Re: [PATCH v3 1/1] pidfd: implement PIDFD_THREAD flag for pidfd_open()

[Christian Brauner <brauner@kernel.org>]

On Wed, Jan 31, 2024 at 02:26:02PM +0100, Oleg Nesterov wrote:
> With this flag:
> 
> 	- pidfd_open() doesn't require that the target task must be
> 	  a thread-group leader
> 
> 	- pidfd_poll() succeeds when the task exits and becomes a
> 	  zombie (iow, passes exit_notify()), even if it is a leader
> 	  and thread-group is not empty.
> 
> 	  This means that the behaviour of pidfd_poll(PIDFD_THREAD,
> 	  pid-of-group-leader) is not well defined if it races with
> 	  exec() from its sub-thread; pidfd_poll() can succeed or not
> 	  depending on whether pidfd_task_exited() is called before
> 	  or after exchange_tids().
> 
> 	  Perhaps we can improve this behaviour later, pidfd_poll()
> 	  can probably take sig->group_exec_task into account. But
> 	  this doesn't really differ from the case when the leader
> 	  exits before other threads (so pidfd_poll() succeeds) and
> 	  then another thread execs and pidfd_poll() will block again.
> 
> thread_group_exited() is no longer used, perhaps it can die.
> 
> Co-developed-by: Tycho Andersen <tycho@tycho.pizza>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>  fs/exec.c                  |  6 +++++-
>  include/linux/pid.h        |  3 ++-
>  include/uapi/linux/pidfd.h |  3 ++-
>  kernel/exit.c              |  7 +++++++
>  kernel/fork.c              | 38 +++++++++++++++++++++++++++++++-------
>  kernel/pid.c               | 14 +++-----------
>  kernel/signal.c            |  6 ++++--
>  7 files changed, 54 insertions(+), 23 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 73e4045df271..0fd7e668c477 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1143,7 +1143,11 @@ static int de_thread(struct task_struct *tsk)
>  
>  		BUG_ON(leader->exit_state != EXIT_ZOMBIE);
>  		leader->exit_state = EXIT_DEAD;
> -
> +		/*
> +		 * leader and tsk exhanged their pids, the old pid dies,
> +		 * wake up the PIDFD_THREAD waiters.
> +		 */
> +		do_notify_pidfd(leader);
>  		/*
>  		 * We are going to release_task()->ptrace_unlink() silently,
>  		 * the tracer can sleep in do_wait(). EXIT_DEAD guarantees
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index e6a041cb8bac..8124d57752b9 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -70,10 +70,11 @@ extern const struct file_operations pidfd_fops;
>  
>  struct file;
>  
> -extern struct pid *pidfd_pid(const struct file *file);
> +struct pid *pidfd_pid(const struct file *file);
>  struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags);
>  struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags);
>  int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret);
> +void do_notify_pidfd(struct task_struct *task);
>  
>  static inline struct pid *get_pid(struct pid *pid)
>  {
> 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..493647fd7c07 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..bea32071fff2 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 */

One more thing that came to my mind. We also support waitid(P_PIDFD,
pidfd). And I just looked through the code and I think it does the right
thing when we pass it a PIDFD_THREAD pidfd because wait_consider_task()
rejects task->exit_signal != SIGCHLD in eligible_child() and
CLONE_THREAD implies task->exit_signal == -1.