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     sigjmp_buf env;
  37 } CoroutineUContext;
  38
  39 /**
  40  * Per-thread coroutine bookkeeping
  41  */
  42 typedef struct {
  43     /** Currently executing coroutine */
  44     Coroutine *current;
  45
  46     /** The default coroutine */
  47     CoroutineUContext leader;
  48
  49     /** Information for the signal handler (trampoline) */
  50     sigjmp_buf tr_reenter;
  51     volatile sig_atomic_t tr_called;
  52     void *tr_handler;
  53 } CoroutineThreadState;
  54
  55 static pthread_key_t thread_state_key;
  56
  57 static CoroutineThreadState *coroutine_get_thread_state(void)
  58 {
  59     CoroutineThreadState *s = pthread_getspecific(thread_state_key);
  60
  61     if (!s) {
  62         s = g_malloc0(sizeof(*s));
  63         s->current = &s->leader.base;
  64         pthread_setspecific(thread_state_key, s);
  65     }
  66     return s;
  67 }
  68
  69 static void qemu_coroutine_thread_cleanup(void *opaque)
  70 {
  71     CoroutineThreadState *s = opaque;
  72
  73     g_free(s);
  74 }
  75
  76 static void __attribute__((constructor)) coroutine_init(void)
  77 {
  78     int ret;
  79
  80     ret = pthread_key_create(&thread_state_key, qemu_coroutine_thread_cleanup);
  81     if (ret != 0) {
  82         fprintf(stderr, "unable to create leader key: %s\n", strerror(errno));
  83         abort();
  84     }
  85 }
  86
  87 /* "boot" function
  88  * This is what starts the coroutine, is called from the trampoline
  89  * (from the signal handler when it is not signal handling, read ahead
  90  * for more information).
  91  */
  92 static void coroutine_bootstrap(CoroutineUContext *self, Coroutine *co)
  93 {
  94     /* Initialize longjmp environment and switch back the caller */
  95     if (!sigsetjmp(self->env, 0)) {
  96         siglongjmp(*(sigjmp_buf *)co->entry_arg, 1);
  97     }
  98
  99     while (true) {
 100         co->entry(co->entry_arg);
 101         qemu_coroutine_switch(co, co->caller, COROUTINE_TERMINATE);
 102     }
 103 }
 104
 105 /*
 106  * This is used as the signal handler. This is called with the brand new stack
 107  * (thanks to sigaltstack). We have to return, given that this is a signal
 108  * handler and the sigmask and some other things are changed.
 109  */
 110 static void coroutine_trampoline(int signal)
 111 {
 112     CoroutineUContext *self;
 113     Coroutine *co;
 114     CoroutineThreadState *coTS;
 115
 116     /* Get the thread specific information */
 117     coTS = coroutine_get_thread_state();
 118     self = coTS->tr_handler;
 119     coTS->tr_called = 1;
 120     co = &self->base;
 121
 122     /*
 123      * Here we have to do a bit of a ping pong between the caller, given that
 124      * this is a signal handler and we have to do a return "soon". Then the
 125      * caller can reestablish everything and do a siglongjmp here again.
 126      */
 127     if (!sigsetjmp(coTS->tr_reenter, 0)) {
 128         return;
 129     }
 130
 131     /*
 132      * Ok, the caller has siglongjmp'ed back to us, so now prepare
 133      * us for the real machine state switching. We have to jump
 134      * into another function here to get a new stack context for
 135      * the auto variables (which have to be auto-variables
 136      * because the start of the thread happens later). Else with
 137      * PIC (i.e. Position Independent Code which is used when PTH
 138      * is built as a shared library) most platforms would
 139      * horrible core dump as experience showed.
 140      */
 141     coroutine_bootstrap(self, co);
 142 }
 143
 144 Coroutine *qemu_coroutine_new(void)
 145 {
 146     const size_t stack_size = 1 << 20;
 147     CoroutineUContext *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 = g_malloc(stack_size);
 168     co->base.entry_arg = &old_env; /* stash away our jmp_buf */
 169
 170     coTS = coroutine_get_thread_state();
 171     coTS->tr_handler = co;
 172
 173     /*
 174      * Preserve the SIGUSR2 signal state, block SIGUSR2,
 175      * and establish our signal handler. The signal will
 176      * later transfer control onto the signal stack.
