[PATCH v4 0/4] pidfd: add CLONE_AUTOREAP and CLONE_PIDFD_AUTOKILL

[PATCH v4 0/4] pidfd: add CLONE_AUTOREAP and CLONE_PIDFD_AUTOKILL

[Christian Brauner]

Add two new clone3() flags for pidfd-based process lifecycle management.

CLONE_AUTOREAP makes a child process auto-reap on exit without ever
becoming a zombie. This is a per-process property in contrast to the
existing auto-reap mechanism via SA_NOCLDWAIT or SIG_IGN for SIGCHLD
which applies to all children of a given parent.

Currently the only way to automatically reap children is to set
SA_NOCLDWAIT or SIG_IGN on SIGCHLD. This is a parent-scoped property
affecting all children which makes it unsuitable for libraries or
applications that need selective auto-reaping of specific children while
still being able to wait() on others.

CLONE_AUTOREAP stores an autoreap flag in the child's signal_struct.
When the child exits do_notify_parent() checks this flag causes
exit_notify() to transition the task directly to EXIT_DEAD. Since the
flag lives on the child it survives reparenting: if the original parent
exits and the child is reparented to a subreaper or init the child still
auto-reaps when it eventually exits. This is cleaner then forcing the
subreaper to get SIGCHLD and then reaping it. If the parent doesn't care
the subreaper won't care. If there's a subreaper that would care it
would be easy enough to add a prctl() that either just turns back on
SIGCHLD and turns of auto-reaping or a prctl() that just notifies the
subreaper whenever a child is reparented to it.

CLONE_AUTOREAP can be combined with CLONE_PIDFD to allow the parent to
monitor the child's exit via poll() and retrieve exit status via
PIDFD_GET_INFO. Without CLONE_PIDFD it provides a fire-and-forget
pattern. No exit signal is delivered so exit_signal must be zero.

The flag is not inherited by the autoreap process's own children. Each
child that should be autoreaped must be explicitly created with
CLONE_AUTOREAP.

CLONE_PIDFD_AUTOKILL ties a child's lifetime to the pidfd returned from
clone3(). When the last reference to the struct file created by clone3()
is closed the kernel sends SIGKILL to the child. A pidfd obtained via
pidfd_open() for the same process does not keep the child alive and does
not trigger autokill - only the specific struct file from clone3() has
this property. This is useful for container runtimes, service managers,
and sandboxed subprocess execution - any scenario where the child must
die if the parent crashes or abandons the pidfd or just wants a
throwaway helper process.

CLONE_PIDFD_AUTOKILL requires both CLONE_PIDFD and CLONE_AUTOREAP. It
requires CLONE_PIDFD because the whole point is tying the child's
lifetime to the pidfd. It requires CLONE_AUTOREAP because a killed child
with no one to reap it would become a zombie - the primary use case is
the parent crashing or abandoning the pidfd so no one is around to call
waitpid().

CLONE_PIDFD_AUTOKILL automatically sets no_new_privs on the child
process. This ensures the child cannot escalate privileges beyond the
parent's credential level via setuid/setgid exec. Because the child can
never can more privileges than the parent the autokill SIGKILL is always
within the parent's authority. This avoids the pdeath_signal trap where
the kernel resets the property during secureexec and commit_creds()
making it useless for container runtimes and service managers that
deprivilege themselves. The no_new_privs restriction only affects the
child. The parent retains full privileges.

The clone3 pidfd is identified by the PIDFD_AUTOKILL file flag set on
the struct file at clone3() time. The pidfs .release handler checks this
flag and sends SIGKILL only when it is set. dup()/fork() share the same
struct file so they extend the child's lifetime until the last reference
drops.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
Changes in v4:
- Set no_new_privs on child when CLONE_PIDFD_AUTOKILL is used. This
  prevents the child from escalating privileges via setuid/setgid exec
  and eliminates the need for magical resets during credential changes.
  The parent retains full privileges.
- Replace autokill_pidfd pointer with PIDFD_AUTOKILL file flag checked
  in pidfs_file_release(). This eliminates the need for pointer
  comparison, stale pointer concerns, and WRITE_ONCE/READ_ONCE pairing
  (Oleg, Jann).
- Reject CLONE_AUTOREAP | CLONE_PARENT to prevent a CLONE_AUTOREAP
  child from creating silent zombies via clone(CLONE_PARENT) (Oleg).
- Link to v3: https://patch.msgid.link/20260217-work-pidfs-autoreap-v3-0-33a403c20111@kernel.org

Changes in v2:
- Add CLONE_PIDFD_AUTOKILL flag
- Decouple CLONE_AUTOREAP from CLONE_PIDFD: the autoreap mechanism has
  no dependency on pidfds. This allows fire-and-forget patterns where
  the parent does not need exit status.
- Link to v1: https://patch.msgid.link/20260216-work-pidfs-autoreap-v1-0-e63f663008f2@kernel.org

---
Christian Brauner (4):
      clone: add CLONE_AUTOREAP
      pidfd: add CLONE_PIDFD_AUTOKILL
      selftests/pidfd: add CLONE_AUTOREAP tests
      selftests/pidfd: add CLONE_PIDFD_AUTOKILL tests

 fs/pidfs.c                                         |  38 +-
 include/linux/sched/signal.h                       |   1 +
 include/uapi/linux/pidfd.h                         |   1 +
 include/uapi/linux/sched.h                         |   2 +
 kernel/fork.c                                      |  34 +-
 kernel/ptrace.c                                    |   3 +-
 kernel/signal.c                                    |   4 +
 tools/testing/selftests/pidfd/.gitignore           |   1 +
 tools/testing/selftests/pidfd/Makefile             |   2 +-
 .../testing/selftests/pidfd/pidfd_autoreap_test.c  | 793 +++++++++++++++++++++
 10 files changed, 868 insertions(+), 11 deletions(-)
---
base-commit: 6de23f81a5e08be8fbf5e8d7e9febc72a5b5f27f
change-id: 20260214-work-pidfs-autoreap-3ee677e240a8

[PATCH v4 1/4] clone: add CLONE_AUTOREAP

[Christian Brauner]

Add a new clone3() flag CLONE_AUTOREAP that makes a child process
auto-reap on exit without ever becoming a zombie. This is a per-process
property in contrast to the existing auto-reap mechanism via
SA_NOCLDWAIT or SIG_IGN for SIGCHLD which applies to all children of a
given parent.

Currently the only way to automatically reap children is to set
SA_NOCLDWAIT or SIG_IGN on SIGCHLD. This is a parent-scoped property
affecting all children which makes it unsuitable for libraries or
applications that need selective auto-reaping of specific children while
still being able to wait() on others.

CLONE_AUTOREAP stores an autoreap flag in the child's signal_struct.
When the child exits do_notify_parent() checks this flag and causes
exit_notify() to transition the task directly to EXIT_DEAD. Since the
flag lives on the child it survives reparenting: if the original parent
exits and the child is reparented to a subreaper or init the child still
auto-reaps when it eventually exits.

CLONE_AUTOREAP can be combined with CLONE_PIDFD to allow the parent to
monitor the child's exit via poll() and retrieve exit status via
PIDFD_GET_INFO. Without CLONE_PIDFD it provides a fire-and-forget
pattern where the parent simply doesn't care about the child's exit
status. No exit signal is delivered so exit_signal must be zero.

CLONE_AUTOREAP is rejected in combination with CLONE_PARENT. If a
CLONE_AUTOREAP child were to clone(CLONE_PARENT) the new grandchild
would inherit exit_signal == 0 from the autoreap parent's group leader
but without signal->autoreap. This grandchild would become a zombie that
never sends a signal and is never autoreaped - confusing and arguably
broken behavior.

The flag is not inherited by the autoreap process's own children. Each
child that should be autoreaped must be explicitly created with
CLONE_AUTOREAP.

