I posted this a few weeks back, took another look at it and refined it a
bit. I'd like some input on the viability of this approach.

A new signalfd setup flag is added, SFD_TASK. This is only valid if used
with SFD_CLOEXEC. If set, the task setting up the signalfd descriptor is
remembered in the signalfd context, and will be the one we use for
checking signals in the poll/read handlers in signalfd.

This is needed to make signalfd useful with io_uring and aio, of which
the former in particular has my interest.

I _think_ this is sane. To prevent the case of a task clearing O_CLOEXEC
on the signalfd descriptor, forking, and then exiting, we grab a
reference to the task when we assign it. If that original task exits, we
catch it in signalfd_flush() and ensure waiters are woken up. The
waiters also hold a task reference, so we don't have to wait for them to
go away.

Need to double check we can't race between original task exiting and new
task grabbing a reference. I don't think this is solid in the version
below. Probably need to add a refcount for ctx->task (the pointer, not
the task) for that.

Comments? Attaching two test programs using io_uring, one using poll and
the other read. Remove SFD_TASK from either of them, and they will fail
ala:

./signalfd-read
Timed out waiting for cqe

and with SFD_TASK set, both will exit silent with a value of 0. You need
liburing installed, then compile them with:

gcc -Wall -O2 -o signalfd-read signalfd-read.c -luring

---

diff --git a/fs/signalfd.c b/fs/signalfd.c
index 44b6845b071c..4bbdab9438c1 100644
--- a/fs/signalfd.c
+++ b/fs/signalfd.c
@@ -50,28 +50,62 @@ void signalfd_cleanup(struct sighand_struct *sighand)
  
  struct signalfd_ctx {
  	sigset_t sigmask;
+	struct task_struct *task;
  };
  
+static int signalfd_flush(struct file *file, void *data)
+{
+	struct signalfd_ctx *ctx = file->private_data;
+	struct task_struct *tsk = ctx->task;
+
+	if (tsk == current) {
+		ctx->task = NULL;
+		wake_up(&tsk->sighand->signalfd_wqh);
+		put_task_struct(tsk);
+	}
+
+	return 0;
+}
+
  static int signalfd_release(struct inode *inode, struct file *file)
  {
-	kfree(file->private_data);
+	struct signalfd_ctx *ctx = file->private_data;
+
+	if (ctx->task)
+		put_task_struct(ctx->task);
+	kfree(ctx);
  	return 0;
  }
  
+static void signalfd_put_task(struct task_struct *tsk)
+{
+	put_task_struct(tsk);
+}
+
+static struct task_struct *signalfd_get_task(struct signalfd_ctx *ctx)
+{
+	struct task_struct *tsk = ctx->task ?: current;
+
+	get_task_struct(tsk);
+	return tsk;
+}
+
  static __poll_t signalfd_poll(struct file *file, poll_table *wait)
  {
  	struct signalfd_ctx *ctx = file->private_data;
+	struct task_struct *tsk = signalfd_get_task(ctx);
  	__poll_t events = 0;
  
-	poll_wait(file, &current->sighand->signalfd_wqh, wait);
+	poll_wait(file, &tsk->sighand->signalfd_wqh, wait);
  
-	spin_lock_irq(&current->sighand->siglock);
-	if (next_signal(&current->pending, &ctx->sigmask) ||
-	    next_signal(&current->signal->shared_pending,
+	spin_lock_irq(&tsk->sighand->siglock);
+	if (next_signal(&tsk->pending, &ctx->sigmask) ||
+	    next_signal(&tsk->signal->shared_pending,
  			&ctx->sigmask))
  		events |= EPOLLIN;
-	spin_unlock_irq(&current->sighand->siglock);
+	spin_unlock_irq(&tsk->sighand->siglock);
  
+	signalfd_put_task(tsk);
  	return events;
  }
  
@@ -167,10 +201,11 @@ static ssize_t signalfd_dequeue(struct signalfd_ctx *ctx, kernel_siginfo_t *info
  				int nonblock)
  {
  	ssize_t ret;
+	struct task_struct *tsk = signalfd_get_task(ctx);
  	DECLARE_WAITQUEUE(wait, current);
  
-	spin_lock_irq(&current->sighand->siglock);
-	ret = dequeue_signal(current, &ctx->sigmask, info);
+	spin_lock_irq(&tsk->sighand->siglock);
+	ret = dequeue_signal(tsk, &ctx->sigmask, info);
  	switch (ret) {
  	case 0:
  		if (!nonblock)
@@ -178,29 +213,35 @@ static ssize_t signalfd_dequeue(struct signalfd_ctx *ctx, kernel_siginfo_t *info
  		ret = -EAGAIN;
  		/* fall through */
  	default:
-		spin_unlock_irq(&current->sighand->siglock);
+		spin_unlock_irq(&tsk->sighand->siglock);
+		signalfd_put_task(tsk);
  		return ret;
  	}
  