 177      */
 178     sigemptyset(&sigs);
 179     sigaddset(&sigs, SIGUSR2);
 180     pthread_sigmask(SIG_BLOCK, &sigs, &osigs);
 181     sa.sa_handler = coroutine_trampoline;
 182     sigfillset(&sa.sa_mask);
 183     sa.sa_flags = SA_ONSTACK;
 184     if (sigaction(SIGUSR2, &sa, &osa) != 0) {
 185         abort();
 186     }
 187
 188     /*
 189      * Set the new stack.
 190      */
 191     ss.ss_sp = co->stack;
 192     ss.ss_size = stack_size;
 193     ss.ss_flags = 0;
 194     if (sigaltstack(&ss, &oss) < 0) {
 195         abort();
 196     }
 197
 198     /*
 199      * Now transfer control onto the signal stack and set it up.
 200      * It will return immediately via "return" after the sigsetjmp()
 201      * was performed. Be careful here with race conditions.  The
 202      * signal can be delivered the first time sigsuspend() is
 203      * called.
 204      */
 205     coTS->tr_called = 0;
 206     pthread_kill(pthread_self(), SIGUSR2);
 207     sigfillset(&sigs);
 208     sigdelset(&sigs, SIGUSR2);
 209     while (!coTS->tr_called) {
 210         sigsuspend(&sigs);
 211     }
 212
 213     /*
 214      * Inform the system that we are back off the signal stack by
 215      * removing the alternative signal stack. Be careful here: It
 216      * first has to be disabled, before it can be removed.
 217      */
 218     sigaltstack(NULL, &ss);
 219     ss.ss_flags = SS_DISABLE;
 220     if (sigaltstack(&ss, NULL) < 0) {
 221         abort();
 222     }
 223     sigaltstack(NULL, &ss);
 224     if (!(oss.ss_flags & SS_DISABLE)) {
 225         sigaltstack(&oss, NULL);
 226     }
 227
 228     /*
 229      * Restore the old SIGUSR2 signal handler and mask
 230      */
 231     sigaction(SIGUSR2, &osa, NULL);
 232     pthread_sigmask(SIG_SETMASK, &osigs, NULL);
 233
 234     /*
 235      * Now enter the trampoline again, but this time not as a signal
 236      * handler. Instead we jump into it directly. The functionally
 237      * redundant ping-pong pointer arithmetic is necessary to avoid
 238      * type-conversion warnings related to the `volatile' qualifier and
 239      * the fact that `jmp_buf' usually is an array type.
 240      */
 241     if (!sigsetjmp(old_env, 0)) {
 242         siglongjmp(coTS->tr_reenter, 1);
 243     }
 244
 245     /*
 246      * Ok, we returned again, so now we're finished
 247      */
 248
 249     return &co->base;
 250 }
 251
 252 void qemu_coroutine_delete(Coroutine *co_)
 253 {
 254     CoroutineUContext *co = DO_UPCAST(CoroutineUContext, base, co_);
 255
 256     g_free(co->stack);
 257     g_free(co);
 258 }
 259
 260 CoroutineAction qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
 261                                       CoroutineAction action)
 262 {
 263     CoroutineUContext *from = DO_UPCAST(CoroutineUContext, base, from_);
 264     CoroutineUContext *to = DO_UPCAST(CoroutineUContext, base, to_);
 265     CoroutineThreadState *s = coroutine_get_thread_state();
 266     int ret;
 267
 268     s->current = to_;
 269
 270     ret = sigsetjmp(from->env, 0);
 271     if (ret == 0) {
 272         siglongjmp(to->env, action);
 273     }
 274     return ret;
 275 }
 276
 277 Coroutine *qemu_coroutine_self(void)
 278 {
 279     CoroutineThreadState *s = coroutine_get_thread_state();
 280
 281     return s->current;
 282 }
 283
 284 bool qemu_in_coroutine(void)
 285 {
 286     CoroutineThreadState *s = pthread_getspecific(thread_state_key);
 287
 288     return s && s->current->caller;
 289 }
 290