Link: https://github.com/uapi-group/kernel-features/issues/45
Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 include/linux/sched/signal.h |  1 +
 include/uapi/linux/sched.h   |  1 +
 kernel/fork.c                | 14 +++++++++++++-
 kernel/ptrace.c              |  3 ++-
 kernel/signal.c              |  4 ++++
 5 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index a22248aebcf9..f842c86b806f 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -132,6 +132,7 @@ struct signal_struct {
 	 */
 	unsigned int		is_child_subreaper:1;
 	unsigned int		has_child_subreaper:1;
+	unsigned int		autoreap:1;
 
 #ifdef CONFIG_POSIX_TIMERS
 
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 359a14cc76a4..8a22ea640817 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -36,6 +36,7 @@
 /* Flags for the clone3() syscall. */
 #define CLONE_CLEAR_SIGHAND 0x100000000ULL /* Clear any signal handler and reset to SIG_DFL. */
 #define CLONE_INTO_CGROUP 0x200000000ULL /* Clone into a specific cgroup given the right permissions. */
+#define CLONE_AUTOREAP 0x400000000ULL /* Auto-reap child on exit. */
 
 /*
  * cloning flags intersect with CSIGNAL so can be used with unshare and clone3
diff --git a/kernel/fork.c b/kernel/fork.c
index e832da9d15a4..0dedf2999f0c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2028,6 +2028,15 @@ __latent_entropy struct task_struct *copy_process(
 			return ERR_PTR(-EINVAL);
 	}
 
+	if (clone_flags & CLONE_AUTOREAP) {
+		if (clone_flags & CLONE_THREAD)
+			return ERR_PTR(-EINVAL);
+		if (clone_flags & CLONE_PARENT)
+			return ERR_PTR(-EINVAL);
+		if (args->exit_signal)
+			return ERR_PTR(-EINVAL);
+	}
+
 	/*
 	 * Force any signals received before this point to be delivered
 	 * before the fork happens.  Collect up signals sent to multiple
@@ -2435,6 +2444,8 @@ __latent_entropy struct task_struct *copy_process(
 			 */
 			p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
 							 p->real_parent->signal->is_child_subreaper;
+			if (clone_flags & CLONE_AUTOREAP)
+				p->signal->autoreap = 1;
 			list_add_tail(&p->sibling, &p->real_parent->children);
 			list_add_tail_rcu(&p->tasks, &init_task.tasks);
 			attach_pid(p, PIDTYPE_TGID);
@@ -2897,7 +2908,8 @@ static bool clone3_args_valid(struct kernel_clone_args *kargs)
 {
 	/* Verify that no unknown flags are passed along. */
 	if (kargs->flags &
-	    ~(CLONE_LEGACY_FLAGS | CLONE_CLEAR_SIGHAND | CLONE_INTO_CGROUP))
+	    ~(CLONE_LEGACY_FLAGS | CLONE_CLEAR_SIGHAND | CLONE_INTO_CGROUP |
+	      CLONE_AUTOREAP))
 		return false;
 
 	/*
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 392ec2f75f01..68c17daef8d4 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -549,7 +549,8 @@ static bool __ptrace_detach(struct task_struct *tracer, struct task_struct *p)
 	if (!dead && thread_group_empty(p)) {
 		if (!same_thread_group(p->real_parent, tracer))
 			dead = do_notify_parent(p, p->exit_signal);
-		else if (ignoring_children(tracer->sighand)) {
+		else if (ignoring_children(tracer->sighand) ||
+			 p->signal->autoreap) {
 			__wake_up_parent(p, tracer);
 			dead = true;
 		}
diff --git a/kernel/signal.c b/kernel/signal.c
index d65d0fe24bfb..e61f39fa8c8a 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2251,6 +2251,10 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
 		if (psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN)
 			sig = 0;
 	}
+	if (!tsk->ptrace && tsk->signal->autoreap) {
+		autoreap = true;
+		sig = 0;
+	}
 	/*
 	 * Send with __send_signal as si_pid and si_uid are in the
 	 * parent's namespaces.

-- 
2.47.3

[PATCH v4 2/4] pidfd: add CLONE_PIDFD_AUTOKILL

[Christian Brauner]

Add a new clone3() flag CLONE_PIDFD_AUTOKILL that ties a child's
lifetime to the pidfd returned from clone3(). When the last reference to
the struct file created by clone3() is closed the kernel sends SIGKILL
to the child. A pidfd obtained via pidfd_open() for the same process
does not keep the child alive and does not trigger autokill - only the
specific struct file from clone3() has this property.

This is useful for container runtimes, service managers, and sandboxed
subprocess execution - any scenario where the child must die if the
parent crashes or abandons the pidfd.

CLONE_PIDFD_AUTOKILL requires both CLONE_PIDFD (the whole point is tying
lifetime to the pidfd file) and CLONE_AUTOREAP (a killed child with no
one to reap it would become a zombie). CLONE_THREAD is rejected because
autokill targets a process not a thread.

The clone3 pidfd is identified by the PIDFD_AUTOKILL file flag set on
the struct file at clone3() time. The pidfs .release handler checks this
flag and sends SIGKILL via do_send_sig_info(SIGKILL, SEND_SIG_PRIV, ...)
only when it is set. Files from pidfd_open() or open_by_handle_at() are
distinct struct files that do not carry this flag. dup()/fork() share the
same struct file so they extend the child's lifetime until the last
reference drops.

CLONE_PIDFD_AUTOKILL automatically sets no_new_privs on the child
process. This ensures the child cannot escalate privileges beyond the
parent's credential level via setuid/setgid exec. Because the child can
never outprivilege the parent the autokill SIGKILL is always within the
parent's natural authority.

This is a deliberate departure from the pdeath_signal model which is
reset during secureexec and commit_creds() rendering it useless for
container runtimes that need to deprivilege themselves. Setting
no_new_privs on the child avoids the need for any such magical resets:
the kill-on-close contract is absolute.

The no_new_privs restriction only affects the child. The parent retains
its full privileges and can continue to execute setuid binaries.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/pidfs.c                 | 38 ++++++++++++++++++++++++++++++++------
 include/uapi/linux/pidfd.h |  1 +
 include/uapi/linux/sched.h |  1 +
 kernel/fork.c              | 22 +++++++++++++++++++---
 4 files changed, 53 insertions(+), 9 deletions(-)

diff --git a/fs/pidfs.c b/fs/pidfs.c
index 318253344b5c..a8d1bca0395d 100644
--- a/fs/pidfs.c
+++ b/fs/pidfs.c
@@ -8,6 +8,8 @@
 #include <linux/mount.h>
 #include <linux/pid.h>
 #include <linux/pidfs.h>
+#include <linux/sched/signal.h>
+#include <linux/signal.h>
 #include <linux/pid_namespace.h>
 #include <linux/poll.h>
 #include <linux/proc_fs.h>
@@ -637,7 +639,28 @@ static long pidfd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	return open_namespace(ns_common);
 }
 
+static int pidfs_file_release(struct inode *inode, struct file *file)
+{
+	struct pid *pid = inode->i_private;
+	struct task_struct *task;
+
+	if (!(file->f_flags & PIDFD_AUTOKILL))
+		return 0;
+
+	guard(rcu)();
+	task = pid_task(pid, PIDTYPE_TGID);
+	if (!task)
+		return 0;
+
+	/* Not available for kthreads or user workers for now. */
+	if (WARN_ON_ONCE(task->flags & (PF_KTHREAD | PF_USER_WORKER)))
+		return 0;
+	do_send_sig_info(SIGKILL, SEND_SIG_PRIV, task, PIDTYPE_TGID);
+	return 0;
+}
+
 static const struct file_operations pidfs_file_operations = {
+	.release	= pidfs_file_release,
 	.poll		= pidfd_poll,
 #ifdef CONFIG_PROC_FS
 	.show_fdinfo	= pidfd_show_fdinfo,
@@ -1093,11 +1116,11 @@ struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
 	int ret;
 