-	add_wait_queue(&current->sighand->signalfd_wqh, &wait);
+	add_wait_queue(&tsk->sighand->signalfd_wqh, &wait);
  	for (;;) {
  		set_current_state(TASK_INTERRUPTIBLE);
-		ret = dequeue_signal(current, &ctx->sigmask, info);
+		ret = dequeue_signal(tsk, &ctx->sigmask, info);
  		if (ret != 0)
  			break;
  		if (signal_pending(current)) {
  			ret = -ERESTARTSYS;
  			break;
  		}
-		spin_unlock_irq(&current->sighand->siglock);
+		spin_unlock_irq(&tsk->sighand->siglock);
  		schedule();
-		spin_lock_irq(&current->sighand->siglock);
+		spin_lock_irq(&tsk->sighand->siglock);
+		if (tsk != current && !ctx->task) {
+			ret = -ESRCH;
+			break;
+		}
  	}
-	spin_unlock_irq(&current->sighand->siglock);
+	spin_unlock_irq(&tsk->sighand->siglock);
  
-	remove_wait_queue(&current->sighand->signalfd_wqh, &wait);
+	remove_wait_queue(&tsk->sighand->signalfd_wqh, &wait);
  	__set_current_state(TASK_RUNNING);
  
+	signalfd_put_task(tsk);
  	return ret;
  }
  
@@ -254,6 +295,7 @@ static const struct file_operations signalfd_fops = {
  #ifdef CONFIG_PROC_FS
  	.show_fdinfo	= signalfd_show_fdinfo,
  #endif
+	.flush		= signalfd_flush,
  	.release	= signalfd_release,
  	.poll		= signalfd_poll,
  	.read		= signalfd_read,
@@ -267,19 +309,26 @@ static int do_signalfd4(int ufd, sigset_t *mask, int flags)
  	/* Check the SFD_* constants for consistency.  */
  	BUILD_BUG_ON(SFD_CLOEXEC != O_CLOEXEC);
  	BUILD_BUG_ON(SFD_NONBLOCK != O_NONBLOCK);
+	BUILD_BUG_ON(SFD_TASK & (SFD_CLOEXEC | SFD_NONBLOCK));
  
-	if (flags & ~(SFD_CLOEXEC | SFD_NONBLOCK))
+	if (flags & ~(SFD_CLOEXEC | SFD_NONBLOCK | SFD_TASK))
+		return -EINVAL;
+	if ((flags & (SFD_CLOEXEC | SFD_TASK)) == SFD_TASK)
  		return -EINVAL;
  
  	sigdelsetmask(mask, sigmask(SIGKILL) | sigmask(SIGSTOP));
  	signotset(mask);
  
  	if (ufd == -1) {
-		ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
+		ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
  		if (!ctx)
  			return -ENOMEM;
  
  		ctx->sigmask = *mask;
+		if (flags & SFD_TASK) {
+			ctx->task = current;
+			get_task_struct(ctx->task);
+		}
  
  		/*
  		 * When we call this, the initialization must be complete, since
@@ -290,6 +339,7 @@ static int do_signalfd4(int ufd, sigset_t *mask, int flags)
  		if (ufd < 0)
  			kfree(ctx);
  	} else {
+		struct task_struct *tsk;
  		struct fd f = fdget(ufd);
  		if (!f.file)
  			return -EBADF;
@@ -298,11 +348,13 @@ static int do_signalfd4(int ufd, sigset_t *mask, int flags)
  			fdput(f);
  			return -EINVAL;
  		}
-		spin_lock_irq(&current->sighand->siglock);
+		tsk = signalfd_get_task(ctx);
+		spin_lock_irq(&tsk->sighand->siglock);
  		ctx->sigmask = *mask;
-		spin_unlock_irq(&current->sighand->siglock);
+		spin_unlock_irq(&tsk->sighand->siglock);
  
-		wake_up(&current->sighand->signalfd_wqh);
+		wake_up(&tsk->sighand->signalfd_wqh);
+		signalfd_put_task(tsk);
  		fdput(f);
  	}
  
diff --git a/include/uapi/linux/signalfd.h b/include/uapi/linux/signalfd.h
index 83429a05b698..064c5dc3eb99 100644
--- a/include/uapi/linux/signalfd.h
+++ b/include/uapi/linux/signalfd.h
@@ -16,6 +16,7 @@
  /* Flags for signalfd4.  */
  #define SFD_CLOEXEC O_CLOEXEC
  #define SFD_NONBLOCK O_NONBLOCK
+#define SFD_TASK 00000001
  
  struct signalfd_siginfo {
  	__u32 ssi_signo;

-- 
Jens Axboe