On Thu, Oct 17, 2024 at 10:05:50PM GMT, Lorenzo Stoakes wrote:

It is useful to be able to utilise the pidfd mechanism to reference the
current thread or process (from a userland point of view - thread group
leader from the kernel's point of view).

Therefore introduce PIDFD_SELF_THREAD to refer to the current thread, and
PIDFD_SELF_THREAD_GROUP to refer to the current thread group leader.

For convenience and to avoid confusion from userland's perspective we alias
these:

* PIDFD_SELF is an alias for PIDFD_SELF_THREAD - This is nearly always what
  the user will want to use, as they would find it surprising if for
  instance fd's were unshared()'d and they wanted to invoke pidfd_getfd()
  and that failed.

* PIDFD_SELF_PROCESS is an alias for PIDFD_SELF_THREAD_GROUP - Most users
  have no concept of thread groups or what a thread group leader is, and
  from userland's perspective and nomenclature this is what userland
  considers to be a process.

Should users use PIDFD_SELF_PROCESS in process_madvise() for self
madvise() (once the support is added)?

[...]

 
+static struct pid *pidfd_get_pid_self(unsigned int pidfd, unsigned int *flags)
+{
+	bool is_thread = pidfd == PIDFD_SELF_THREAD;
+	enum pid_type type = is_thread ? PIDTYPE_PID : PIDTYPE_TGID;
+	struct pid *pid = *task_pid_ptr(current, type);
+
+	/* The caller expects an elevated reference count. */
+	get_pid(pid);

Do you want this helper to work for scenarios where pid is used across
context? Otherwise can't we get rid of this get and later put for self?

+	return pid;
+}
+

Overall looks good to me.

Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>