coredump: add support for %d=__get_dumpable() in core name

Some coredump handlers want to create a core file in a way compatible with
standard behavior.  Standard behavior with fs.suid_dumpable = 2 is to
create core file with uid=gid=0.  However, there was no way for coredump
handler to know that the process being dumped was suid'ed.

This patch adds the new %d specifier for format_corename() which simply
reports __get_dumpable(mm->flags), this is compatible with
/proc/sys/fs/suid_dumpable we already have.

Addresses https://bugzilla.redhat.com/show_bug.cgi?id=787135

Developed during a discussion with Denys Vlasenko.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Alex Kelly <alex.page.kelly@gmail.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Cong Wang <amwang@redhat.com>
Cc: Jiri Moskovcak <jmoskovc@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/sysctl/kernel.txt

@@ -181,6 +181,8 @@ core_pattern is used to specify a core dumpfile pattern name.
 	%p	pid
 	%u	uid
 	%g	gid
+	%d	dump mode, matches PR_SET_DUMPABLE and
+		/proc/sys/fs/suid_dumpable
 	%s	signal number
 	%t	UNIX time of dump
 	%h	hostname

fs/coredump.c

@@ -149,7 +149,7 @@ static int cn_print_exe_file(struct core_name *cn)
  * name into corename, which must have space for at least
  * CORENAME_MAX_SIZE bytes plus one byte for the zero terminator.
  */
-static int format_corename(struct core_name *cn, long signr)
+static int format_corename(struct core_name *cn, struct coredump_params *cprm)
 {
 	const struct cred *cred = current_cred();
 	const char *pat_ptr = core_pattern;
@@ -194,9 +194,13 @@ static int format_corename(struct core_name *cn, long signr)
 			case 'g':
 				err = cn_printf(cn, "%d", cred->gid);
 				break;
+			case 'd':
+				err = cn_printf(cn, "%d",
+					__get_dumpable(cprm->mm_flags));
+				break;
 			/* signal that caused the coredump */
 			case 's':
-				err = cn_printf(cn, "%ld", signr);
+				err = cn_printf(cn, "%ld", cprm->signr);
 				break;
 			/* UNIX time of coredump */
 			case 't': {
@@ -515,7 +519,7 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
 	 */
 	clear_thread_flag(TIF_SIGPENDING);
 
-	ispipe = format_corename(&cn, signr);
+	ispipe = format_corename(&cn, &cprm);
 
  	if (ispipe) {
 		int dump_count;