coredump: add %i/%I in core_pattern to report the tid of the crashed thread

format_corename() can only pass the leader's pid to the core handler,
but there is no simple way to figure out which thread originated the
coredump.

As Jan explains, this also means that there is no simple way to create
the backtrace of the crashed process:

As programs are mostly compiled with implicit gcc -fomit-frame-pointer
one needs program's .eh_frame section (equivalently PT_GNU_EH_FRAME
segment) or .debug_frame section.  .debug_frame usually is present only
in separate debug info files usually not even installed on the system.
While .eh_frame is a part of the executable/library (and it is even
always mapped for C++ exceptions unwinding) it no longer has to be
present anywhere on the disk as the program could be upgraded in the
meantime and the running instance has its executable file already
unlinked from disk.

One possibility is to echo 0x3f >/proc/*/coredump_filter and dump all
the file-backed memory including the exec...

Documentation/sysctl/kernel.txt

@@ -190,6 +190,8 @@ core_pattern is used to specify a core dumpfile pattern name.
 	%%	output one '%'
 	%p	pid
 	%P	global pid (init PID namespace)
+	%i	tid
+	%I	global tid (init PID namespace)
 	%u	uid
 	%g	gid
 	%d	dump mode, matches PR_SET_DUMPABLE and

fs/coredump.c

@@ -199,6 +199,14 @@ static int format_corename(struct core_name *cn, struct coredump_params *cprm)
 				err = cn_printf(cn, "%d",
 					      task_tgid_nr(current));
 				break;
+			case 'i':
+				err = cn_printf(cn, "%d",
+					      task_pid_vnr(current));
+				break;
+			case 'I':
+				err = cn_printf(cn, "%d",
+					      task_pid_nr(current));
+				break;
 			/* uid */
 			case 'u':
 				err = cn_printf(cn, "%d", cred->uid);