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Subject[RFC PATCH v2] waitfd
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

--CJD

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);


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