pidfs: never refuse ppid == 0 in PIDFD_GET_INFO

JIRA: https://issues.redhat.com/browse/RHEL-113598

commit b55eb6e
Author: Mike Yuan <me@yhndnzj.com>
Date:   Wed, 4 Jun 2025 15:03:42 +0000

    pidfs: never refuse ppid == 0 in PIDFD_GET_INFO

    In systemd we spotted an issue after switching to ioctl(PIDFD_GET_INFO)
    for obtaining pid number the pidfd refers to, that for processes
    with a parent from outer pidns PIDFD_GET_INFO unexpectedly yields
    -ESRCH [1]. It turned out that there's an arbitrary check blocking
    this, which is not really sensible given getppid() happily returns
    0 for such processes. Just drop the spurious check and userspace
    ought to handle ppid == 0 properly everywhere.

    [1] systemd/systemd#37715

    Fixes: cdda1f2 ("pidfd: add ioctl to retrieve pid info")
    Signed-off-by: Mike Yuan <me@yhndnzj.com>
    Link: https://lore.kernel.org/20250604150238.42664-1-me@yhndnzj.com
    Cc: Christian Brauner <brauner@kernel.org>
    Cc: Luca Boccassi <luca.boccassi@gmail.com>
    Cc: stable@vger.kernel.org
    Signed-off-by: Christian Brauner <brauner@kernel.org>

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

fs/pidfs.c

@@ -335,7 +335,7 @@ static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg)
 	kinfo.pid = task_pid_vnr(task);
 	kinfo.mask |= PIDFD_INFO_PID;
 
-	if (kinfo.pid == 0 || kinfo.tgid == 0 || (kinfo.ppid == 0 && kinfo.pid != 1))
+	if (kinfo.pid == 0 || kinfo.tgid == 0)
 		return -ESRCH;
 
 copy_out: