xref: /openbmc/linux/fs/signalfd.c (revision e868d61272caa648214046a096e5a6bfc068dc8c)
1 /*
2  *  fs/signalfd.c
3  *
4  *  Copyright (C) 2003  Linus Torvalds
5  *
6  *  Mon Mar 5, 2007: Davide Libenzi <davidel@xmailserver.org>
7  *      Changed ->read() to return a siginfo strcture instead of signal number.
8  *      Fixed locking in ->poll().
9  *      Added sighand-detach notification.
10  *      Added fd re-use in sys_signalfd() syscall.
11  *      Now using anonymous inode source.
12  *      Thanks to Oleg Nesterov for useful code review and suggestions.
13  *      More comments and suggestions from Arnd Bergmann.
14  */
15 
16 #include <linux/file.h>
17 #include <linux/poll.h>
18 #include <linux/init.h>
19 #include <linux/fs.h>
20 #include <linux/sched.h>
21 #include <linux/kernel.h>
22 #include <linux/signal.h>
23 #include <linux/list.h>
24 #include <linux/anon_inodes.h>
25 #include <linux/signalfd.h>
26 
27 struct signalfd_ctx {
28 	struct list_head lnk;
29 	wait_queue_head_t wqh;
30 	sigset_t sigmask;
31 	struct task_struct *tsk;
32 };
33 
34 struct signalfd_lockctx {
35 	struct task_struct *tsk;
36 	unsigned long flags;
37 };
38 
39 /*
40  * Tries to acquire the sighand lock. We do not increment the sighand
41  * use count, and we do not even pin the task struct, so we need to
42  * do it inside an RCU read lock, and we must be prepared for the
43  * ctx->tsk going to NULL (in signalfd_deliver()), and for the sighand
44  * being detached. We return 0 if the sighand has been detached, or
45  * 1 if we were able to pin the sighand lock.
46  */
47 static int signalfd_lock(struct signalfd_ctx *ctx, struct signalfd_lockctx *lk)
48 {
49 	struct sighand_struct *sighand = NULL;
50 
51 	rcu_read_lock();
52 	lk->tsk = rcu_dereference(ctx->tsk);
53 	if (likely(lk->tsk != NULL))
54 		sighand = lock_task_sighand(lk->tsk, &lk->flags);
55 	rcu_read_unlock();
56 
57 	if (sighand && !ctx->tsk) {
58 		unlock_task_sighand(lk->tsk, &lk->flags);
59 		sighand = NULL;
60 	}
61 
62 	return sighand != NULL;
63 }
64 
65 static void signalfd_unlock(struct signalfd_lockctx *lk)
66 {
67 	unlock_task_sighand(lk->tsk, &lk->flags);
68 }
69 
70 /*
71  * This must be called with the sighand lock held.
72  */
73 void signalfd_deliver(struct task_struct *tsk, int sig)
74 {
75 	struct sighand_struct *sighand = tsk->sighand;
76 	struct signalfd_ctx *ctx, *tmp;
77 
78 	BUG_ON(!sig);
79 	list_for_each_entry_safe(ctx, tmp, &sighand->signalfd_list, lnk) {
80 		/*
81 		 * We use a negative signal value as a way to broadcast that the
82 		 * sighand has been orphaned, so that we can notify all the
83 		 * listeners about this. Remember the ctx->sigmask is inverted,
84 		 * so if the user is interested in a signal, that corresponding
85 		 * bit will be zero.
86 		 */
87 		if (sig < 0) {
88 			if (ctx->tsk == tsk) {
89 				ctx->tsk = NULL;
90 				list_del_init(&ctx->lnk);
91 				wake_up(&ctx->wqh);
92 			}
93 		} else {
94 			if (!sigismember(&ctx->sigmask, sig))
95 				wake_up(&ctx->wqh);
96 		}
97 	}
98 }
99 
100 static void signalfd_cleanup(struct signalfd_ctx *ctx)
101 {
102 	struct signalfd_lockctx lk;
103 
104 	/*
105 	 * This is tricky. If the sighand is gone, we do not need to remove
106 	 * context from the list, the list itself won't be there anymore.
107 	 */
108 	if (signalfd_lock(ctx, &lk)) {
109 		list_del(&ctx->lnk);
110 		signalfd_unlock(&lk);
111 	}
112 	kfree(ctx);
113 }
114 
115 static int signalfd_release(struct inode *inode, struct file *file)
116 {
117 	signalfd_cleanup(file->private_data);
118 	return 0;
119 }
120 
121 static unsigned int signalfd_poll(struct file *file, poll_table *wait)
122 {
123 	struct signalfd_ctx *ctx = file->private_data;
124 	unsigned int events = 0;
125 	struct signalfd_lockctx lk;
126 
127 	poll_wait(file, &ctx->wqh, wait);
128 
129 	/*
130 	 * Let the caller get a POLLIN in this case, ala socket recv() when
131 	 * the peer disconnects.
