xref: /qemu/util/coroutine-sigaltstack.c (revision 9afa888c)
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 
coroutine_get_thread_state(void)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 
qemu_coroutine_thread_cleanup(void * opaque)73 static void qemu_coroutine_thread_cleanup(void *opaque)
74 {
75     CoroutineThreadState *s = opaque;
76 
77     g_free(s);
78 }
79 
coroutine_init(void)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  */
coroutine_bootstrap(CoroutineSigAltStack * self,Coroutine * co)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  */
coroutine_trampoline(int signal)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 
qemu_coroutine_new(void)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 
qemu_coroutine_delete(Coroutine * co_)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 
qemu_coroutine_switch(Coroutine * from_,Coroutine * to_,CoroutineAction action)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 
qemu_coroutine_self(void)290 Coroutine *qemu_coroutine_self(void)
291 {
292     CoroutineThreadState *s = coroutine_get_thread_state();
293 
294     return s->current;
295 }
296 
qemu_in_coroutine(void)297 bool qemu_in_coroutine(void)
298 {
299     CoroutineThreadState *s = pthread_getspecific(thread_state_key);
300 
301     return s && s->current->caller;
302 }
303 
304