coredump: add new %P variable in core_pattern

Add a new %P variable to be used in core_pattern.  This variable contains
the global PID (PID in the init namespace) as %p contains the PID in the
current namespace which isn't always what we want.

The main use for this is to make it easier to handle crashes that happened
within a container.  With that new variables it's possible to have the
crashes dumped into the container or forwarded to the host with the right
PID (from the host's point of view).

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Reported-by: Hans Feldt <hans.feldt@ericsson.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Andy Whitcroft <apw@canonical.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/sysctl/kernel.txt

@@ -182,6 +182,7 @@ core_pattern is used to specify a core dumpfile pattern name.
 	%<NUL>	'%' is dropped
 	%%	output one '%'
 	%p	pid
+	%P	global pid (init PID namespace)
 	%u	uid
 	%g	gid
 	%d	dump mode, matches PR_SET_DUMPABLE and

fs/coredump.c

@@ -190,6 +190,11 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm)
 				err = cn_printf(cn, "%d",
 					      task_tgid_vnr(current));
 				break;
+			/* global pid */
+			case 'P':
+				err = cn_printf(cn, "%d",
+					      task_tgid_nr(current));
+				break;
 			/* uid */
 			case 'u':
 				err = cn_printf(cn, "%d", cred->uid);