 	/*
-	 * Ensure that PIDFD_STALE can be passed as a flag without
-	 * overloading other uapi pidfd flags.
+	 * Ensure that internal pidfd flags don't overlap with each
+	 * other or with uapi pidfd flags.
 	 */
-	BUILD_BUG_ON(PIDFD_STALE == PIDFD_THREAD);
-	BUILD_BUG_ON(PIDFD_STALE == PIDFD_NONBLOCK);
+	BUILD_BUG_ON(hweight32(PIDFD_THREAD | PIDFD_NONBLOCK |
+				PIDFD_STALE | PIDFD_AUTOKILL) != 4);
 
 	ret = path_from_stashed(&pid->stashed, pidfs_mnt, get_pid(pid), &path);
 	if (ret < 0)
@@ -1108,9 +1131,12 @@ struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
 	flags &= ~PIDFD_STALE;
 	flags |= O_RDWR;
 	pidfd_file = dentry_open(&path, flags, current_cred());
-	/* Raise PIDFD_THREAD explicitly as do_dentry_open() strips it. */
+	/*
+	 * Raise PIDFD_THREAD and PIDFD_AUTOKILL explicitly as
+	 * do_dentry_open() strips O_EXCL and O_TRUNC.
+	 */
 	if (!IS_ERR(pidfd_file))
-		pidfd_file->f_flags |= (flags & PIDFD_THREAD);
+		pidfd_file->f_flags |= (flags & (PIDFD_THREAD | PIDFD_AUTOKILL));
 
 	return pidfd_file;
 }
diff --git a/include/uapi/linux/pidfd.h b/include/uapi/linux/pidfd.h
index ea9a6811fc76..9281956a9f32 100644
--- a/include/uapi/linux/pidfd.h
+++ b/include/uapi/linux/pidfd.h
@@ -13,6 +13,7 @@
 #ifdef __KERNEL__
 #include <linux/sched.h>
 #define PIDFD_STALE CLONE_PIDFD
+#define PIDFD_AUTOKILL O_TRUNC
 #endif
 
 /* Flags for pidfd_send_signal(). */
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index 8a22ea640817..b1aea8a86e2f 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -37,6 +37,7 @@
 #define CLONE_CLEAR_SIGHAND 0x100000000ULL /* Clear any signal handler and reset to SIG_DFL. */
 #define CLONE_INTO_CGROUP 0x200000000ULL /* Clone into a specific cgroup given the right permissions. */
 #define CLONE_AUTOREAP 0x400000000ULL /* Auto-reap child on exit. */
+#define CLONE_PIDFD_AUTOKILL 0x800000000ULL /* Kill child when clone pidfd closes. */
 
 /*
  * cloning flags intersect with CSIGNAL so can be used with unshare and clone3
diff --git a/kernel/fork.c b/kernel/fork.c
index 0dedf2999f0c..778aed24e01d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2037,6 +2037,15 @@ __latent_entropy struct task_struct *copy_process(
 			return ERR_PTR(-EINVAL);
 	}
 
+	if (clone_flags & CLONE_PIDFD_AUTOKILL) {
+		if (!(clone_flags & CLONE_PIDFD))
+			return ERR_PTR(-EINVAL);
+		if (!(clone_flags & CLONE_AUTOREAP))
+			return ERR_PTR(-EINVAL);
+		if (clone_flags & CLONE_THREAD)
+			return ERR_PTR(-EINVAL);
+	}
+
 	/*
 	 * Force any signals received before this point to be delivered
 	 * before the fork happens.  Collect up signals sent to multiple
@@ -2259,13 +2268,20 @@ __latent_entropy struct task_struct *copy_process(
 	 * if the fd table isn't shared).
 	 */
 	if (clone_flags & CLONE_PIDFD) {
-		int flags = (clone_flags & CLONE_THREAD) ? PIDFD_THREAD : 0;
+		unsigned flags = PIDFD_STALE;
+
+		if (clone_flags & CLONE_THREAD)
+			flags |= PIDFD_THREAD;
+		if (clone_flags & CLONE_PIDFD_AUTOKILL) {
+			task_set_no_new_privs(p);
+			flags |= PIDFD_AUTOKILL;
+		}
 
 		/*
 		 * Note that no task has been attached to @pid yet indicate
 		 * that via CLONE_PIDFD.
 		 */
-		retval = pidfd_prepare(pid, flags | PIDFD_STALE, &pidfile);
+		retval = pidfd_prepare(pid, flags, &pidfile);
 		if (retval < 0)
 			goto bad_fork_free_pid;
 		pidfd = retval;
@@ -2909,7 +2925,7 @@ static bool clone3_args_valid(struct kernel_clone_args *kargs)
 	/* Verify that no unknown flags are passed along. */
 	if (kargs->flags &
 	    ~(CLONE_LEGACY_FLAGS | CLONE_CLEAR_SIGHAND | CLONE_INTO_CGROUP |
-	      CLONE_AUTOREAP))
+	      CLONE_AUTOREAP | CLONE_PIDFD_AUTOKILL))
 		return false;
 
 	/*

-- 
2.47.3

Re: [PATCH v4 2/4] pidfd: add CLONE_PIDFD_AUTOKILL

[Oleg Nesterov]

On 02/23, Christian Brauner wrote:
>
> @@ -2259,13 +2268,20 @@ __latent_entropy struct task_struct *copy_process(
>  	 * if the fd table isn't shared).
>  	 */
>  	if (clone_flags & CLONE_PIDFD) {
> -		int flags = (clone_flags & CLONE_THREAD) ? PIDFD_THREAD : 0;
> +		unsigned flags = PIDFD_STALE;
> +
> +		if (clone_flags & CLONE_THREAD)
> +			flags |= PIDFD_THREAD;
> +		if (clone_flags & CLONE_PIDFD_AUTOKILL) {
> +			task_set_no_new_privs(p);
> +			flags |= PIDFD_AUTOKILL;
> +		}
>  
>  		/*
>  		 * Note that no task has been attached to @pid yet indicate
>  		 * that via CLONE_PIDFD.
>  		 */
> -		retval = pidfd_prepare(pid, flags | PIDFD_STALE, &pidfile);
> +		retval = pidfd_prepare(pid, flags, &pidfile);

Confused... I think you also need to change pidfs_alloc_file() to restore
O_TRUNC after do_dentry_open() clears this flag? Just like it curently does

	pidfd_file->f_flags |= (flags & PIDFD_THREAD);

?

Oleg.

Re: [PATCH v4 2/4] pidfd: add CLONE_PIDFD_AUTOKILL

[Oleg Nesterov]

On 02/23, Oleg Nesterov wrote:
>
> On 02/23, Christian Brauner wrote:
> >
> > @@ -2259,13 +2268,20 @@ __latent_entropy struct task_struct *copy_process(
> >  	 * if the fd table isn't shared).
> >  	 */
> >  	if (clone_flags & CLONE_PIDFD) {
> > -		int flags = (clone_flags & CLONE_THREAD) ? PIDFD_THREAD : 0;
> > +		unsigned flags = PIDFD_STALE;
> > +
> > +		if (clone_flags & CLONE_THREAD)
> > +			flags |= PIDFD_THREAD;
> > +		if (clone_flags & CLONE_PIDFD_AUTOKILL) {
> > +			task_set_no_new_privs(p);
> > +			flags |= PIDFD_AUTOKILL;
> > +		}
> >
> >  		/*
> >  		 * Note that no task has been attached to @pid yet indicate
> >  		 * that via CLONE_PIDFD.
> >  		 */
> > -		retval = pidfd_prepare(pid, flags | PIDFD_STALE, &pidfile);
> > +		retval = pidfd_prepare(pid, flags, &pidfile);
>
> Confused... I think you also need to change pidfs_alloc_file() to restore
> O_TRUNC after do_dentry_open() clears this flag? Just like it curently does
>
> 	pidfd_file->f_flags |= (flags & PIDFD_THREAD);

Aah! please ignore me. Somehow I missed exactly this change in your patch.

Sorry for noise!

