Date:	Thu, 15 Oct 2015 10:42:08 +0900
From:	Damian Hobson-Garcia <dhobsong@...l.co.jp>
To:	viro@...iv.linux.org.uk, sustrik@...bpm.com
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-api@...r.kernel.org, netdev@...r.kernel.org,
	David.Laight@...lab.com, Damian Hobson-Garcia <dhobsong@...l.co.jp>
Subject: [PATCH v3 1/1] eventfd: implementation of EFD_MASK flag

From: Martin Sustrik <sustrik@...bpm.com>

When implementing network protocols in user space, one has to implement
fake file descriptors to represent the sockets for the protocol.

Polling on such fake file descriptors is a problem (poll/select/epoll
accept only true file descriptors) and forces protocol implementers to use
various workarounds resulting in complex, non-standard and convoluted APIs.

More generally, ability to create full-blown file descriptors for
userspace-to-userspace signalling is missing. While eventfd(2) goes half
the way towards this goal it has follwoing shorcomings:

I.  There's no way to signal POLLPRI, POLLHUP etc.
II. There's no way to signal arbitrary combination of POLL* flags. Most
    notably, simultaneous !POLLIN and !POLLOUT, which is a perfectly valid
    combination for a network protocol (rx buffer is empty and tx buffer is
    full), cannot be signaled using eventfd.

This patch implements new EFD_MASK flag which solves the above problems.

The semantics of EFD_MASK are as follows:

eventfd(2):

If eventfd is created with EFD_MASK flag set, it is initialised in such a
way as to signal no events on the file descriptor when it is polled on.
The 'initval' argument is ignored.

write(2):

User is allowed to write only buffers containing a 32-bit value
representing any combination of event flags as defined by the poll(2)
function (POLLIN, POLLOUT, POLLERR, POLLHUP etc.). Specified events
will be signaled when polling (select, poll, epoll) on the eventfd is
done later on.

read(2):

read is not supported and will fail with EINVAL.

select(2), poll(2) and similar:

When polling on the eventfd marked by EFD_MASK flag, all the events
specified in last written event flags shall be signaled.

Signed-off-by: Martin Sustrik <sustrik@...bpm.com>

[dhobsong@...l.co.jp: Rebased, and resubmitted for Linux 4.3]
Signed-off-by: Damian Hobson-Garcia <dhobsong@...l.co.jp>
---
 fs/eventfd.c                 | 102 ++++++++++++++++++++++++++++++++++++++-----
 include/linux/eventfd.h      |  16 +------
 include/uapi/linux/eventfd.h |  33 ++++++++++++++
 3 files changed, 126 insertions(+), 25 deletions(-)
 create mode 100644 include/uapi/linux/eventfd.h

diff --git a/fs/eventfd.c b/fs/eventfd.c
index 8d0c0df..1310779 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -2,6 +2,7 @@
  *  fs/eventfd.c
  *
  *  Copyright (C) 2007  Davide Libenzi <davidel@...ilserver.org>
+ *  Copyright (C) 2013  Martin Sustrik <sustrik@...bpm.com>
  *
  */
 
@@ -22,18 +23,31 @@
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>
 
+#define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
+#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE | EFD_MASK)
+#define EFD_MASK_VALID_EVENTS (POLLIN | POLLPRI | POLLOUT | POLLERR | POLLHUP)
+
 struct eventfd_ctx {
 	struct kref kref;
 	wait_queue_head_t wqh;
-	/*
-	 * Every time that a write(2) is performed on an eventfd, the
-	 * value of the __u64 being written is added to "count" and a
-	 * wakeup is performed on "wqh". A read(2) will return the "count"
-	 * value to userspace, and will reset "count" to zero. The kernel
-	 * side eventfd_signal() also, adds to the "count" counter and
-	 * issue a wakeup.
-	 */
-	__u64 count;
+	union {
+		/*
+		 * Every time that a write(2) is performed on an eventfd, the
+		 * value of the __u64 being written is added to "count" and a
+		 * wakeup is performed on "wqh". A read(2) will return the
+		 * "count" value to userspace, and will reset "count" to zero.
+		 * The kernel side eventfd_signal() also, adds to the "count"
+		 * counter and issue a wakeup.
+		 */
+		__u64 count;
+
+		/*
+		 * When using eventfd in EFD_MASK mode this stracture stores the
+		 * current events to be signaled on the eventfd (events member)
+		 * along with opaque user-defined data (data member).
+		 */
+		__u32 events;
+	};
 	unsigned int flags;
 };
 
@@ -134,6 +148,14 @@ static unsigned int eventfd_poll(struct file *file, poll_table *wait)
 	return events;
 }
 
+static unsigned int eventfd_mask_poll(struct file *file, poll_table *wait)
+{
+	struct eventfd_ctx *ctx = file->private_data;
+
+	poll_wait(file, &ctx->wqh, wait);
+	return ctx->events;
+}
+
 static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
 {
 	*cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
@@ -239,6 +261,14 @@ static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
 	return put_user(cnt, (__u64 __user *) buf) ? -EFAULT : sizeof(cnt);
 }
 
+static ssize_t eventfd_mask_read(struct file *file, char __user *buf,
+			    size_t count,
+			    loff_t *ppos)
+{
+	return -EINVAL;
+}
+
+
 static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
 			     loff_t *ppos)
 {
@@ -286,6 +316,28 @@ static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t c
 	return res;
 }
 
