pidfs: return -EREMOTE when PIDFD_GET_INFO is called on another ns

Currently it is not possible to distinguish between the case where a
process has already exited and the case where a process is in a
different namespace, as both return -ESRCH.
glibc's pidfd_getpid() procfs-based implementation returns -EREMOTE
in the latter, so that distinguishing the two is possible, as the
fdinfo in procfs will list '0' as the PID in that case:

https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/pidfd_getpid.c;h=860829cf07da2267484299ccb02861822c0d07b4;hb=HEAD#l121

Change the error code so that the kernel also returns -EREMOTE in
that case.

Fixes: 7477d7d ("pidfs: allow to retrieve exit information")

Signed-off-by: Luca Boccassi <luca.boccassi@gmail.com>
Link: https://patch.msgid.link/20260127225209.2293342-1-luca.boccassi@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: bluca

fs/pidfs.c

@@ -329,7 +329,7 @@ static long pidfd_info(struct file *file, unsigned int cmd, unsigned long arg)
 	 * namespace hierarchy.
 	 */
 	if (!pid_in_current_pidns(pid))
-		return -ESRCH;
+		return -EREMOTE;
 
 	attr = READ_ONCE(pid->attr);
 	if (mask & PIDFD_INFO_EXIT) {