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pidfd: implement PIDFD_THREAD flag for pidfd_open()

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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>
Link: https://lore.kernel.org/r/20240131132602.GA23641@redhat.com
Tested-by: Tycho Andersen <tandersen@netflix.com>
Reviewed-by: Tycho Andersen <tandersen@netflix.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Oleg Nesterov
2024-01-31 14:26:02 +01:00
committed by Christian Brauner
parent 21e25205d7
commit 64bef697d3
7 changed files with 54 additions and 23 deletions
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14
kernel/pid.c
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@@ -552,11 +552,6 @@ struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags)
* Return the task associated with @pidfd. The function takes a reference on
* the returned task. The caller is responsible for releasing that reference.
*
* Currently, the process identified by @pidfd is always a thread-group leader.
* This restriction currently exists for all aspects of pidfds including pidfd
* creation (CLONE_PIDFD cannot be used with CLONE_THREAD) and pidfd polling
* (only supports thread group leaders).
*
* Return: On success, the task_struct associated with the pidfd.
* On error, a negative errno number will be returned.
*/
@@ -615,11 +610,8 @@ static int pidfd_create(struct pid *pid, unsigned int flags)
* @flags: flags to pass
*
* This creates a new pid file descriptor with the O_CLOEXEC flag set for
* the process identified by @pid. Currently, the process identified by
* @pid must be a thread-group leader. This restriction currently exists
* for all aspects of pidfds including pidfd creation (CLONE_PIDFD cannot
* be used with CLONE_THREAD) and pidfd polling (only supports thread group
* leaders).
* the task identified by @pid. Without PIDFD_THREAD flag the target task
* must be a thread-group leader.
*
* Return: On success, a cloexec pidfd is returned.
* On error, a negative errno number will be returned.
@@ -629,7 +621,7 @@ 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)