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 #ifdef CONFIG_SAFESTACK
  34 #error "SafeStack is not compatible with code run in alternate signal stacks"
  35 #endif
  36
  37 typedef struct {
  38     Coroutine base;
  39     void *stack;
  40     size_t stack_size;
  41     sigjmp_buf env;
  42 } CoroutineSigAltStack;
  43
  44 /**
  45  * Per-thread coroutine bookkeeping
  46  */
  47 typedef struct {
  48     /** Currently executing coroutine */
  49     Coroutine *current;
  50
  51     /** The default coroutine */
  52     CoroutineSigAltStack leader;
  53
  54     /** Information for the signal handler (trampoline) */
  55     sigjmp_buf tr_reenter;
  56     volatile sig_atomic_t tr_called;
  57     void *tr_handler;
  58 } CoroutineThreadState;
  59
  60 static pthread_key_t thread_state_key;
  61
  62 static CoroutineThreadState *coroutine_get_thread_state(void)
  63 {
  64     CoroutineThreadState *s = pthread_getspecific(thread_state_key);
  65
  66     if (!s) {
  67         s = g_malloc0(sizeof(*s));
  68         s->current = &s->leader.base;
  69         pthread_setspecific(thread_state_key, s);
  70     }
  71     return s;
  72 }
  73
  74 static void qemu_coroutine_thread_cleanup(void *opaque)
  75 {
  76     CoroutineThreadState *s = opaque;
  77
  78     g_free(s);
  79 }
  80
  81 static void __attribute__((constructor)) coroutine_init(void)
  82 {
  83     int ret;
  84
  85     ret = pthread_key_create(&thread_state_key, qemu_coroutine_thread_cleanup);
  86     if (ret != 0) {
  87         fprintf(stderr, "unable to create leader key: %s\n", strerror(errno));
  88         abort();
  89     }
  90 }
  91
  92 /* "boot" function
  93  * This is what starts the coroutine, is called from the trampoline
  94  * (from the signal handler when it is not signal handling, read ahead
  95  * for more information).
  96  */
  97 static void coroutine_bootstrap(CoroutineSigAltStack *self, Coroutine *co)
  98 {
  99     /* Initialize longjmp environment and switch back the caller */
 100     if (!sigsetjmp(self->env, 0)) {
 101         siglongjmp(*(sigjmp_buf *)co->entry_arg, 1);
 102     }
 103
 104     while (true) {
 105         co->entry(co->entry_arg);
 106         qemu_coroutine_switch(co, co->caller, COROUTINE_TERMINATE);
 107     }
 108 }
 109
 110 /*
 111  * This is used as the signal handler. This is called with the brand new stack
 112  * (thanks to sigaltstack). We have to return, given that this is a signal
 113  * handler and the sigmask and some other things are changed.
 114  */
 115 static void coroutine_trampoline(int signal)
 116 {
 117     CoroutineSigAltStack *self;
 118     Coroutine *co;
 119     CoroutineThreadState *coTS;
 120
 121     /* Get the thread specific information */
 122     coTS = coroutine_get_thread_state();
 123     self = coTS->tr_handler;
 124     coTS->tr_called = 1;
 125     co = &self->base;
 126
 127     /*
 128      * Here we have to do a bit of a ping pong between the caller, given that
 129      * this is a signal handler and we have to do a return "soon". Then the
 130      * caller can reestablish everything and do a siglongjmp here again.
 131      */
 132     if (!sigsetjmp(coTS->tr_reenter, 0)) {
 133         return;
 134     }
 135
 136     /*
 137      * Ok, the caller has siglongjmp'ed back to us, so now prepare
 138      * us for the real machine state switching. We have to jump
 139      * into another function here to get a new stack context for
 140      * the auto variables (which have to be auto-variables
 141      * because the start of the thread happens later). Else with
 142      * PIC (i.e. Position Independent Code which is used when PTH
 143      * is built as a shared library) most platforms would
 144      * horrible core dump as experience showed.
