Re: [PATCH v3 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

Please the the incomplete/untested patch below.

	- The change in exit_notify() is sub-optimal, we can do better
	  to avoid 2 do_notify_pidfd() calls from exit_notify(). But
	  so far this is only for discussion, lets keep it simple.

	- __pidfd_prepare() needs some minor cleanups regardless of
	  this change, I'll send the patch...

What do you think?

And why is thread_group_exited() exported?

Oleg.

diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h
index 5406fbc13074..2e6461459877 100644
--- a/include/uapi/linux/pidfd.h
+++ b/include/uapi/linux/pidfd.h

@@ -7,6 +7,7 @@
 #include <linux/fcntl.h>
 
 /* Flags for pidfd_open().  */
-#define PIDFD_NONBLOCK O_NONBLOCK
+#define PIDFD_NONBLOCK	O_NONBLOCK
+#define PIDFD_THREAD	O_EXCL	// or anything else not used by anon_inode's

 
 #endif /* _UAPI_LINUX_PIDFD_H */

diff --git a/kernel/exit.c b/kernel/exit.c
index dfb963d2f862..9f8526b7d717 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c

@@ -752,6 +752,10 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 		autoreap = true;
 	}
 
+	/* unnecessary if do_notify_parent() was already called,
+	   we can do better */
+	do_notify_pidfd(tsk);
+
 	if (autoreap) {
 		tsk->exit_state = EXIT_DEAD;
 		list_add(&tsk->ptrace_entry, &dead);

diff --git a/kernel/fork.c b/kernel/fork.c
index c981fa6171c1..38f2c7423fb4 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c

@@ -101,6 +101,7 @@
 #include <linux/user_events.h>
 #include <linux/iommu.h>
 #include <linux/rseq.h>
+#include <uapi/linux/pidfd.h>
 
 #include <asm/pgalloc.h>
 #include <linux/uaccess.h>

@@ -2068,12 +2069,27 @@ static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
 }
 #endif
 
+static bool xxx_exited(struct pid *pid, int excl)
+{
+	struct task_struct *task;
+	bool exited;
+
+	rcu_read_lock();
+	task = pid_task(pid, PIDTYPE_PID);
+	exited = !task ||
+		(READ_ONCE(task->exit_state) && (excl || thread_group_empty(task)));
+	rcu_read_unlock();
+
+	return exited;
+}
+
 /*
  * Poll support for process exit notification.
  */
 static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
 {
 	struct pid *pid = file->private_data;
+	int excl = file->f_flags & PIDFD_THREAD;
 	__poll_t poll_flags = 0;
 
 	poll_wait(file, &pid->wait_pidfd, pts);

@@ -2083,7 +2099,7 @@ static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
 	 * If the thread group leader exits before all other threads in the
 	 * group, then poll(2) should block, similar to the wait(2) family.
 	 */
-	if (thread_group_exited(pid))
+	if (xxx_exited(pid, excl))
 		poll_flags = EPOLLIN | EPOLLRDNORM;
 
 	return poll_flags;

@@ -2129,7 +2145,9 @@ static int __pidfd_prepare(struct pid *pid, unsigned int flags, struct file **re
 {
 	int pidfd;
 	struct file *pidfd_file;
+	unsigned excl = flags & PIDFD_THREAD;
 
+	flags &= ~PIDFD_THREAD;
 	if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
 		return -EINVAL;
 

@@ -2144,6 +2162,7 @@ static int __pidfd_prepare(struct pid *pid, unsigned int flags, struct file **re
 		return PTR_ERR(pidfd_file);
 	}
 	get_pid(pid); /* held by pidfd_file now */
+	pidfd_file->f_flags |= excl;
 	*ret = pidfd_file;
 	return pidfd;
 }

@@ -2176,7 +2195,9 @@ static int __pidfd_prepare(struct pid *pid, unsigned int flags, struct file **re
  */
 int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret)
 {
-	if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
+	unsigned excl = flags & PIDFD_THREAD;
+
+	if (!pid || !pid_has_task(pid, excl ? PIDTYPE_PID : PIDTYPE_TGID))
 		return -EINVAL;
 
 	return __pidfd_prepare(pid, flags, ret);

diff --git a/kernel/pid.c b/kernel/pid.c
index b52b10865454..5257197f9493 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c

@@ -629,7 +629,7 @@ SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
 	int fd;
 	struct pid *p;
 
-	if (flags & ~PIDFD_NONBLOCK)
+	if (flags & ~(PIDFD_NONBLOCK | PIDFD_THREAD))
 		return -EINVAL;
 
 	if (pid <= 0)