Oleg.

pidfd && O_RDWR

[Oleg Nesterov]

On 02/23, Oleg Nesterov wrote:
>
> Sorry for noise!

Yes, but let me add more (off-topic) noise to this thread...

pidfd_prepare() does pidfs_alloc_file(pid, flags | O_RDWR) and "| O_RDWR"
makes no sense because pidfs_alloc_file() itself does

	flags |= O_RDWR;

I was going to send the trivial cleanup, but why a pidfs file needs
O_RDWR/FMODE_WRITE ?

Actually the same question about some anon_inode_getfile_fmode(O_RDWR)
users, for example signalfd.c.

Can you explain just for my education?

Oleg.

Re: pidfd && O_RDWR

[David Laight]

On Mon, 23 Feb 2026 18:05:44 +0100
Oleg Nesterov <oleg@redhat.com> wrote:

> On 02/23, Oleg Nesterov wrote:
> >
> > Sorry for noise!  
> 
> Yes, but let me add more (off-topic) noise to this thread...
> 
> pidfd_prepare() does pidfs_alloc_file(pid, flags | O_RDWR) and "| O_RDWR"
> makes no sense because pidfs_alloc_file() itself does
> 
> 	flags |= O_RDWR;
> 
> I was going to send the trivial cleanup, but why a pidfs file needs
> O_RDWR/FMODE_WRITE ?

Or why any program that gets that far through the code 'wins'
write access.

	David

Re: pidfd && O_RDWR

[Oleg Nesterov]

On 02/23, Oleg Nesterov wrote:
>
> pidfd_prepare() does pidfs_alloc_file(pid, flags | O_RDWR) and "| O_RDWR"
> makes no sense because pidfs_alloc_file() itself does
>
> 	flags |= O_RDWR;
>
> I was going to send the trivial cleanup, but why a pidfs file needs
> O_RDWR/FMODE_WRITE ?
>
> Actually the same question about some anon_inode_getfile_fmode(O_RDWR)
> users, for example signalfd.c.

perhaps an accidental legacy from 628ff7c1d8d8 ("anonfd: Allow making anon
files read-only") ?

Oleg.

Re: pidfd && O_RDWR

[Christian Brauner]

On Mon, Feb 23, 2026 at 08:21:02PM +0100, Oleg Nesterov wrote:
> On 02/23, Oleg Nesterov wrote:
> >
> > pidfd_prepare() does pidfs_alloc_file(pid, flags | O_RDWR) and "| O_RDWR"
> > makes no sense because pidfs_alloc_file() itself does
> >
> > 	flags |= O_RDWR;
> >
> > I was going to send the trivial cleanup, but why a pidfs file needs
> > O_RDWR/FMODE_WRITE ?
> >
> > Actually the same question about some anon_inode_getfile_fmode(O_RDWR)
> > users, for example signalfd.c.
> 
> perhaps an accidental legacy from 628ff7c1d8d8 ("anonfd: Allow making anon
> files read-only") ?

It was always a possibility that we would support some form of
write-like operation eventually. And we have support for setting trusted
extended attributes on pidfds for some time now (trusted xattrs require
global cap_sys_admin).

Re: pidfd && O_RDWR

[David Laight]

On Mon, 23 Feb 2026 22:39:22 +0100
Christian Brauner <brauner@kernel.org> wrote:

> On Mon, Feb 23, 2026 at 08:21:02PM +0100, Oleg Nesterov wrote:
> > On 02/23, Oleg Nesterov wrote:  
> > >
> > > pidfd_prepare() does pidfs_alloc_file(pid, flags | O_RDWR) and "| O_RDWR"
> > > makes no sense because pidfs_alloc_file() itself does
> > >
> > > 	flags |= O_RDWR;
> > >
> > > I was going to send the trivial cleanup, but why a pidfs file needs
> > > O_RDWR/FMODE_WRITE ?
> > >
> > > Actually the same question about some anon_inode_getfile_fmode(O_RDWR)
> > > users, for example signalfd.c.  
> > 
> > perhaps an accidental legacy from 628ff7c1d8d8 ("anonfd: Allow making anon
> > files read-only") ?  
> 
> It was always a possibility that we would support some form of
> write-like operation eventually. And we have support for setting trusted
> extended attributes on pidfds for some time now (trusted xattrs require
> global cap_sys_admin).
> 

Isn't 'sending a signal' a write-like operation?

	David

Re: pidfd && O_RDWR

[Oleg Nesterov]

On 02/23, Christian Brauner wrote:
>
> On Mon, Feb 23, 2026 at 08:21:02PM +0100, Oleg Nesterov wrote:
> > On 02/23, Oleg Nesterov wrote:
> > >
> > > pidfd_prepare() does pidfs_alloc_file(pid, flags | O_RDWR) and "| O_RDWR"
> > > makes no sense because pidfs_alloc_file() itself does
> > >
> > > 	flags |= O_RDWR;
> > >
> > > I was going to send the trivial cleanup, but why a pidfs file needs
> > > O_RDWR/FMODE_WRITE ?
> > >
> > > Actually the same question about some anon_inode_getfile_fmode(O_RDWR)
> > > users, for example signalfd.c.
> >
> > perhaps an accidental legacy from 628ff7c1d8d8 ("anonfd: Allow making anon
> > files read-only") ?
>
> It was always a possibility that we would support some form of
> write-like operation eventually. And we have support for setting trusted
> extended attributes on pidfds for some time now (trusted xattrs require
> global cap_sys_admin).

But why do we need O_RDWR right now? That was my question.

I can be easily wrong, but I think that pidfs_xattr_handlers logic doesn't
need it...

OK, I won't pretend I understand fs, I'll send the trivial cleanup which just
removes the unnecessary "flags | O_RDWR" in pidfd_prepare().

Oleg.

Re: pidfd && O_RDWR

[Christian Brauner]

On Tue, Feb 24, 2026 at 11:17:43AM +0100, Oleg Nesterov wrote:
> On 02/23, Christian Brauner wrote:
> >
> > On Mon, Feb 23, 2026 at 08:21:02PM +0100, Oleg Nesterov wrote:
> > > On 02/23, Oleg Nesterov wrote:
> > > >
> > > > pidfd_prepare() does pidfs_alloc_file(pid, flags | O_RDWR) and "| O_RDWR"
> > > > makes no sense because pidfs_alloc_file() itself does
> > > >
> > > > 	flags |= O_RDWR;
> > > >
> > > > I was going to send the trivial cleanup, but why a pidfs file needs
> > > > O_RDWR/FMODE_WRITE ?
> > > >
> > > > Actually the same question about some anon_inode_getfile_fmode(O_RDWR)
> > > > users, for example signalfd.c.
> > >
> > > perhaps an accidental legacy from 628ff7c1d8d8 ("anonfd: Allow making anon
> > > files read-only") ?
> >
> > It was always a possibility that we would support some form of
> > write-like operation eventually. And we have support for setting trusted
> > extended attributes on pidfds for some time now (trusted xattrs require
> > global cap_sys_admin).
> 
> But why do we need O_RDWR right now? That was my question.
> 
> I can be easily wrong, but I think that pidfs_xattr_handlers logic doesn't
> need it...
> 
> OK, I won't pretend I understand fs, I'll send the trivial cleanup which just
> removes the unnecessary "flags | O_RDWR" in pidfd_prepare().

xattrs don't need FMODE_WRITE. You can use O_RDONLY fds with the
justification that it's metadata (most likely). Although I always found
that rather weird. Sending signals is technically also equivalent to
writing and I think that was the original reason this was done. If you
want to remove it then be my guest.