 145      */
 146     coroutine_bootstrap(self, co);
 147 }
 148
 149 Coroutine *qemu_coroutine_new(void)
 150 {
 151     CoroutineSigAltStack *co;
 152     CoroutineThreadState *coTS;
 153     struct sigaction sa;
 154     struct sigaction osa;
 155     stack_t ss;
 156     stack_t oss;
 157     sigset_t sigs;
 158     sigset_t osigs;
 159     sigjmp_buf old_env;
 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     if (sigaction(SIGUSR2, &sa, &osa) != 0) {
 190         abort();
 191     }
 192
 193     /*
 194      * Set the new stack.
 195      */
 196     ss.ss_sp = co->stack;
 197     ss.ss_size = co->stack_size;
 198     ss.ss_flags = 0;
 199     if (sigaltstack(&ss, &oss) < 0) {
 200         abort();
 201     }
 202
 203     /*
 204      * Now transfer control onto the signal stack and set it up.
 205      * It will return immediately via "return" after the sigsetjmp()
 206      * was performed. Be careful here with race conditions.  The
 207      * signal can be delivered the first time sigsuspend() is
 208      * called.
 209      */
 210     coTS->tr_called = 0;
 211     pthread_kill(pthread_self(), SIGUSR2);
 212     sigfillset(&sigs);
 213     sigdelset(&sigs, SIGUSR2);
 214     while (!coTS->tr_called) {
 215         sigsuspend(&sigs);
 216     }
 217
 218     /*
 219      * Inform the system that we are back off the signal stack by
 220      * removing the alternative signal stack. Be careful here: It
 221      * first has to be disabled, before it can be removed.
 222      */
 223     sigaltstack(NULL, &ss);
 224     ss.ss_flags = SS_DISABLE;
 225     if (sigaltstack(&ss, NULL) < 0) {
 226         abort();
 227     }
 228     sigaltstack(NULL, &ss);
 229     if (!(oss.ss_flags & SS_DISABLE)) {
 230         sigaltstack(&oss, NULL);
 231     }
 232
 233     /*
 234      * Restore the old SIGUSR2 signal handler and mask
 235      */
 236     sigaction(SIGUSR2, &osa, NULL);
 237     pthread_sigmask(SIG_SETMASK, &osigs, NULL);
 238
 239     /*
 240      * Now enter the trampoline again, but this time not as a signal
 241      * handler. Instead we jump into it directly. The functionally
 242      * redundant ping-pong pointer arithmetic is necessary to avoid
 243      * type-conversion warnings related to the `volatile' qualifier and
 244      * the fact that `jmp_buf' usually is an array type.
 245      */
 246     if (!sigsetjmp(old_env, 0)) {
 247         siglongjmp(coTS->tr_reenter, 1);
 248     }
 249
 250     /*
 251      * Ok, we returned again, so now we're finished
 252      */
 253
 254     return &co->base;
 255 }
 256
 257 void qemu_coroutine_delete(Coroutine *co_)
 258 {
 259     CoroutineSigAltStack *co = DO_UPCAST(CoroutineSigAltStack, base, co_);
 260
 261     qemu_free_stack(co->stack, co->stack_size);
 262     g_free(co);
 263 }
 264
 265 CoroutineAction qemu_coroutine_switch(Coroutine *from_, Coroutine *to_,
 266                                       CoroutineAction action)
 267 {
 268     CoroutineSigAltStack *from = DO_UPCAST(CoroutineSigAltStack, base, from_);
 269     CoroutineSigAltStack *to = DO_UPCAST(CoroutineSigAltStack, base, to_);
 270     CoroutineThreadState *s = coroutine_get_thread_state();
 271     int ret;
 272
 273     s->current = to_;
 274
 275     ret = sigsetjmp(from->env, 0);
 276     if (ret == 0) {
 277         siglongjmp(to->env, action);
 278     }
 279     return ret;
 280 }
 281
 282 Coroutine *qemu_coroutine_self(void)
 283 {
 284     CoroutineThreadState *s = coroutine_get_thread_state();
 285
 286     return s->current;
 287 }
 288
 289 bool qemu_in_coroutine(void)
 290 {
 291     CoroutineThreadState *s = pthread_getspecific(thread_state_key);
 292
 293     return s && s->current->caller;
 294 }
 295