signal.c

  1// SPDX-License-Identifier: GPL-2.0
  2/*
  3 * Copyright (C) 2015 Anton Ivanov (aivanov@{brocade.com,kot-begemot.co.uk})
  4 * Copyright (C) 2015 Thomas Meyer (thomas@m3y3r.de)
  5 * Copyright (C) 2004 PathScale, Inc
  6 * Copyright (C) 2004 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
  7 */
  8
  9#include <stdlib.h>
 10#include <stdarg.h>
 11#include <stdbool.h>
 12#include <errno.h>
 13#include <signal.h>
 14#include <string.h>
 15#include <strings.h>
 16#include <as-layout.h>
 17#include <kern_util.h>
 18#include <os.h>
 19#include <sysdep/mcontext.h>
 20#include <um_malloc.h>
 21#include <sys/ucontext.h>
 22#include <timetravel.h>
 23#include "internal.h"
 24
 25void (*sig_info[NSIG])(int, struct siginfo *, struct uml_pt_regs *, void *mc) = {
 26	[SIGTRAP]	= relay_signal,
 27	[SIGFPE]	= relay_signal,
 28	[SIGILL]	= relay_signal,
 29	[SIGWINCH]	= winch,
 30	[SIGBUS]	= relay_signal,
 31	[SIGSEGV]	= segv_handler,
 32	[SIGIO]		= sigio_handler,
 33	[SIGCHLD]	= sigchld_handler,
 34};
 35
 36static void sig_handler_common(int sig, struct siginfo *si, mcontext_t *mc)
 37{
 38	struct uml_pt_regs r;
 39
 40	r.is_user = 0;
 41	if (sig == SIGSEGV) {
 42		/* For segfaults, we want the data from the sigcontext. */
 43		get_regs_from_mc(&r, mc);
 44		GET_FAULTINFO_FROM_MC(r.faultinfo, mc);
 45	}
 46
 47	/* enable signals if sig isn't IRQ signal */
 48	if ((sig != SIGIO) && (sig != SIGWINCH) && (sig != SIGCHLD))
 49		unblock_signals_trace();
 50
 51	(*sig_info[sig])(sig, si, &r, mc);
 52}
 53
 54/*
 55 * These are the asynchronous signals.  SIGPROF is excluded because we want to
 56 * be able to profile all of UML, not just the non-critical sections.  If
 57 * profiling is not thread-safe, then that is not my problem.  We can disable
 58 * profiling when SMP is enabled in that case.
 59 */
 60#define SIGIO_BIT 0
 61#define SIGIO_MASK (1 << SIGIO_BIT)
 62
 63#define SIGALRM_BIT 1
 64#define SIGALRM_MASK (1 << SIGALRM_BIT)
 65
 66#define SIGCHLD_BIT 2
 67#define SIGCHLD_MASK (1 << SIGCHLD_BIT)
 68
 69__thread int signals_enabled;
 70#if IS_ENABLED(CONFIG_UML_TIME_TRAVEL_SUPPORT)
 71static int signals_blocked, signals_blocked_pending;
 72#endif
 73static __thread unsigned int signals_pending;
 74static __thread unsigned int signals_active;
 75
 76static void sig_handler(int sig, struct siginfo *si, mcontext_t *mc)
 77{
 78	int enabled = signals_enabled;
 79
 80#if IS_ENABLED(CONFIG_UML_TIME_TRAVEL_SUPPORT)
 81	if ((signals_blocked ||
 82	     __atomic_load_n(&signals_blocked_pending, __ATOMIC_SEQ_CST)) &&
 83	    (sig == SIGIO)) {
 84		/* increment so unblock will do another round */
 85		__atomic_add_fetch(&signals_blocked_pending, 1,
 86				   __ATOMIC_SEQ_CST);
 87		return;
 88	}
 89#endif
 90
 91	if (!enabled && (sig == SIGIO)) {
 92		/*
 93		 * In TT_MODE_EXTERNAL, need to still call time-travel
 94		 * handlers. This will mark signals_pending by itself
 95		 * (only if necessary.)