[PATCH v4 3/4] selftests/pidfd: add CLONE_AUTOREAP tests

[Christian Brauner]

Add tests for the new CLONE_AUTOREAP clone3() flag:

- autoreap_without_pidfd: CLONE_AUTOREAP without CLONE_PIDFD works
  (fire-and-forget)
- autoreap_rejects_exit_signal: CLONE_AUTOREAP with non-zero
  exit_signal fails
- autoreap_rejects_parent: CLONE_AUTOREAP with CLONE_PARENT fails
- autoreap_rejects_thread: CLONE_AUTOREAP with CLONE_THREAD fails
- autoreap_basic: child exits, pidfd poll works, PIDFD_GET_INFO returns
  correct exit code, waitpid() returns -ECHILD
- autoreap_signaled: child killed by signal, exit info correct via pidfd
- autoreap_reparent: autoreap grandchild reparented to subreaper still
  auto-reaps
- autoreap_multithreaded: autoreap process with sub-threads auto-reaps
  after last thread exits
- autoreap_no_inherit: grandchild forked without CLONE_AUTOREAP becomes
  a regular zombie

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/pidfd/.gitignore           |   1 +
 tools/testing/selftests/pidfd/Makefile             |   2 +-
 .../testing/selftests/pidfd/pidfd_autoreap_test.c  | 507 +++++++++++++++++++++
 3 files changed, 509 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/pidfd/.gitignore b/tools/testing/selftests/pidfd/.gitignore
index 144e7ff65d6a..4cd8ec7fd349 100644
--- a/tools/testing/selftests/pidfd/.gitignore
+++ b/tools/testing/selftests/pidfd/.gitignore
@@ -12,3 +12,4 @@ pidfd_info_test
 pidfd_exec_helper
 pidfd_xattr_test
 pidfd_setattr_test
+pidfd_autoreap_test
diff --git a/tools/testing/selftests/pidfd/Makefile b/tools/testing/selftests/pidfd/Makefile
index 764a8f9ecefa..4211f91e9af8 100644
--- a/tools/testing/selftests/pidfd/Makefile
+++ b/tools/testing/selftests/pidfd/Makefile
@@ -4,7 +4,7 @@ CFLAGS += -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES) -pthread -Wall
 TEST_GEN_PROGS := pidfd_test pidfd_fdinfo_test pidfd_open_test \
 	pidfd_poll_test pidfd_wait pidfd_getfd_test pidfd_setns_test \
 	pidfd_file_handle_test pidfd_bind_mount pidfd_info_test \
-	pidfd_xattr_test pidfd_setattr_test
+	pidfd_xattr_test pidfd_setattr_test pidfd_autoreap_test
 
