[RESEND][RFC PATCH v2] waitfd

[Casey Dahlin <cdahlin@redhat.com>]

Same as last time, but the formatting should be right now.

Original description:
Linux now exposes signals, timers, and events via file descriptors through signalfd, timerfd, and eventfd. This means programmers can use a single select/[e]poll call to monitor all change in their program. This patch aims to expose child death via the same mechanism.

waitfd provides a file descriptor out of which may be read a series of siginfo_t objects describing child death. A child process is reaped as soon as its information is read. This means child monitoring too can be performed with that same poll call.

Patch is against v2.6.28 

diff --git a/arch/x86/include/asm/unistd_32.h b/arch/x86/include/asm/unistd_32.h
index f2bba78..134d83c 100644
--- a/arch/x86/include/asm/unistd_32.h
+++ b/arch/x86/include/asm/unistd_32.h
@@ -338,6 +338,7 @@
 #define __NR_dup3		330
 #define __NR_pipe2		331
 #define __NR_inotify_init1	332
+#define __NR_waitfd		333
 
 #ifdef __KERNEL__
 
diff --git a/arch/x86/include/asm/unistd_64.h b/arch/x86/include/asm/unistd_64.h
index d2e415e..b28eb07 100644
--- a/arch/x86/include/asm/unistd_64.h
+++ b/arch/x86/include/asm/unistd_64.h
@@ -653,6 +653,8 @@ __SYSCALL(__NR_dup3, sys_dup3)
 __SYSCALL(__NR_pipe2, sys_pipe2)
 #define __NR_inotify_init1			294
 __SYSCALL(__NR_inotify_init1, sys_inotify_init1)
+#define __NR_waitfd				295
+__SYSCALL(__NR_waitfd, sys_waitfd)
 