+static ssize_t eventfd_mask_write(struct file *file, const char __user *buf,
+			     size_t count,
+			     loff_t *ppos)
+{
+	struct eventfd_ctx *ctx = file->private_data;
+	__u32 events;
+
+	if (count < sizeof(events))
+		return -EINVAL;
+	if (copy_from_user(&events, buf, sizeof(events)))
+		return -EFAULT;
+	if (events & ~EFD_MASK_VALID_EVENTS)
+		return -EINVAL;
+	spin_lock_irq(&ctx->wqh.lock);
+	memcpy(&ctx->events, &events, sizeof(ctx->events));
+	if (waitqueue_active(&ctx->wqh))
+		wake_up_locked_poll(&ctx->wqh,
+			(unsigned long)ctx->events);
+	spin_unlock_irq(&ctx->wqh.lock);
+	return sizeof(ctx->events);
+}
+
 #ifdef CONFIG_PROC_FS
 static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
 {
@@ -296,6 +348,16 @@ static void eventfd_show_fdinfo(struct seq_file *m, struct file *f)
 		   (unsigned long long)ctx->count);
 	spin_unlock_irq(&ctx->wqh.lock);
 }
+
+static void eventfd_mask_show_fdinfo(struct seq_file *m, struct file *f)
+{
+	struct eventfd_ctx *ctx = f->private_data;
+
+	spin_lock_irq(&ctx->wqh.lock);
+	seq_printf(m, "eventfd-mask: %x\n",
+		ctx->events);
+	spin_unlock_irq(&ctx->wqh.lock);
+}
 #endif
 
 static const struct file_operations eventfd_fops = {
@@ -309,6 +371,17 @@ static const struct file_operations eventfd_fops = {
 	.llseek		= noop_llseek,
 };
 
+static const struct file_operations eventfd_mask_fops = {
+#ifdef CONFIG_PROC_FS
+	.show_fdinfo	= eventfd_mask_show_fdinfo,
+#endif
+	.release	= eventfd_release,
+	.poll		= eventfd_mask_poll,
+	.read		= eventfd_mask_read,
+	.write		= eventfd_mask_write,
+	.llseek		= noop_llseek,
+};
+
 /**
  * eventfd_fget - Acquire a reference of an eventfd file descriptor.
  * @fd: [in] Eventfd file descriptor.
@@ -392,6 +465,7 @@ struct file *eventfd_file_create(unsigned int count, int flags)
 {
 	struct file *file;
 	struct eventfd_ctx *ctx;
+	const struct file_operations *fops;
 
 	/* Check the EFD_* constants for consistency.  */
 	BUILD_BUG_ON(EFD_CLOEXEC != O_CLOEXEC);
@@ -406,10 +480,16 @@ struct file *eventfd_file_create(unsigned int count, int flags)
 
 	kref_init(&ctx->kref);
 	init_waitqueue_head(&ctx->wqh);
-	ctx->count = count;
+	if (flags & EFD_MASK) {
+		ctx->events = 0;
+		fops = &eventfd_mask_fops;
+	} else {
+		ctx->count = count;
+		fops = &eventfd_fops;
+	}
 	ctx->flags = flags;
 
-	file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx,
+	file = anon_inode_getfile("[eventfd]", fops, ctx,
 				  O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS));
 	if (IS_ERR(file))
 		eventfd_free_ctx(ctx);
diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
index ff0b981..87de343 100644
--- a/include/linux/eventfd.h
+++ b/include/linux/eventfd.h
@@ -8,23 +8,11 @@
 #ifndef _LINUX_EVENTFD_H
 #define _LINUX_EVENTFD_H
 
+#include <uapi/linux/eventfd.h>
+
 #include <linux/fcntl.h>
 #include <linux/wait.h>
 
-/*
- * CAREFUL: Check include/uapi/asm-generic/fcntl.h when defining
- * new flags, since they might collide with O_* ones. We want
- * to re-use O_* flags that couldn't possibly have a meaning
- * from eventfd, in order to leave a free define-space for
- * shared O_* flags.
- */
-#define EFD_SEMAPHORE (1 << 0)
-#define EFD_CLOEXEC O_CLOEXEC
-#define EFD_NONBLOCK O_NONBLOCK
-
-#define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
-#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
-
 struct file;
 
 #ifdef CONFIG_EVENTFD
diff --git a/include/uapi/linux/eventfd.h b/include/uapi/linux/eventfd.h
new file mode 100644
index 0000000..097dcad
--- /dev/null
+++ b/include/uapi/linux/eventfd.h
@@ -0,0 +1,33 @@
+/*
+ *  Copyright (C) 2013 Martin Sustrik <sustrik@...bpm.com>
+ *
+ *  This program is free software; you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation; either version 2 of the License, or
+ *  (at your option) any later version.
+ */
+
+#ifndef _UAPI_LINUX_EVENTFD_H
+#define _UAPI_LINUX_EVENTFD_H
+
+/* For O_CLOEXEC */
+#include <linux/fcntl.h>
+#include <linux/types.h>
+
+/*
+ * CAREFUL: Check include/asm-generic/fcntl.h when defining
+ * new flags, since they might collide with O_* ones. We want
+ * to re-use O_* flags that couldn't possibly have a meaning
+ * from eventfd, in order to leave a free define-space for
+ * shared O_* flags.
+ */
+
+/* Provide semaphore-like semantics for reads from the eventfd. */
+#define EFD_SEMAPHORE (1 << 0)
+/* Provide event mask semantics for the eventfd. */
+#define EFD_MASK (1 << 1)
+/*  Set the close-on-exec (FD_CLOEXEC) flag on the eventfd. */
+#define EFD_CLOEXEC O_CLOEXEC
+/*  Create the eventfd in non-blocking mode. */
+#define EFD_NONBLOCK O_NONBLOCK
+#endif /* _UAPI_LINUX_EVENTFD_H */
-- 
1.9.1

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