 TEST_GEN_PROGS_EXTENDED := pidfd_exec_helper
 
diff --git a/tools/testing/selftests/pidfd/pidfd_autoreap_test.c b/tools/testing/selftests/pidfd/pidfd_autoreap_test.c
new file mode 100644
index 000000000000..9e52a16239ea
--- /dev/null
+++ b/tools/testing/selftests/pidfd/pidfd_autoreap_test.c
@@ -0,0 +1,507 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2026 Christian Brauner <brauner@kernel.org>
+
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <linux/types.h>
+#include <poll.h>
+#include <pthread.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <syscall.h>
+#include <sys/ioctl.h>
+#include <sys/prctl.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "pidfd.h"
+#include "kselftest_harness.h"
+
+#ifndef CLONE_AUTOREAP
+#define CLONE_AUTOREAP 0x400000000ULL
+#endif
+
+static pid_t create_autoreap_child(int *pidfd)
+{
+	struct __clone_args args = {
+		.flags		= CLONE_PIDFD | CLONE_AUTOREAP,
+		.exit_signal	= 0,
+		.pidfd		= ptr_to_u64(pidfd),
+	};
+
+	return sys_clone3(&args, sizeof(args));
+}
+
+/*
+ * Test that CLONE_AUTOREAP works without CLONE_PIDFD (fire-and-forget).
+ */
+TEST(autoreap_without_pidfd)
+{
+	struct __clone_args args = {
+		.flags		= CLONE_AUTOREAP,
+		.exit_signal	= 0,
+	};
+	pid_t pid;
+	int ret;
+
+	pid = sys_clone3(&args, sizeof(args));
+	if (pid < 0 && errno == EINVAL)
+		SKIP(return, "CLONE_AUTOREAP not supported");
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0)
+		_exit(0);
+
+	/*
+	 * Give the child a moment to exit and be autoreaped.
+	 * Then verify no zombie remains.
+	 */
+	usleep(200000);
+	ret = waitpid(pid, NULL, WNOHANG);
+	ASSERT_EQ(ret, -1);
+	ASSERT_EQ(errno, ECHILD);
+}
+
+/*
+ * Test that CLONE_AUTOREAP with a non-zero exit_signal fails.
+ */
+TEST(autoreap_rejects_exit_signal)
+{
+	struct __clone_args args = {
+		.flags		= CLONE_PIDFD | CLONE_AUTOREAP,
+		.exit_signal	= SIGCHLD,
+	};
+	int pidfd = -1;
+	pid_t pid;
+
+	args.pidfd = ptr_to_u64(&pidfd);
+
+	pid = sys_clone3(&args, sizeof(args));
+	ASSERT_EQ(pid, -1);
+	ASSERT_EQ(errno, EINVAL);
+}
+
+/*
+ * Test that CLONE_AUTOREAP with CLONE_PARENT fails.
+ */
+TEST(autoreap_rejects_parent)
+{
+	struct __clone_args args = {
+		.flags		= CLONE_PIDFD | CLONE_AUTOREAP | CLONE_PARENT,
+		.exit_signal	= 0,
+	};
+	int pidfd = -1;
+	pid_t pid;
+
+	args.pidfd = ptr_to_u64(&pidfd);
+	pid = sys_clone3(&args, sizeof(args));
+	ASSERT_EQ(pid, -1);
+	ASSERT_EQ(errno, EINVAL);
+}
+
+/*
+ * Test that CLONE_AUTOREAP with CLONE_THREAD fails.
+ */
+TEST(autoreap_rejects_thread)
+{
+	struct __clone_args args = {
+		.flags		= CLONE_PIDFD | CLONE_AUTOREAP |
+				  CLONE_THREAD | CLONE_SIGHAND |
+				  CLONE_VM,
+		.exit_signal	= 0,
+	};
+	int pidfd = -1;
+	pid_t pid;
+
+	args.pidfd = ptr_to_u64(&pidfd);
+
+	pid = sys_clone3(&args, sizeof(args));
+	ASSERT_EQ(pid, -1);
+	ASSERT_EQ(errno, EINVAL);
+}
+
+/*
+ * Basic test: create an autoreap child, let it exit, verify:
+ * - pidfd becomes readable (poll returns POLLIN)
+ * - PIDFD_GET_INFO returns the correct exit code
+ * - waitpid() returns -1/ECHILD (no zombie)
+ */
+TEST(autoreap_basic)
+{
+	struct pidfd_info info = { .mask = PIDFD_INFO_EXIT };
+	int pidfd = -1, ret;
+	struct pollfd pfd;
+	pid_t pid;
+
+	pid = create_autoreap_child(&pidfd);
+	if (pid < 0 && errno == EINVAL)
+		SKIP(return, "CLONE_AUTOREAP not supported");
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0)
+		_exit(42);
+
+	ASSERT_GE(pidfd, 0);
+
+	/* Wait for the child to exit via pidfd poll. */
+	pfd.fd = pidfd;
+	pfd.events = POLLIN;
+	ret = poll(&pfd, 1, 5000);
+	ASSERT_EQ(ret, 1);
+	ASSERT_TRUE(pfd.revents & POLLIN);
+
+	/* Verify exit info via PIDFD_GET_INFO. */
+	ret = ioctl(pidfd, PIDFD_GET_INFO, &info);
+	ASSERT_EQ(ret, 0);
+	ASSERT_TRUE(info.mask & PIDFD_INFO_EXIT);
+	/*
+	 * exit_code is in waitpid format: for _exit(42),
+	 * WIFEXITED is true and WEXITSTATUS is 42.
+	 */
+	ASSERT_TRUE(WIFEXITED(info.exit_code));
+	ASSERT_EQ(WEXITSTATUS(info.exit_code), 42);
+
+	/* Verify no zombie: waitpid should fail with ECHILD. */
+	ret = waitpid(pid, NULL, WNOHANG);
+	ASSERT_EQ(ret, -1);
+	ASSERT_EQ(errno, ECHILD);
+
+	close(pidfd);
+}
+
+/*
+ * Test that an autoreap child killed by a signal reports
+ * the correct exit info.
+ */
+TEST(autoreap_signaled)
+{
+	struct pidfd_info info = { .mask = PIDFD_INFO_EXIT };
+	int pidfd = -1, ret;
+	struct pollfd pfd;
+	pid_t pid;
+
+	pid = create_autoreap_child(&pidfd);
+	if (pid < 0 && errno == EINVAL)
+		SKIP(return, "CLONE_AUTOREAP not supported");
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		pause();
+		_exit(1);
+	}
+
+	ASSERT_GE(pidfd, 0);
+
+	/* Kill the child. */
+	ret = sys_pidfd_send_signal(pidfd, SIGKILL, NULL, 0);
+	ASSERT_EQ(ret, 0);
+
+	/* Wait for exit via pidfd. */
+	pfd.fd = pidfd;
+	pfd.events = POLLIN;
+	ret = poll(&pfd, 1, 5000);
+	ASSERT_EQ(ret, 1);
+	ASSERT_TRUE(pfd.revents & POLLIN);
+
+	/* Verify signal info. */
+	ret = ioctl(pidfd, PIDFD_GET_INFO, &info);
+	ASSERT_EQ(ret, 0);
+	ASSERT_TRUE(info.mask & PIDFD_INFO_EXIT);
+	ASSERT_TRUE(WIFSIGNALED(info.exit_code));
+	ASSERT_EQ(WTERMSIG(info.exit_code), SIGKILL);
+
+	/* No zombie. */
+	ret = waitpid(pid, NULL, WNOHANG);
+	ASSERT_EQ(ret, -1);
+	ASSERT_EQ(errno, ECHILD);
+
+	close(pidfd);
+}
+
+/*
+ * Test autoreap survives reparenting: middle process creates an
+ * autoreap grandchild, then exits. The grandchild gets reparented
+ * to us (the grandparent, which is a subreaper). When the grandchild
+ * exits, it should still be autoreaped - no zombie under us.
+ */
+TEST(autoreap_reparent)
+{
+	int ipc_sockets[2], ret;
+	int pidfd = -1;
+	struct pollfd pfd;
+	pid_t mid_pid, grandchild_pid;
+	char buf[32] = {};
+
+	/* Make ourselves a subreaper so reparented children come to us. */
+	ret = prctl(PR_SET_CHILD_SUBREAPER, 1);
+	ASSERT_EQ(ret, 0);
+
+	ret = socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets);
+	ASSERT_EQ(ret, 0);
+
+	mid_pid = fork();
+	ASSERT_GE(mid_pid, 0);
+
+	if (mid_pid == 0) {
+		/* Middle child: create an autoreap grandchild. */
+		int gc_pidfd = -1;
+
+		close(ipc_sockets[0]);
+
+		grandchild_pid = create_autoreap_child(&gc_pidfd);
+		if (grandchild_pid < 0) {
+			write_nointr(ipc_sockets[1], "E", 1);
+			close(ipc_sockets[1]);
+			_exit(1);
+		}
+
+		if (grandchild_pid == 0) {
+			/* Grandchild: wait for signal to exit. */
+			close(ipc_sockets[1]);
+			if (gc_pidfd >= 0)
+				close(gc_pidfd);
+			pause();
+			_exit(0);
+		}
+
+		/* Send grandchild PID to grandparent. */
+		snprintf(buf, sizeof(buf), "%d", grandchild_pid);
+		write_nointr(ipc_sockets[1], buf, strlen(buf));
+		close(ipc_sockets[1]);
+		if (gc_pidfd >= 0)
+			close(gc_pidfd);
+
+		/* Middle child exits, grandchild gets reparented. */
+		_exit(0);
+	}
+
+	close(ipc_sockets[1]);
+
+	/* Read grandchild's PID. */
+	ret = read_nointr(ipc_sockets[0], buf, sizeof(buf) - 1);
+	close(ipc_sockets[0]);
+	ASSERT_GT(ret, 0);
+
+	if (buf[0] == 'E') {
+		waitpid(mid_pid, NULL, 0);
+		prctl(PR_SET_CHILD_SUBREAPER, 0);
+		SKIP(return, "CLONE_AUTOREAP not supported");
+	}
+
+	grandchild_pid = atoi(buf);
+	ASSERT_GT(grandchild_pid, 0);
+
+	/* Wait for the middle child to exit. */
+	ret = waitpid(mid_pid, NULL, 0);
+	ASSERT_EQ(ret, mid_pid);
+
+	/*
+	 * Now the grandchild is reparented to us (subreaper).
+	 * Open a pidfd for the grandchild and kill it.
+	 */
+	pidfd = sys_pidfd_open(grandchild_pid, 0);
+	ASSERT_GE(pidfd, 0);
+
+	ret = sys_pidfd_send_signal(pidfd, SIGKILL, NULL, 0);
+	ASSERT_EQ(ret, 0);
+
+	/* Wait for it to exit via pidfd poll. */
+	pfd.fd = pidfd;
+	pfd.events = POLLIN;
+	ret = poll(&pfd, 1, 5000);
+	ASSERT_EQ(ret, 1);
+	ASSERT_TRUE(pfd.revents & POLLIN);
+
+	/*
+	 * The grandchild should have been autoreaped even though
+	 * we (the new parent) haven't set SA_NOCLDWAIT.
+	 * waitpid should return -1/ECHILD.
+	 */
+	ret = waitpid(grandchild_pid, NULL, WNOHANG);
+	EXPECT_EQ(ret, -1);
+	EXPECT_EQ(errno, ECHILD);
+
+	close(pidfd);
+
+	/* Clean up subreaper status. */
+	prctl(PR_SET_CHILD_SUBREAPER, 0);
+}
+
+static int thread_sock_fd;
+
+static void *thread_func(void *arg)
+{
+	/* Signal parent we're running. */
+	write_nointr(thread_sock_fd, "1", 1);
+
+	/* Give main thread time to call _exit() first. */
+	usleep(200000);
+
+	return NULL;
+}
+
+/*
+ * Test that an autoreap child with multiple threads is properly
+ * autoreaped only after all threads have exited.
+ */
+TEST(autoreap_multithreaded)
+{
+	struct pidfd_info info = { .mask = PIDFD_INFO_EXIT };
+	int ipc_sockets[2], ret;
+	int pidfd = -1;
+	struct pollfd pfd;
+	pid_t pid;
+	char c;
+
+	ret = socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets);
+	ASSERT_EQ(ret, 0);
+
+	pid = create_autoreap_child(&pidfd);
+	if (pid < 0 && errno == EINVAL) {
+		close(ipc_sockets[0]);
+		close(ipc_sockets[1]);
+		SKIP(return, "CLONE_AUTOREAP not supported");
+	}
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		pthread_t thread;
+
+		close(ipc_sockets[0]);
+
+		/*
+		 * Create a sub-thread that outlives the main thread.
+		 * The thread signals readiness, then sleeps.
+		 * The main thread waits briefly, then calls _exit().
+		 */
+		thread_sock_fd = ipc_sockets[1];
+		pthread_create(&thread, NULL, thread_func, NULL);
+		pthread_detach(thread);
+
+		/* Wait for thread to be running. */
+		usleep(100000);
+
+		/* Main thread exits; sub-thread is still alive. */
+		_exit(99);
+	}
+
+	close(ipc_sockets[1]);
+
+	/* Wait for the sub-thread to signal readiness. */
+	ret = read_nointr(ipc_sockets[0], &c, 1);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(ret, 1);
+
+	/* Wait for the process to fully exit via pidfd poll. */
+	pfd.fd = pidfd;
+	pfd.events = POLLIN;
+	ret = poll(&pfd, 1, 5000);
+	ASSERT_EQ(ret, 1);
+	ASSERT_TRUE(pfd.revents & POLLIN);
+
+	/* Verify exit info. */
+	ret = ioctl(pidfd, PIDFD_GET_INFO, &info);
+	ASSERT_EQ(ret, 0);
+	ASSERT_TRUE(info.mask & PIDFD_INFO_EXIT);
+	ASSERT_TRUE(WIFEXITED(info.exit_code));
+	ASSERT_EQ(WEXITSTATUS(info.exit_code), 99);
+
+	/* No zombie. */
+	ret = waitpid(pid, NULL, WNOHANG);
+	ASSERT_EQ(ret, -1);
+	ASSERT_EQ(errno, ECHILD);
+
+	close(pidfd);
+}
+
+/*
+ * Test that autoreap is NOT inherited by grandchildren.
+ */
+TEST(autoreap_no_inherit)
+{
+	int ipc_sockets[2], ret;
+	int pidfd = -1;
+	pid_t pid;
+	char buf[2] = {};
+	struct pollfd pfd;
+
+	ret = socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets);
+	ASSERT_EQ(ret, 0);
+
+	pid = create_autoreap_child(&pidfd);
+	if (pid < 0 && errno == EINVAL) {
+		close(ipc_sockets[0]);
+		close(ipc_sockets[1]);
+		SKIP(return, "CLONE_AUTOREAP not supported");
+	}
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		pid_t gc;
+		int status;
+
+		close(ipc_sockets[0]);
+
+		/* Autoreap child forks a grandchild (without autoreap). */
+		gc = fork();
+		if (gc < 0) {
+			write_nointr(ipc_sockets[1], "E", 1);
+			_exit(1);
+		}
+		if (gc == 0) {
+			/* Grandchild: exit immediately. */
+			close(ipc_sockets[1]);
+			_exit(77);
+		}
+
+		/*
+		 * The grandchild should become a regular zombie
+		 * since it was NOT created with CLONE_AUTOREAP.
+		 * Wait for it to verify.
+		 */
+		ret = waitpid(gc, &status, 0);
+		if (ret == gc && WIFEXITED(status) &&
+		    WEXITSTATUS(status) == 77) {
+			write_nointr(ipc_sockets[1], "P", 1);
+		} else {
+			write_nointr(ipc_sockets[1], "F", 1);
+		}
+		close(ipc_sockets[1]);
+		_exit(0);
+	}
+
+	close(ipc_sockets[1]);
+
+	ret = read_nointr(ipc_sockets[0], buf, 1);
+	close(ipc_sockets[0]);
+	ASSERT_EQ(ret, 1);
+
+	/*
+	 * 'P' means the autoreap child was able to waitpid() its
+	 * grandchild (correct - grandchild should be a normal zombie,
+	 * not autoreaped).
+	 */
+	ASSERT_EQ(buf[0], 'P');
+
+	/* Wait for the autoreap child to exit. */
+	pfd.fd = pidfd;
+	pfd.events = POLLIN;
+	ret = poll(&pfd, 1, 5000);
+	ASSERT_EQ(ret, 1);
+
+	/* Autoreap child itself should be autoreaped. */
+	ret = waitpid(pid, NULL, WNOHANG);
+	ASSERT_EQ(ret, -1);
+	ASSERT_EQ(errno, ECHILD);
+
+	close(pidfd);
+}
+
+TEST_HARNESS_MAIN