 
 #ifndef __NO_STUBS
diff --git a/arch/x86/kernel/syscall_table_32.S b/arch/x86/kernel/syscall_table_32.S
index d44395f..c796a8b 100644
--- a/arch/x86/kernel/syscall_table_32.S
+++ b/arch/x86/kernel/syscall_table_32.S
@@ -332,3 +332,4 @@ ENTRY(sys_call_table)
 	.long sys_dup3			/* 330 */
 	.long sys_pipe2
 	.long sys_inotify_init1
+	.long sys_waitfd
diff --git a/fs/Makefile b/fs/Makefile
index d9f8afe..74c31fb 100644
--- a/fs/Makefile
+++ b/fs/Makefile
@@ -25,6 +25,7 @@ obj-$(CONFIG_INOTIFY_USER)	+= inotify_user.o
 obj-$(CONFIG_EPOLL)		+= eventpoll.o
 obj-$(CONFIG_ANON_INODES)	+= anon_inodes.o
 obj-$(CONFIG_SIGNALFD)		+= signalfd.o
+obj-$(CONFIG_WAITFD)		+= waitfd.o
 obj-$(CONFIG_TIMERFD)		+= timerfd.o
 obj-$(CONFIG_EVENTFD)		+= eventfd.o
 obj-$(CONFIG_AIO)               += aio.o
diff --git a/fs/waitfd.c b/fs/waitfd.c
new file mode 100644
index 0000000..0155a83
--- /dev/null
+++ b/fs/waitfd.c
@@ -0,0 +1,117 @@
+/*
+ *  fs/waitfd.c
+ *
+ *  Copyright (C) 2008  Red Hat, Casey Dahlin <cdahlin@redhat.com>
+ *
+ *  Largely derived from fs/signalfd.c
+ */
+
+#include <linux/file.h>
+#include <linux/poll.h>
+#include <linux/init.h>
+#include <linux/fs.h>
+#include <linux/sched.h>
+#include <linux/kernel.h>
+#include <linux/signal.h>
+#include <linux/list.h>
+#include <linux/anon_inodes.h>
+#include <linux/syscalls.h>
+
+long do_waitid(int which, pid_t upid,
+	       struct siginfo __user *infop, int options,
+	       struct rusage __user *ru);
+
+struct waitfd_ctx {
+	int ops;
+	int which;
+	pid_t upid;
+};
+
+static int waitfd_release(struct inode *inode, struct file *file)
+{
+	kfree(file->private_data);
+	return 0;
+}
+
+static unsigned int waitfd_poll(struct file *file, poll_table *wait)
+{
+	struct waitfd_ctx *ctx = file->private_data;
+	long value;
+
+	poll_wait(file, &current->signal->wait_chldexit, wait);
+
+	value = do_waitid(ctx->which, ctx->upid, NULL,
+			   ctx->ops | WNOHANG | WNOWAIT, NULL);
+	if (value > 0 || value == -ECHILD)
+		return POLLIN;
+
+	return 0;
+}
+
+/*
+ * Returns a multiple of the size of a struct siginfo, or a negative
+ * error code. The "count" parameter must be at least sizeof(struct siginfo)
+ */
+static ssize_t waitfd_read(struct file *file, char __user *buf, size_t count,
+			     loff_t *ppos)
+{
+	struct waitfd_ctx *ctx = file->private_data;
+	struct siginfo __user *info_addr = (struct siginfo *)buf;
+	int flags = ctx->ops;
+	ssize_t ret, total = 0;
+
+	count /= sizeof(struct siginfo);
+	if (!count)
+		return -EINVAL;
+
+	do {
+		ret = do_waitid(ctx->which, ctx->upid, info_addr, flags, NULL);
+		if (ret == 0)
+			ret = -EAGAIN;
+		if (ret == -ECHILD)
+			ret = 0;
+		if (ret <= 0)
+			break;
+
+		info_addr++;
+		total += sizeof(struct siginfo);
+		flags |= WNOHANG;
+	} while (--count);
+
+	return total ? total: ret;
+}
+
+static const struct file_operations waitfd_fops = {
+	.release	= waitfd_release,
+	.poll		= waitfd_poll,
+	.read		= waitfd_read,
+};
+
+asmlinkage long sys_waitfd(int which, pid_t upid, int options, int unused)
+{
+	int ufd;
+	struct waitfd_ctx *ctx;
+
+	/* Just to make sure we don't end up with a sys_waitfd4 */
+	(void)unused;
+
+	if (options & ~(WNOHANG|WEXITED|WSTOPPED|WCONTINUED))
+		return -EINVAL;
+	if (!(options & (WEXITED|WSTOPPED|WCONTINUED)))
+		return -EINVAL;
+
+	ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
+	if (!ctx)
+		return -ENOMEM;
+
+	ctx->ops = options;
+	ctx->upid = upid;
+	ctx->which = which;
+
+	ufd = anon_inode_getfd("[waitfd]", &waitfd_fops, ctx,
+			       (options & WNOHANG) ? O_NONBLOCK : 0);
+	if (ufd < 0)
+		kfree(ctx);
+
+	return ufd;
+}
diff --git a/init/Kconfig b/init/Kconfig
index f763762..bc34871 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -683,6 +683,16 @@ config EPOLL
 	  Disabling this option will cause the kernel to be built without
 	  support for epoll family of system calls.
 
+config WAITFD
+	bool "Enable waitfd() system call" if EMBEDDED
+	select ANON_INODES
+	default y
+	help
+	  Enable the waitfd() system call that allows receving child state
+	  changes from a file descriptor.
+
+	  If unsure, say Y.
+
 config SIGNALFD
 	bool "Enable signalfd() system call" if EMBEDDED
 	select ANON_INODES
diff --git a/kernel/exit.c b/kernel/exit.c
index 2d8be7e..b53e8ba 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1233,18 +1233,20 @@ static int wait_noreap_copyout(struct task_struct *p, pid_t pid, uid_t uid,
 	int retval = rusagep ? getrusage(p, RUSAGE_BOTH, rusagep) : 0;
 
