Add new vnode dumper to support live minidumps

This dumper can instantiate and write the dump's contents to a
file-backed vnode.

Unlike existing disk or network dumpers, the vnode dumper should not be
invoked during a system panic, and therefore is not added to the global
dumper_configs list. Instead, the vnode dumper is constructed ad-hoc
when a live dump is requested using the new ioctl on /dev/mem. This is
similar in spirit to a kgdb session against the live system via
/dev/mem.

As described briefly in the mem(4) man page, live dumps are not
guaranteed to result in a usuable output file, but offer some debugging
value where forcefully panicing a system to dump its memory is not
desirable/feasible.

A future change to savecore(8) will add an option to save a live dump.

Reviewed by:	markj, Pau Amma <pauamma@gundo.com> (manpages)
Discussed with:	kib
MFC after:	3 weeks
Sponsored by:	Juniper Networks, Inc.
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D33813

(cherry picked from commit c9114f9f86f92742eacd1d802c34009a57e81055)

1 files changed, 12 insertions, 2 deletions

diff --git a/sys/kern/kern_shutdown.c b/sys/kern/kern_shutdown.c
index 37544e364ee2..0f31622903bf 100644
--- a/sys/kern/kern_shutdown.c
+++ b/sys/kern/kern_shutdown.c
@@ -383,6 +383,17 @@ print_uptime(void)
 	printf("%lds\n", (long)ts.tv_sec);
 }
 
+/*
+ * Set up a context that can be extracted from the dump.
+ */
+void
+dump_savectx(void)
+{
+
+	savectx(&dumppcb);
+	dumptid = curthread->td_tid;
+}
+
 int
 doadump(boolean_t textdump)
 {
@@ -395,8 +406,7 @@ doadump(boolean_t textdump)
 	if (TAILQ_EMPTY(&dumper_configs))
 		return (ENXIO);
 
-	savectx(&dumppcb);
-	dumptid = curthread->td_tid;
+	dump_savectx();
 	dumping++;
 
 	coredump = TRUE;