-- 
2.47.3

[PATCH v4 4/4] selftests/pidfd: add CLONE_PIDFD_AUTOKILL tests

[Christian Brauner]

Add tests for the new CLONE_PIDFD_AUTOKILL clone3() flag:

- autokill_sets_no_new_privs: child created with CLONE_PIDFD_AUTOKILL
  has no_new_privs set, parent does not
- autoreap_no_new_privs_unset: plain CLONE_AUTOREAP child does not get
  no_new_privs (only CLONE_PIDFD_AUTOKILL sets it)
- autokill_basic: child blocks in pause(), parent closes clone3 pidfd,
  child is killed and autoreaped
- autokill_requires_pidfd: CLONE_PIDFD_AUTOKILL without CLONE_PIDFD
  fails with EINVAL
- autokill_requires_autoreap: CLONE_PIDFD_AUTOKILL without
  CLONE_AUTOREAP fails with EINVAL
- autokill_rejects_thread: CLONE_PIDFD_AUTOKILL with CLONE_THREAD fails
  with EINVAL
- autokill_pidfd_open_no_effect: closing a pidfd_open() fd does not kill
  the child, closing the clone3 pidfd does

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 .../testing/selftests/pidfd/pidfd_autoreap_test.c  | 286 +++++++++++++++++++++
 1 file changed, 286 insertions(+)

diff --git a/tools/testing/selftests/pidfd/pidfd_autoreap_test.c b/tools/testing/selftests/pidfd/pidfd_autoreap_test.c
index 9e52a16239ea..9037542eef2a 100644
--- a/tools/testing/selftests/pidfd/pidfd_autoreap_test.c
+++ b/tools/testing/selftests/pidfd/pidfd_autoreap_test.c
@@ -28,6 +28,10 @@
 #define CLONE_AUTOREAP 0x400000000ULL
 #endif
 
+#ifndef CLONE_PIDFD_AUTOKILL
+#define CLONE_PIDFD_AUTOKILL 0x800000000ULL
+#endif
+
 static pid_t create_autoreap_child(int *pidfd)
 {
 	struct __clone_args args = {
@@ -504,4 +508,286 @@ TEST(autoreap_no_inherit)
 	close(pidfd);
 }
 