 	put_task_struct(p);
-	if (!retval)
-		retval = put_user(SIGCHLD, &infop->si_signo);
-	if (!retval)
-		retval = put_user(0, &infop->si_errno);
-	if (!retval)
-		retval = put_user((short)why, &infop->si_code);
-	if (!retval)
-		retval = put_user(pid, &infop->si_pid);
-	if (!retval)
-		retval = put_user(uid, &infop->si_uid);
-	if (!retval)
-		retval = put_user(status, &infop->si_status);
+	if (infop) {
+		if (!retval)
+			retval = put_user(SIGCHLD, &infop->si_signo);
+		if (!retval)
+			retval = put_user(0, &infop->si_errno);
+		if (!retval)
+			retval = put_user((short)why, &infop->si_code);
+		if (!retval)
+			retval = put_user(pid, &infop->si_pid);
+		if (!retval)
+			retval = put_user(uid, &infop->si_uid);
+		if (!retval)
+			retval = put_user(status, &infop->si_status);
+	}
 	if (!retval)
 		retval = pid;
 	return retval;
@@ -1727,35 +1729,12 @@ repeat:
 end:
 	current->state = TASK_RUNNING;
 	remove_wait_queue(&current->signal->wait_chldexit,&wait);
-	if (infop) {
-		if (retval > 0)
-			retval = 0;
-		else {
-			/*
-			 * For a WNOHANG return, clear out all the fields
-			 * we would set so the user can easily tell the
-			 * difference.
-			 */
-			if (!retval)
-				retval = put_user(0, &infop->si_signo);
-			if (!retval)
-				retval = put_user(0, &infop->si_errno);
-			if (!retval)
-				retval = put_user(0, &infop->si_code);
-			if (!retval)
-				retval = put_user(0, &infop->si_pid);
-			if (!retval)
-				retval = put_user(0, &infop->si_uid);
-			if (!retval)
-				retval = put_user(0, &infop->si_status);
-		}
-	}
 	return retval;
 }
 
-asmlinkage long sys_waitid(int which, pid_t upid,
-			   struct siginfo __user *infop, int options,
-			   struct rusage __user *ru)
+long do_waitid(int which, pid_t upid,
+	       struct siginfo __user *infop, int options,
+	       struct rusage __user *ru)
 {
 	struct pid *pid = NULL;
 	enum pid_type type;
@@ -1789,6 +1768,39 @@ asmlinkage long sys_waitid(int which, pid_t upid,
 	ret = do_wait(type, pid, options, infop, NULL, ru);
 	put_pid(pid);
 
+	return ret;
+}
+
+asmlinkage long sys_waitid(int which, pid_t upid,
+			   struct siginfo __user *infop, int options,
+			   struct rusage __user *ru)
+{
+	long ret;
+
+	ret = do_waitid(which, upid, infop, options, ru);
+
+	if (ret > 0)
+		ret = 0;
+	else {
+		/*
+		 * For a WNOHANG return, clear out all the fields
+		 * we would set so the user can easily tell the
+		 * difference.
+		 */
+		if (!ret)
+			ret = put_user(0, &infop->si_signo);
+		if (!ret)
+			ret = put_user(0, &infop->si_errno);
+		if (!ret)
+			ret = put_user(0, &infop->si_code);
+		if (!ret)
+			ret = put_user(0, &infop->si_pid);
+		if (!ret)
+			ret = put_user(0, &infop->si_uid);
+		if (!ret)
+			ret = put_user(0, &infop->si_status);
+	}
+
 	/* avoid REGPARM breakage on x86: */
 	asmlinkage_protect(5, ret, which, upid, infop, options, ru);
 	return ret;
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index e14a232..e8d4da6 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -163,6 +163,7 @@ cond_syscall(sys_ioprio_set);
 cond_syscall(sys_ioprio_get);
 
 /* New file descriptors */
+cond_syscall(sys_waitfd);
 cond_syscall(sys_signalfd);
 cond_syscall(sys_signalfd4);
 cond_syscall(compat_sys_signalfd);