_free_stack(co->stack, co->stack_size);
 258     g_free(co);
 259 }
 260
 261 CoroutineAction qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
 262                                       CoroutineAction action)
 263 {
 264     CoroutineSigAltStack *from = DO_UPCAST(CoroutineSigAltStack, base, from_);
 265     CoroutineSigAltStack *to = DO_UPCAST(CoroutineSigAltStack, base, to_);
 266     CoroutineThreadState *s = coroutine_get_thread_state();
 267     int ret;
 268
 269     s->current = to_;
 270
 271     ret = sigsetjmp(from->env, 0);
 272     if (ret == 0) {
 273         siglongjmp(to->env, action);
 274     }
 275     return ret;
 276 }
 277
 278 Coroutine *qemu_coroutine_self(void)
 279 {
 280     CoroutineThreadState *s = coroutine_get_thread_state();
 281
 282     return s->current;
 283 }
 284
 285 bool qemu_in_coroutine(void)
 286 {
 287     CoroutineThreadState *s = pthread_getspecific(thread_state_key);
 288
 289     return s && s->current->caller;
 290 }
 291