From: "Joel Fernandes (Google)" joel@joelfernandes.org
[ Upstream commit b53b0b9d9a613c418057f6cb921c2f40a6f78c24 ]
This patch adds polling support to pidfd.
Android low memory killer (LMK) needs to know when a process dies once it is sent the kill signal. It does so by checking for the existence of /proc/pid which is both racy and slow. For example, if 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. Using the polling support, LMK will be able to get notified when a process exists in race-free and fast way, and allows the LMK to do other things (such as by polling on other fds) while awaiting the process being killed to die.
For notification to polling processes, we follow the same existing mechanism in the kernel used when the parent of the task group is to be notified of a child's death (do_notify_parent). This is precisely when the tasks waiting on a poll of pidfd are also awakened in this patch.
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_thread(), the new thread group leader 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.
Cc: Andy Lutomirski luto@amacapital.net Cc: Steven Rostedt rostedt@goodmis.org Cc: Daniel Colascione dancol@google.com Cc: Jann Horn jannh@google.com Cc: Tim Murray timmurray@google.com Cc: Jonathan Kowalski bl0pbl33p@gmail.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Al Viro viro@zeniv.linux.org.uk Cc: Kees Cook keescook@chromium.org Cc: David Howells dhowells@redhat.com Cc: Oleg Nesterov oleg@redhat.com Cc: kernel-team@android.com Reviewed-by: Oleg Nesterov oleg@redhat.com Co-developed-by: Daniel Colascione dancol@google.com Signed-off-by: Daniel Colascione dancol@google.com Signed-off-by: Joel Fernandes (Google) joel@joelfernandes.org Signed-off-by: Christian Brauner christian@brauner.io Cc: stable@vger.kernel.org # 4.19.x Signed-off-by: Wen Yang wenyang@linux.alibaba.com --- include/linux/pid.h | 3 +++ kernel/fork.c | 26 ++++++++++++++++++++++++++ kernel/pid.c | 2 ++ kernel/signal.c | 11 +++++++++++ 4 files changed, 42 insertions(+)
diff --git a/include/linux/pid.h b/include/linux/pid.h index 29c0a99..a82d2f7 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 notifications */ + wait_queue_head_t wait_pidfd; struct rcu_head rcu; struct upid numbers[1]; }; diff --git a/kernel/fork.c b/kernel/fork.c index e419891..33dc746 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1688,8 +1688,34 @@ static void pidfd_show_fdinfo(struct seq_file *m, struct file *f) } #endif
+/* + * Poll support for process exit notification. + */ +static unsigned int pidfd_poll(struct file *file, struct poll_table_struct *pts) +{ + struct task_struct *task; + struct pid *pid = file->private_data; + int poll_flags = 0; + + poll_wait(file, &pid->wait_pidfd, pts); + + rcu_read_lock(); + task = pid_task(pid, PIDTYPE_PID); + /* + * Inform pollers only when the whole thread group exits. + * 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 (!task || (task->exit_state && thread_group_empty(task))) + poll_flags = POLLIN | POLLRDNORM; + rcu_read_unlock(); + + return poll_flags; +} + const struct file_operations pidfd_fops = { .release = pidfd_release, + .poll = pidfd_poll, #ifdef CONFIG_PROC_FS .show_fdinfo = pidfd_show_fdinfo, #endif diff --git a/kernel/pid.c b/kernel/pid.c index b88fe5e..3ba6fcb 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 a02a25a..22a04795 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1810,6 +1810,14 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type) return ret; }
+static void do_notify_pidfd(struct task_struct *task) +{ + struct pid *pid; + + pid = task_pid(task); + wake_up_all(&pid->wait_pidfd); +} + /* * Let a parent know about the death of a child. * For a stopped/continued status change, use do_notify_parent_cldstop instead. @@ -1833,6 +1841,9 @@ 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_notify_pidfd(tsk); + if (sig != SIGCHLD) { /* * This is only possible if parent == real_parent.