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

From: Oleg Nesterov
Date: Wed Jan 31 2024 - 08:30:23 EST


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@xxxxxxxxxx>
---
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 */
+
#ifdef CONFIG_PID_NS
seq_put_decimal_ll(m, "\nNSpid:\t", nr);
if (nr > 0) {
@@ -2068,22 +2071,35 @@ static void pidfd_show_fdinfo(struct seq_file *m, struct file *f)
}
#endif

+static bool pidfd_task_exited(struct pid *pid, bool thread)
+{
+ struct task_struct *task;
+ bool exited;
+
+ rcu_read_lock();
+ task = pid_task(pid, PIDTYPE_PID);
+ exited = !task ||
+ (READ_ONCE(task->exit_state) && (thread || thread_group_empty(task)));
+ rcu_read_unlock();
+
+ return exited;
+}
+
/*
* Poll support for process exit notification.
*/
static __poll_t pidfd_poll(struct file *file, struct poll_table_struct *pts)
{
struct pid *pid = file->private_data;
+ bool thread = file->f_flags & PIDFD_THREAD;
__poll_t poll_flags = 0;

poll_wait(file, &pid->wait_pidfd, pts);
-
/*
- * 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.
+ * Depending on PIDFD_THREAD, inform pollers when the thread
+ * or the whole thread-group exits.
*/
- if (thread_group_exited(pid))
+ if (pidfd_task_exited(pid, thread))
poll_flags = EPOLLIN | EPOLLRDNORM;

return poll_flags;
@@ -2141,6 +2157,11 @@ static int __pidfd_prepare(struct pid *pid, unsigned int flags, struct file **re
return PTR_ERR(pidfd_file);
}
get_pid(pid); /* held by pidfd_file now */
+ /*
+ * anon_inode_getfile() ignores everything outside of the
+ * O_ACCMODE | O_NONBLOCK mask, set PIDFD_THREAD manually.
+ */
+ pidfd_file->f_flags |= (flags & PIDFD_THREAD);
*ret = pidfd_file;
return pidfd;
}
@@ -2154,7 +2175,8 @@ static int __pidfd_prepare(struct pid *pid, unsigned int flags, struct file **re
* Allocate a new file that stashes @pid and reserve a new pidfd number in the
* caller's file descriptor table. The pidfd is reserved but not installed yet.
*
- * The helper verifies that @pid is used as a thread group leader.
+ * The helper verifies that @pid is still in use, without PIDFD_THREAD the
+ * task identified by @pid must be a thread-group leader.
*
* If this function returns successfully the caller is responsible to either
* call fd_install() passing the returned pidfd and pidfd file as arguments in
@@ -2173,7 +2195,9 @@ static int __pidfd_prepare(struct pid *pid, unsigned int flags, struct file **re
*/
int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret)
{
- if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
+ bool thread = flags & PIDFD_THREAD;
+
+ if (!pid || !pid_has_task(pid, thread ? PIDTYPE_PID : PIDTYPE_TGID));
return -EINVAL;

return __pidfd_prepare(pid, flags, ret);
diff --git a/kernel/pid.c b/kernel/pid.c
index c7a3e359f8f5..e11144466828 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -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)
diff --git a/kernel/signal.c b/kernel/signal.c
index 9561a3962ca6..9b40109f0c56 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2019,7 +2019,7 @@ int send_sigqueue(struct sigqueue *q, struct pid *pid, enum pid_type type)
return ret;
}

-static void do_notify_pidfd(struct task_struct *task)
+void do_notify_pidfd(struct task_struct *task)
{
struct pid *pid;

@@ -2051,7 +2051,8 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
WARN_ON_ONCE(!tsk->ptrace &&
(tsk->group_leader != tsk || !thread_group_empty(tsk)));
/*
- * tsk is a group leader and has no threads, wake up the pidfd waiters.
+ * tsk is a group leader and has no threads, wake up the
+ * non-PIDFD_THREAD waiters.
*/
if (thread_group_empty(tsk))
do_notify_pidfd(tsk);
@@ -3926,6 +3927,7 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
prepare_kill_siginfo(sig, &kinfo);
}

+ /* TODO: respect PIDFD_THREAD */
ret = kill_pid_info(sig, &kinfo, pid);

err:
--
2.25.1.362.g51ebf55