 96		 * Note we won't get here if signals are hard-blocked
 97		 * (which is handled above), in that case the hard-
 98		 * unblock will handle things.
 99		 */
100		if (time_travel_mode == TT_MODE_EXTERNAL)
101			sigio_run_timetravel_handlers();
102		else
103			signals_pending |= SIGIO_MASK;
104		return;
105	}
106
107	if (!enabled && (sig == SIGCHLD)) {
108		signals_pending |= SIGCHLD_MASK;
109		return;
110	}
111
112	block_signals_trace();
113
114	sig_handler_common(sig, si, mc);
115
116	um_set_signals_trace(enabled);
117}
118
119static void timer_real_alarm_handler(mcontext_t *mc)
120{
121	struct uml_pt_regs regs;
122
123	if (mc != NULL)
124		get_regs_from_mc(&regs, mc);
125	else
126		memset(&regs, 0, sizeof(regs));
127	timer_handler(SIGALRM, NULL, &regs);
128}
129
130static void timer_alarm_handler(int sig, struct siginfo *unused_si, mcontext_t *mc)
131{
132	int enabled;
133
134	enabled = signals_enabled;
135	if (!signals_enabled) {
136		signals_pending |= SIGALRM_MASK;
137		return;
138	}
139
140	block_signals_trace();
141
142	signals_active |= SIGALRM_MASK;
143
144	timer_real_alarm_handler(mc);
145
146	signals_active &= ~SIGALRM_MASK;
147
148	um_set_signals_trace(enabled);
149}
150
151void deliver_alarm(void) {
152    timer_alarm_handler(SIGALRM, NULL, NULL);
153}
154
155void timer_set_signal_handler(void)
156{
157	set_handler(SIGALRM);
158}
159
160int timer_alarm_pending(void)
161{
162	return !!(signals_pending & SIGALRM_MASK);
163}
164
165void set_sigstack(void *sig_stack, int size)
166{
167	stack_t stack = {
168		.ss_flags = 0,
169		.ss_sp = sig_stack,
170		.ss_size = size
171	};
172
173	if (sigaltstack(&stack, NULL) != 0)
174		panic("enabling signal stack failed, errno = %d\n", errno);
175}
176
177static void sigusr1_handler(int sig, struct siginfo *unused_si, mcontext_t *mc)
178{
179	uml_pm_wake();
180}
181
182void register_pm_wake_signal(void)
183{
184	set_handler(SIGUSR1);
185}
186
187static void (*handlers[_NSIG])(int sig, struct siginfo *si, mcontext_t *mc) = {
188	[SIGSEGV] = sig_handler,
189	[SIGBUS] = sig_handler,
190	[SIGILL] = sig_handler,
191	[SIGFPE] = sig_handler,
192	[SIGTRAP] = sig_handler,
193
194	[SIGIO] = sig_handler,
195	[SIGWINCH] = sig_handler,
196	/* SIGCHLD is only actually registered in seccomp mode. */
197	[SIGCHLD] = sig_handler,
198	[SIGALRM] = timer_alarm_handler,
199
200	[SIGUSR1] = sigusr1_handler,
201};
202
203static void hard_handler(int sig, siginfo_t *si, void *p)
204{
205	ucontext_t *uc = p;
206	mcontext_t *mc = &uc->uc_mcontext;
207	int save_errno = errno;
208
209	(*handlers[sig])(sig, (struct siginfo *)si, mc);
210
211	errno = save_errno;
212}
213
214void set_handler(int sig)
215{
216	struct sigaction action;
217	int flags = SA_SIGINFO | SA_ONSTACK;
218	sigset_t sig_mask;
219
220	action.sa_sigaction = hard_handler;
221
222	/* block irq ones */
223	sigemptyset(&action.sa_mask);
224	sigaddset(&action.sa_mask, SIGIO);
225	sigaddset(&action.sa_mask, SIGWINCH);
226	sigaddset(&action.sa_mask, SIGALRM);
227
228	if (sig == SIGSEGV)