+/*
+ * Helper: create a child with CLONE_PIDFD | CLONE_PIDFD_AUTOKILL | CLONE_AUTOREAP.
+ */
+static pid_t create_autokill_child(int *pidfd)
+{
+	struct __clone_args args = {
+		.flags		= CLONE_PIDFD | CLONE_PIDFD_AUTOKILL |
+				  CLONE_AUTOREAP,
+		.exit_signal	= 0,
+		.pidfd		= ptr_to_u64(pidfd),
+	};
+
+	return sys_clone3(&args, sizeof(args));
+}
+
+/*
+ * Test that CLONE_PIDFD_AUTOKILL sets no_new_privs on the child.
+ * The child checks via prctl(PR_GET_NO_NEW_PRIVS) and reports back.
+ * The parent must NOT have no_new_privs set afterwards.
+ */
+TEST(autokill_sets_no_new_privs)
+{
+	struct pidfd_info info = { .mask = PIDFD_INFO_EXIT };
+	int pidfd = -1, ret;
+	struct pollfd pfd;
+	pid_t pid;
+
+	/* Ensure parent does not already have no_new_privs. */
+	ret = prctl(PR_GET_NO_NEW_PRIVS, 0, 0, 0, 0);
+	ASSERT_EQ(ret, 0) {
+		TH_LOG("Parent already has no_new_privs set, cannot run test");
+	}
+
+	pid = create_autokill_child(&pidfd);
+	if (pid < 0 && errno == EINVAL)
+		SKIP(return, "CLONE_PIDFD_AUTOKILL not supported");
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/*
+		 * Child: check no_new_privs. Exit 0 if set, 1 if not.
+		 */
+		ret = prctl(PR_GET_NO_NEW_PRIVS, 0, 0, 0, 0);
+		_exit(ret == 1 ? 0 : 1);
+	}
+
+	ASSERT_GE(pidfd, 0);
+
+	/* Parent must still NOT have no_new_privs. */
+	ret = prctl(PR_GET_NO_NEW_PRIVS, 0, 0, 0, 0);
+	ASSERT_EQ(ret, 0) {
+		TH_LOG("Parent got no_new_privs after creating autokill child");
+	}
+
+	/* Wait for child to exit. */
+	pfd.fd = pidfd;
+	pfd.events = POLLIN;
+	ret = poll(&pfd, 1, 5000);
+	ASSERT_EQ(ret, 1);
+
+	/* Verify child exited with 0 (no_new_privs was set). */
+	ret = ioctl(pidfd, PIDFD_GET_INFO, &info);
+	ASSERT_EQ(ret, 0);
+	ASSERT_TRUE(info.mask & PIDFD_INFO_EXIT);
+	ASSERT_TRUE(WIFEXITED(info.exit_code));
+	ASSERT_EQ(WEXITSTATUS(info.exit_code), 0) {
+		TH_LOG("Child did not have no_new_privs set");
+	}
+
+	close(pidfd);
+}
+
+/*
+ * Test that a plain CLONE_AUTOREAP child does NOT get no_new_privs.
+ * Only CLONE_PIDFD_AUTOKILL should set it.
+ */
+TEST(autoreap_no_new_privs_unset)
+{
+	struct pidfd_info info = { .mask = PIDFD_INFO_EXIT };
+	int pidfd = -1, ret;
+	struct pollfd pfd;
+	pid_t pid;
+
+	pid = create_autoreap_child(&pidfd);
+	if (pid < 0 && errno == EINVAL)
+		SKIP(return, "CLONE_AUTOREAP not supported");
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		/*
+		 * Child: check no_new_privs. Exit 0 if NOT set, 1 if set.
+		 */
+		ret = prctl(PR_GET_NO_NEW_PRIVS, 0, 0, 0, 0);
+		_exit(ret == 0 ? 0 : 1);
+	}
+
+	ASSERT_GE(pidfd, 0);
+
+	pfd.fd = pidfd;
+	pfd.events = POLLIN;
+	ret = poll(&pfd, 1, 5000);
+	ASSERT_EQ(ret, 1);
+
+	ret = ioctl(pidfd, PIDFD_GET_INFO, &info);
+	ASSERT_EQ(ret, 0);
+	ASSERT_TRUE(info.mask & PIDFD_INFO_EXIT);
+	ASSERT_TRUE(WIFEXITED(info.exit_code));
+	ASSERT_EQ(WEXITSTATUS(info.exit_code), 0) {
+		TH_LOG("Plain autoreap child unexpectedly has no_new_privs");
+	}
+
+	close(pidfd);
+}
+
+/*
+ * Basic autokill test: child blocks in pause(), parent closes the
+ * clone3 pidfd, child should be killed and autoreaped.
+ */
+TEST(autokill_basic)
+{
+	int pidfd = -1, pollfd_fd = -1, ret;
+	struct pollfd pfd;
+	pid_t pid;
+
+	pid = create_autokill_child(&pidfd);
+	if (pid < 0 && errno == EINVAL)
+		SKIP(return, "CLONE_PIDFD_AUTOKILL not supported");
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		pause();
+		_exit(1);
+	}
+
+	ASSERT_GE(pidfd, 0);
+
+	/*
+	 * Open a second pidfd via pidfd_open() so we can observe the
+	 * child's death after closing the clone3 pidfd.
+	 */
+	pollfd_fd = sys_pidfd_open(pid, 0);
+	ASSERT_GE(pollfd_fd, 0);
+
+	/* Close the clone3 pidfd — this should trigger autokill. */
+	close(pidfd);
+
+	/* Wait for the child to die via the pidfd_open'd fd. */
+	pfd.fd = pollfd_fd;
+	pfd.events = POLLIN;
+	ret = poll(&pfd, 1, 5000);
+	ASSERT_EQ(ret, 1);
+	ASSERT_TRUE(pfd.revents & POLLIN);
+
+	/* Child should be autoreaped — no zombie. */
+	usleep(100000);
+	ret = waitpid(pid, NULL, WNOHANG);
+	ASSERT_EQ(ret, -1);
+	ASSERT_EQ(errno, ECHILD);
+
+	close(pollfd_fd);
+}
+
+/*
+ * CLONE_PIDFD_AUTOKILL without CLONE_PIDFD must fail with EINVAL.
+ */
+TEST(autokill_requires_pidfd)
+{
+	struct __clone_args args = {
+		.flags		= CLONE_PIDFD_AUTOKILL | CLONE_AUTOREAP,
+		.exit_signal	= 0,
+	};
+	pid_t pid;
+
+	pid = sys_clone3(&args, sizeof(args));
+	ASSERT_EQ(pid, -1);
+	ASSERT_EQ(errno, EINVAL);
+}
+
+/*
+ * CLONE_PIDFD_AUTOKILL without CLONE_AUTOREAP must fail with EINVAL.
+ */
+TEST(autokill_requires_autoreap)
+{
+	struct __clone_args args = {
+		.flags		= CLONE_PIDFD | CLONE_PIDFD_AUTOKILL,
+		.exit_signal	= SIGCHLD,
+	};
+	int pidfd = -1;
+	pid_t pid;
+
+	args.pidfd = ptr_to_u64(&pidfd);
+
+	pid = sys_clone3(&args, sizeof(args));
+	ASSERT_EQ(pid, -1);
+	ASSERT_EQ(errno, EINVAL);
+}
+
+/*
+ * CLONE_PIDFD_AUTOKILL with CLONE_THREAD must fail with EINVAL.
+ */
+TEST(autokill_rejects_thread)
+{
+	struct __clone_args args = {
+		.flags		= CLONE_PIDFD | CLONE_PIDFD_AUTOKILL |
+				  CLONE_AUTOREAP | CLONE_THREAD |
+				  CLONE_SIGHAND | CLONE_VM,
+		.exit_signal	= 0,
+	};
+	int pidfd = -1;
+	pid_t pid;
+
+	args.pidfd = ptr_to_u64(&pidfd);
+
+	pid = sys_clone3(&args, sizeof(args));
+	ASSERT_EQ(pid, -1);
+	ASSERT_EQ(errno, EINVAL);
+}
+
+/*
+ * Test that only the clone3 pidfd triggers autokill, not pidfd_open().
+ * Close the pidfd_open'd fd first — child should survive.
+ * Then close the clone3 pidfd — child should be killed and autoreaped.
+ */
+TEST(autokill_pidfd_open_no_effect)
+{
+	int pidfd = -1, open_fd = -1, ret;
+	struct pollfd pfd;
+	pid_t pid;
+
+	pid = create_autokill_child(&pidfd);
+	if (pid < 0 && errno == EINVAL)
+		SKIP(return, "CLONE_PIDFD_AUTOKILL not supported");
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		pause();
+		_exit(1);
+	}
+
+	ASSERT_GE(pidfd, 0);
+
+	/* Open a second pidfd via pidfd_open(). */
+	open_fd = sys_pidfd_open(pid, 0);
+	ASSERT_GE(open_fd, 0);
+
+	/*
+	 * Close the pidfd_open'd fd — child should survive because
+	 * only the clone3 pidfd has autokill.
+	 */
+	close(open_fd);
+	usleep(200000);
+
+	/* Verify child is still alive by polling the clone3 pidfd. */
+	pfd.fd = pidfd;
+	pfd.events = POLLIN;
+	ret = poll(&pfd, 1, 0);
+	ASSERT_EQ(ret, 0) {
+		TH_LOG("Child died after closing pidfd_open fd — should still be alive");
+	}
+
+	/* Open another observation fd before triggering autokill. */
+	open_fd = sys_pidfd_open(pid, 0);
+	ASSERT_GE(open_fd, 0);
+
+	/* Now close the clone3 pidfd — this triggers autokill. */
+	close(pidfd);
+
+	pfd.fd = open_fd;
+	pfd.events = POLLIN;
+	ret = poll(&pfd, 1, 5000);
+	ASSERT_EQ(ret, 1);
+	ASSERT_TRUE(pfd.revents & POLLIN);
+
+	/* Child should be autoreaped — no zombie. */
+	usleep(100000);
+	ret = waitpid(pid, NULL, WNOHANG);
+	ASSERT_EQ(ret, -1);
+	ASSERT_EQ(errno, ECHILD);
+
+	close(open_fd);
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3