Re: [PATCH RFC 1/2] Add polling support to pidfd

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Apr 11, 2019 at 01:50:42PM -0400, Joel Fernandes (Google) wrote:
> pidfd are /proc/pid directory file descriptors referring to a task group
> leader. Android low memory killer (LMK) needs pidfd polling support to
> replace code that currently checks for existence of /proc/pid for
> knowing a process that is signalled to be killed has died, which is both
> racy and slow. The pidfd poll approach is race-free, and also allows the
> LMK to do other things (such as by polling on other fds) while awaiting
> the process being killed to die.

It appears to me that the "pidfd" now will be an anon inode fd, and not based
on /proc/, based on discussions with Linus. So I'll rework the patches
accordingly. However that is relatively independent of this patch so this
version can also be reviewed before I send out the reworked version.

thanks,

 - Joel

> 
> It prevents a situation where a PID is reused between when LMK sends a
> kill signal and checks for existence of the PID, since the wrong PID is
> now possibly checked for existence.
> 
> In this patch, we follow the same mechanism used uhen the parent of the
> task group is to be notified, that is when the tasks waiting on a poll
> of pidfd are also awakened.
> 
> We have decided to include the waitqueue in struct pid for the following
> reasons:
> 1. The wait queue has to survive for the lifetime of the poll. Including
> it in task_struct would not be option in this case because the task can
> be reaped and destroyed before the poll returns.
> 
> 2. By including the struct pid for the waitqueue means that during
> de_exec, the thread doing de_thread() automatically gets the new
> waitqueue/pid even though its task_struct is different.
> 
> Appropriate test cases are added in the second patch to provide coverage
> of all the cases the patch is handling.
> 
> Andy had a similar patch [1] in the past which was a good reference
> however this patch tries to handle different situations properly related
> to thread group existence, and how/where it notifies. And also solves
> other bugs (existence of taks_struct).  Daniel had a similar patch [2]
> recently which this patch supercedes.
> 
> [1] https://lore.kernel.org/patchwork/patch/345098/
> [2] https://lore.kernel.org/lkml/20181029175322.189042-1-dancol@google.com/
> 
> Cc: luto@amacapital.net
> Cc: rostedt@goodmis.org
> Cc: dancol@google.com
> Cc: christian@brauner.io
> Cc: jannh@google.com
> Cc: surenb@google.com
> Cc: torvalds@linux-foundation.org
> Co-developed-by: Daniel Colascione <dancol@google.com>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> 
> ---
>  fs/proc/base.c      | 39 +++++++++++++++++++++++++++++++++++++++
>  include/linux/pid.h |  3 +++
>  kernel/exit.c       |  1 -
>  kernel/pid.c        |  2 ++
>  kernel/signal.c     | 14 ++++++++++++++
>  5 files changed, 58 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 6a803a0b75df..879900082647 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3069,8 +3069,47 @@ static int proc_tgid_base_readdir(struct file *file, struct dir_context *ctx)
>  				   tgid_base_stuff, ARRAY_SIZE(tgid_base_stuff));
>  }
>  
> +static unsigned int proc_tgid_base_poll(struct file *file, struct poll_table_struct *pts)
> +{
> +	int poll_flags = 0;
> +	struct task_struct *task;
> +	struct pid *pid;
> +
> +	task = get_proc_task(file->f_path.dentry->d_inode);
> +
> +	WARN_ON_ONCE(task && !thread_group_leader(task));
> +
> +	/*
> +	 * tasklist_lock must be held because to avoid racing with
> +	 * changes in exit_state and wake up. Basically to avoid:
> +	 *
> +	 * P0: read exit_state = 0
> +	 * P1: write exit_state = EXIT_DEAD
> +	 * P1: Do a wake up - wq is empty, so do nothing
> +	 * P0: Queue for polling - wait forever.
> +	 */
> +	read_lock(&tasklist_lock);
> +	if (!task)
> +		poll_flags = POLLIN | POLLRDNORM | POLLERR;
> +	else if (task->exit_state == EXIT_DEAD)
> +		poll_flags = POLLIN | POLLRDNORM;
> +	else if (task->exit_state == EXIT_ZOMBIE && thread_group_empty(task))
> +		poll_flags = POLLIN | POLLRDNORM;
> +
> +	if (!poll_flags) {
> +		pid = proc_pid(file->f_path.dentry->d_inode);
> +		poll_wait(file, &pid->wait_pidfd, pts);
> +	}
> +	read_unlock(&tasklist_lock);
> +
> +	if (task)
> +		put_task_struct(task);
> +	return poll_flags;
> +}
> +
>  static const struct file_operations proc_tgid_base_operations = {
>  	.read		= generic_read_dir,
> +	.poll		= proc_tgid_base_poll,
>  	.iterate_shared	= proc_tgid_base_readdir,
>  	.llseek		= generic_file_llseek,
>  };
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index b6f4ba16065a..2e0dcbc6d14e 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -3,6 +3,7 @@
>  #define _LINUX_PID_H
>  
>  #include <linux/rculist.h>
> +#include <linux/wait.h>
>  
>  enum pid_type
>  {
> @@ -60,6 +61,8 @@ struct pid
>  	unsigned int level;
>  	/* lists of tasks that use this pid */
>  	struct hlist_head tasks[PIDTYPE_MAX];
> +	/* wait queue for pidfd pollers */
> +	wait_queue_head_t wait_pidfd;
>  	struct rcu_head rcu;
>  	struct upid numbers[1];
>  };
> diff --git a/kernel/exit.c b/kernel/exit.c
> index 2166c2d92ddc..c386ec52687d 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -181,7 +181,6 @@ static void delayed_put_task_struct(struct rcu_head *rhp)
>  	put_task_struct(tsk);
>  }
>  
> -
>  void release_task(struct task_struct *p)
>  {
>  	struct task_struct *leader;
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 20881598bdfa..5c90c239242f 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -214,6 +214,8 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>  	for (type = 0; type < PIDTYPE_MAX; ++type)
>  		INIT_HLIST_HEAD(&pid->tasks[type]);
>  
> +	init_waitqueue_head(&pid->wait_pidfd);
> +
>  	upid = pid->numbers + ns->level;
>  	spin_lock_irq(&pidmap_lock);
>  	if (!(ns->pid_allocated & PIDNS_ADDING))
> diff --git a/kernel/signal.c b/kernel/signal.c
> index f98448cf2def..e3781703ef7e 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1800,6 +1800,17 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
>  	return ret;
>  }
>  
> +static void do_wakeup_pidfd_pollers(struct task_struct *task)
> +{
> +	struct pid *pid;
> +
> +	lockdep_assert_held(&tasklist_lock);
> +
> +	pid = get_task_pid(task, PIDTYPE_PID);
> +	wake_up_all(&pid->wait_pidfd);
> +	put_pid(pid);
> +}
> +
>  /*
>   * Let a parent know about the death of a child.
>   * For a stopped/continued status change, use do_notify_parent_cldstop instead.
> @@ -1823,6 +1834,9 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
>  	BUG_ON(!tsk->ptrace &&
>  	       (tsk->group_leader != tsk || !thread_group_empty(tsk)));
>  
> +	/* Wake up all pidfd waiters */
> +	do_wakeup_pidfd_pollers(tsk);
> +
>  	if (sig != SIGCHLD) {
>  		/*
>  		 * This is only possible if parent == real_parent.
> -- 
> 2.21.0.392.gf8f6787159e-goog
>