229		flags |= SA_NODEFER;
230
231	if (sigismember(&action.sa_mask, sig))
232		flags |= SA_RESTART; /* if it's an irq signal */
233
234	action.sa_flags = flags;
235	action.sa_restorer = NULL;
236	if (sigaction(sig, &action, NULL) < 0)
237		panic("sigaction failed - errno = %d\n", errno);
238
239	sigemptyset(&sig_mask);
240	sigaddset(&sig_mask, sig);
241	if (sigprocmask(SIG_UNBLOCK, &sig_mask, NULL) < 0)
242		panic("sigprocmask failed - errno = %d\n", errno);
243}
244
245void send_sigio_to_self(void)
246{
247	kill(os_getpid(), SIGIO);
248}
249
250int change_sig(int signal, int on)
251{
252	sigset_t sigset;
253
254	sigemptyset(&sigset);
255	sigaddset(&sigset, signal);
256	if (sigprocmask(on ? SIG_UNBLOCK : SIG_BLOCK, &sigset, NULL) < 0)
257		return -errno;
258
259	return 0;
260}
261
262static inline void __block_signals(void)
263{
264	if (!signals_enabled)
265		return;
266
267	os_local_ipi_disable();
268	barrier();
269	signals_enabled = 0;
270}
271
272static inline void __unblock_signals(void)
273{
274	if (signals_enabled)
275		return;
276
277	signals_enabled = 1;
278	barrier();
279	os_local_ipi_enable();
280}
281
282void block_signals(void)
283{
284	__block_signals();
285	/*
286	 * This must return with signals disabled, so this barrier
287	 * ensures that writes are flushed out before the return.
288	 * This might matter if gcc figures out how to inline this and
289	 * decides to shuffle this code into the caller.
290	 */
291	barrier();
292}
293
294void unblock_signals(void)
295{
296	int save_pending;
297
298	if (signals_enabled == 1)
299		return;
300
301	__unblock_signals();
302
303#if IS_ENABLED(CONFIG_UML_TIME_TRAVEL_SUPPORT)
304	deliver_time_travel_irqs();
305#endif
306
307	/*
308	 * We loop because the IRQ handler returns with interrupts off.  So,
309	 * interrupts may have arrived and we need to re-enable them and
310	 * recheck signals_pending.
311	 */
312	while (1) {
313		/*
314		 * Save and reset save_pending after enabling signals.  This
315		 * way, signals_pending won't be changed while we're reading it.
316		 *
317		 * Setting signals_enabled and reading signals_pending must
318		 * happen in this order, so have the barrier here.
319		 */
320		barrier();
321
322		save_pending = signals_pending;
323		if (save_pending == 0)
324			return;
325
326		signals_pending = 0;
327
328		/*
329		 * We have pending interrupts, so disable signals, as the
330		 * handlers expect them off when they are called.  They will
331		 * be enabled again above. We need to trace this, as we're
332		 * expected to be enabling interrupts already, but any more
333		 * tracing that happens inside the handlers we call for the
334		 * pending signals will mess up the tracing state.
335		 */
336		__block_signals();
337		um_trace_signals_off();
338
339		/*
340		 * Deal with SIGIO first because the alarm handler might
341		 * schedule, leaving the pending SIGIO stranded until we come
342		 * back here.
343		 *
344		 * SIGIO's handler doesn't use siginfo or mcontext,
345		 * so they can be NULL.