132 	 */
133 	if (signalfd_lock(ctx, &lk)) {
134 		if (next_signal(&lk.tsk->pending, &ctx->sigmask) > 0 ||
135 		    next_signal(&lk.tsk->signal->shared_pending,
136 				&ctx->sigmask) > 0)
137 			events |= POLLIN;
138 		signalfd_unlock(&lk);
139 	} else
140 		events |= POLLIN;
141 
142 	return events;
143 }
144 
145 /*
146  * Copied from copy_siginfo_to_user() in kernel/signal.c
147  */
148 static int signalfd_copyinfo(struct signalfd_siginfo __user *uinfo,
149 			     siginfo_t const *kinfo)
150 {
151 	long err;
152 
153 	BUILD_BUG_ON(sizeof(struct signalfd_siginfo) != 128);
154 
155 	/*
156 	 * Unused memebers should be zero ...
157 	 */
158 	err = __clear_user(uinfo, sizeof(*uinfo));
159 
160 	/*
161 	 * If you change siginfo_t structure, please be sure
162 	 * this code is fixed accordingly.
163 	 */
164 	err |= __put_user(kinfo->si_signo, &uinfo->signo);
165 	err |= __put_user(kinfo->si_errno, &uinfo->err);
166 	err |= __put_user((short)kinfo->si_code, &uinfo->code);
167 	switch (kinfo->si_code & __SI_MASK) {
168 	case __SI_KILL:
169 		err |= __put_user(kinfo->si_pid, &uinfo->pid);
170 		err |= __put_user(kinfo->si_uid, &uinfo->uid);
171 		break;
172 	case __SI_TIMER:
173 		 err |= __put_user(kinfo->si_tid, &uinfo->tid);
174 		 err |= __put_user(kinfo->si_overrun, &uinfo->overrun);
175 		 err |= __put_user((long)kinfo->si_ptr, &uinfo->svptr);
176 		break;
177 	case __SI_POLL:
178 		err |= __put_user(kinfo->si_band, &uinfo->band);
179 		err |= __put_user(kinfo->si_fd, &uinfo->fd);
180 		break;
181 	case __SI_FAULT:
182 		err |= __put_user((long)kinfo->si_addr, &uinfo->addr);
183 #ifdef __ARCH_SI_TRAPNO
184 		err |= __put_user(kinfo->si_trapno, &uinfo->trapno);
185 #endif
186 		break;
187 	case __SI_CHLD:
188 		err |= __put_user(kinfo->si_pid, &uinfo->pid);
189 		err |= __put_user(kinfo->si_uid, &uinfo->uid);
190 		err |= __put_user(kinfo->si_status, &uinfo->status);
191 		err |= __put_user(kinfo->si_utime, &uinfo->utime);
192 		err |= __put_user(kinfo->si_stime, &uinfo->stime);
193 		break;
194 	case __SI_RT: /* This is not generated by the kernel as of now. */
195 	case __SI_MESGQ: /* But this is */
196 		err |= __put_user(kinfo->si_pid, &uinfo->pid);
197 		err |= __put_user(kinfo->si_uid, &uinfo->uid);
198 		err |= __put_user((long)kinfo->si_ptr, &uinfo->svptr);
199 		break;
200 	default: /* this is just in case for now ... */
201 		err |= __put_user(kinfo->si_pid, &uinfo->pid);
202 		err |= __put_user(kinfo->si_uid, &uinfo->uid);
203 		break;
204 	}
205 
206 	return err ? -EFAULT: sizeof(*uinfo);
207 }
208 
209 /*
210  * Returns either the size of a "struct signalfd_siginfo", or zero if the
211  * sighand we are attached to, has been orphaned. The "count" parameter
212  * must be at least the size of a "struct signalfd_siginfo".
