Re: [RFC PATCH] pidfd: implement PIDFD_THREAD flag for pidfd_open()

[Christian Brauner <brauner@kernel.org>]

On Mon, Jan 29, 2024 at 12:23:15PM +0100, Oleg Nesterov wrote:
> On 01/27, Oleg Nesterov wrote:
> >
> > I'll (hopefully) send v2 on top of
> >
> > 	pidfd: cleanup the usage of __pidfd_prepare's flags
> > 	pidfd: don't do_notify_pidfd() if !thread_group_empty()
> >
> > on Monday
> 
> Sorry, I don't have time to finish v2 today, I need to update the comments
> and write the changelog.
> 
> But the patch itself is ready, I am sending it for review.
> 
> Tycho, Christian, any comments?
> 
> Oleg.
> 
> 
> From c31780f6c1136a72048d24701ac6d8401fc1afda Mon Sep 17 00:00:00 2001
> From: Oleg Nesterov <oleg@redhat.com>
> Date: Sat, 27 Jan 2024 16:59:18 +0100
> Subject: [PATCH] pidfd: implement PIDFD_THREAD flag for pidfd_open()
> 
> ---
>  include/uapi/linux/pidfd.h |  3 ++-
>  kernel/exit.c              |  7 +++++++
>  kernel/fork.c              | 29 +++++++++++++++++++++++++++--
>  kernel/pid.c               |  2 +-
>  kernel/signal.c            |  4 +++-
>  5 files changed, 40 insertions(+), 5 deletions(-)
> 
> 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
>  
>  #endif /* _UAPI_LINUX_PIDFD_H */
> diff --git a/kernel/exit.c b/kernel/exit.c
> index dfb963d2f862..74fe6bfb9577 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -739,6 +739,13 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
>  		kill_orphaned_pgrp(tsk->group_leader, NULL);
>  
>  	tsk->exit_state = EXIT_ZOMBIE;
> +	/*
> +	 * sub-thread or delay_group_leader(), wake up the PIDFD_THREAD
> +	 * waiters.
> +	 */
> +	if (!thread_group_empty(tsk))
> +		do_notify_pidfd(tsk);
> +
>  	if (unlikely(tsk->ptrace)) {
>  		int sig = thread_group_leader(tsk) &&
>  				thread_group_empty(tsk) &&
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 347641398f9d..977b58c0eac6 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>
> @@ -2050,6 +2051,8 @@ static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
>  
>  	seq_put_decimal_ll(m, "Pid:\t", nr);
>  
> +	/* TODO: report PIDFD_THREAD */

Ah yes, very good point. We should give userspace an indicator whether
something is thread pidfd or not.

> +
>  #ifdef CONFIG_PID_NS
>  	seq_put_decimal_ll(m, "\nNSpid:\t", nr);
>  	if (nr > 0) {
> @@ -2068,12 +2071,27 @@ static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
>  }
>  #endif
>  
> +static bool pidfd_task_exited(struct pid *pid, bool thread)
> +{
> +	struct task_struct *task;
> +	bool exited;
> +
> +	rcu_read_lock();
> +	task = pid_task(pid, PIDTYPE_PID);
> +	exited = !task ||
> +		(READ_ONCE(task->exit_state) && (thread || 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;
> +	bool thread = file->f_flags & PIDFD_THREAD;
>  	__poll_t poll_flags = 0;
>  
>  	poll_wait(file, &pid->wait_pidfd, pts);
> @@ -2083,7 +2101,7 @@ 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 (pidfd_task_exited(pid, thread))
>  		poll_flags = EPOLLIN | EPOLLRDNORM;
>  
>  	return poll_flags;
> @@ -2141,6 +2159,11 @@ 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 */
> +	/*
> +	 * anon_inode_getfile() ignores everything outside of the
> +	 * O_ACCMODE | O_NONBLOCK mask, set PIDFD_THREAD manually.
> +	 */
> +	pidfd_file->f_flags |= (flags & PIDFD_THREAD);
>  	*ret = pidfd_file;
>  	return pidfd;
>  }
> @@ -2173,7 +2196,9 @@ 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))
> +	bool thread = flags & PIDFD_THREAD;
> +
> +	if (!pid || !pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID));
>  		return -EINVAL;
>  
>  	return __pidfd_prepare(pid, flags, ret);
> diff --git a/kernel/pid.c b/kernel/pid.c
> index c7a3e359f8f5..04bdd5ecf183 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)
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 9561a3962ca6..919cd33a0405 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -2051,7 +2051,8 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
>  	WARN_ON_ONCE(!tsk->ptrace &&
>  	       (tsk->group_leader != tsk || !thread_group_empty(tsk)));
>  	/*
> -	 * tsk is a group leader and has no threads, wake up the pidfd waiters.
> +	 * tsk is a group leader and has no threads, wake up the !PIDFD_THREAD
> +	 * waiters.
>  	 */
>  	if (thread_group_empty(tsk))
>  		do_notify_pidfd(tsk);
> @@ -3926,6 +3927,7 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
>  		prepare_kill_siginfo(sig, &kinfo);
>  	}
>  
> +	/* TODO: respect PIDFD_THREAD */

So I've been thinking about this at the end of last week. Do we need to
give userspace a way to send a thread-group wide signal even when a
PIDFD_THREAD pidfd is passed? Or should we just not worry about this
right now and wait until someone needs this?

>  	ret = kill_pid_info(sig, &kinfo, pid);

Otherwise this looks good to me!