346		 */
347		if (save_pending & SIGIO_MASK)
348			sig_handler_common(SIGIO, NULL, NULL);
349
350		if (save_pending & SIGCHLD_MASK) {
351			struct uml_pt_regs regs = {};
352
353			sigchld_handler(SIGCHLD, NULL, &regs, NULL);
354		}
355
356		/* Do not reenter the handler */
357
358		if ((save_pending & SIGALRM_MASK) && (!(signals_active & SIGALRM_MASK)))
359			timer_real_alarm_handler(NULL);
360
361		/* Rerun the loop only if there is still pending SIGIO and not in TIMER handler */
362
363		if (!(signals_pending & SIGIO_MASK) && (signals_active & SIGALRM_MASK))
364			return;
365
366		/* Re-enable signals and trace that we're doing so. */
367		um_trace_signals_on();
368		__unblock_signals();
369	}
370}
371
372int um_get_signals(void)
373{
374	return signals_enabled;
375}
376
377int um_set_signals(int enable)
378{
379	int ret;
380	if (signals_enabled == enable)
381		return enable;
382
383	ret = signals_enabled;
384	if (enable)
385		unblock_signals();
386	else block_signals();
387
388	return ret;
389}
390
391int um_set_signals_trace(int enable)
392{
393	int ret;
394	if (signals_enabled == enable)
395		return enable;
396
397	ret = signals_enabled;
398	if (enable)
399		unblock_signals_trace();
400	else
401		block_signals_trace();
402
403	return ret;
404}
405
406#if IS_ENABLED(CONFIG_UML_TIME_TRAVEL_SUPPORT)
407void mark_sigio_pending(void)
408{
409	/*
410	 * It would seem that this should be atomic so
411	 * it isn't a read-modify-write with a signal
412	 * that could happen in the middle, losing the
413	 * value set by the signal.
414	 *
415	 * However, this function is only called when in
416	 * time-travel=ext simulation mode, in which case
417	 * the only signal ever pending is SIGIO, which
418	 * is blocked while this can be called, and the
419	 * timer signal (SIGALRM) cannot happen.
420	 */
421	signals_pending |= SIGIO_MASK;
422}
423
424void block_signals_hard(void)
425{
426	signals_blocked++;
427	barrier();
428}
429
430void unblock_signals_hard(void)
431{
432	static bool unblocking;
433
434	if (!signals_blocked)
435		panic("unblocking signals while not blocked");
436
437	if (--signals_blocked)
438		return;
439	/*
440	 * Must be set to 0 before we check pending so the
441	 * SIGIO handler will run as normal unless we're still
442	 * going to process signals_blocked_pending.
443	 */
444	barrier();
445
446	/*
447	 * Note that block_signals_hard()/unblock_signals_hard() can be called
448	 * within the unblock_signals()/sigio_run_timetravel_handlers() below.
449	 * This would still be prone to race conditions since it's actually a
450	 * call _within_ e.g. vu_req_read_message(), where we observed this
451	 * issue, which loops. Thus, if the inner call handles the recorded
452	 * pending signals, we can get out of the inner call with the real
453	 * signal hander no longer blocked, and still have a race. Thus don't
454	 * handle unblocking in the inner call, if it happens, but only in
455	 * the outermost call - 'unblocking' serves as an ownership for the
456	 * signals_blocked_pending decrement.
457	 */
458	if (unblocking)
459		return;
460	unblocking = true;
461
462	while (__atomic_load_n(&signals_blocked_pending, __ATOMIC_SEQ_CST)) {
463		if (signals_enabled) {
464			/* signals are enabled so we can touch this */
465			signals_pending |= SIGIO_MASK;
466			/*
467			 * this is a bit inefficient, but that's
468			 * not really important
469			 */
470			block_signals();
471			unblock_signals();
472		} else {
473			/*
474			 * we need to run time-travel handlers even
475			 * if not enabled
476			 */
477			sigio_run_timetravel_handlers();
478		}
479
480		/*
481		 * The decrement of signals_blocked_pending must be atomic so
482		 * that the signal handler will either happen before or after
483		 * the decrement, not during a read-modify-write:
484		 *  - If it happens before, it can increment it and we'll
485		 *    decrement it and do another round in the loop.
486		 *  - If it happens after it'll see 0 for both signals_blocked
487		 *    and signals_blocked_pending and thus run the handler as
488		 *    usual (subject to signals_enabled, but that's unrelated.)
489		 *
490		 * Note that a call to unblock_signals_hard() within the calls
491		 * to unblock_signals() or sigio_run_timetravel_handlers() above
492		 * will do nothing due to the 'unblocking' state, so this cannot
493		 * underflow as the only one decrementing will be the outermost
494		 * one.
495		 */
496		if (__atomic_sub_fetch(&signals_blocked_pending, 1,
497				       __ATOMIC_SEQ_CST) < 0)
498			panic("signals_blocked_pending underflow");
499	}
500
501	unblocking = false;
502}
503#endif