213  */
214 static ssize_t signalfd_read(struct file *file, char __user *buf, size_t count,
215 			     loff_t *ppos)
216 {
217 	struct signalfd_ctx *ctx = file->private_data;
218 	ssize_t res = 0;
219 	int locked, signo;
220 	siginfo_t info;
221 	struct signalfd_lockctx lk;
222 	DECLARE_WAITQUEUE(wait, current);
223 
224 	if (count < sizeof(struct signalfd_siginfo))
225 		return -EINVAL;
226 	locked = signalfd_lock(ctx, &lk);
227 	if (!locked)
228 		return 0;
229 	res = -EAGAIN;
230 	signo = dequeue_signal(lk.tsk, &ctx->sigmask, &info);
231 	if (signo == 0 && !(file->f_flags & O_NONBLOCK)) {
232 		add_wait_queue(&ctx->wqh, &wait);
233 		for (;;) {
234 			set_current_state(TASK_INTERRUPTIBLE);
235 			signo = dequeue_signal(lk.tsk, &ctx->sigmask, &info);
236 			if (signo != 0)
237 				break;
238 			if (signal_pending(current)) {
239 				res = -ERESTARTSYS;
240 				break;
241 			}
242 			signalfd_unlock(&lk);
243 			schedule();
244 			locked = signalfd_lock(ctx, &lk);
245 			if (unlikely(!locked)) {
246 				/*
247 				 * Let the caller read zero byte, ala socket
248 				 * recv() when the peer disconnect. This test
249 				 * must be done before doing a dequeue_signal(),
250 				 * because if the sighand has been orphaned,
251 				 * the dequeue_signal() call is going to crash.
252 				 */
253 				res = 0;
254 				break;
255 			}
256 		}
257 		remove_wait_queue(&ctx->wqh, &wait);
258 		__set_current_state(TASK_RUNNING);
259 	}
260 	if (likely(locked))
261 		signalfd_unlock(&lk);
262 	if (likely(signo))
263 		res = signalfd_copyinfo((struct signalfd_siginfo __user *) buf,
264 					&info);
265 
266 	return res;
267 }
268 
269 static const struct file_operations signalfd_fops = {
270 	.release	= signalfd_release,
271 	.poll		= signalfd_poll,
272 	.read		= signalfd_read,
273 };
274 
275 /*
276  * Create a file descriptor that is associated with our signal
277  * state. We can pass it around to others if we want to, but
278  * it will always be _our_ signal state.
279  */
280 asmlinkage long sys_signalfd(int ufd, sigset_t __user *user_mask, size_t sizemask)
281 {
282 	int error;
283 	sigset_t sigmask;
284 	struct signalfd_ctx *ctx;
285 	struct sighand_struct *sighand;
286 	struct file *file;
287 	struct inode *inode;
288 	struct signalfd_lockctx lk;
289 
290 	if (sizemask != sizeof(sigset_t) ||
291 	    copy_from_user(&sigmask, user_mask, sizeof(sigmask)))
292 		return error = -EINVAL;
293 	sigdelsetmask(&sigmask, sigmask(SIGKILL) | sigmask(SIGSTOP));
294 	signotset(&sigmask);
295 
296 	if (ufd == -1) {
297 		ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
298 		if (!ctx)
299 			return -ENOMEM;
300 
301 		init_waitqueue_head(&ctx->wqh);
302 		ctx->sigmask = sigmask;
303 		ctx->tsk = current;
304 
305 		sighand = current->sighand;
306 		/*
307 		 * Add this fd to the list of signal listeners.
308 		 */
309 		spin_lock_irq(&sighand->siglock);
310 		list_add_tail(&ctx->lnk, &sighand->signalfd_list);
311 		spin_unlock_irq(&sighand->siglock);
312 
313 		/*
314 		 * When we call this, the initialization must be complete, since
315 		 * anon_inode_getfd() will install the fd.
316 		 */
317 		error = anon_inode_getfd(&ufd, &inode, &file, "[signalfd]",
318 					 &signalfd_fops, ctx);
319 		if (error)
320 			goto err_fdalloc;
321 	} else {
322 		file = fget(ufd);
323 		if (!file)
324 			return -EBADF;
325 		ctx = file->private_data;
326 		if (file->f_op != &signalfd_fops) {
327 			fput(file);
328 			return -EINVAL;
329 		}
330 		/*
331 		 * We need to be prepared of the fact that the sighand this fd
332 		 * is attached to, has been detched. In that case signalfd_lock()
333 		 * will return 0, and we'll just skip setting the new mask.
334 		 */
335 		if (signalfd_lock(ctx, &lk)) {
336 			ctx->sigmask = sigmask;
337 			signalfd_unlock(&lk);
338 		}
339 		wake_up(&ctx->wqh);
340 		fput(file);
341 	}
342 
343 	return ufd;
344 
345 err_fdalloc:
346 	signalfd_cleanup(ctx);
347 	return error;
348 }
349 
350