kerneldump: Inline dump_savectx() into its callers

The callers of dump_savectx() (i.e., doadump() and livedump_start())
subsequently call dumpsys()/minidumpsys(), which dump the calling
thread's stack when writing the dump.  If dump_savectx() gets its own
stack frame, that frame might be clobbered when its caller later calls
dumpsys()/minidumpsys(), making it difficult for debuggers to unwind the
stack.

Fix this by making dump_savectx() a macro, so that savectx() is always
called directly by the function which subsequently calls
dumpsys()/minidumpsys().

This fixes stack unwinding for the panicking thread from arm64
minidumps.  The same happened to work on amd64, but kgdb reports the
dump_savectx() calls as coming from dumpsys(), so in that case it
appears to work by accident.

Fixes:	c9114f9f86f9 ("Add new vnode dumper to support live minidumps")
Reviewed by:	mhorne, jhb
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D39151

(cherry picked from commit c3179891f897d840f578a5139839fcacb587c96d)

1 files changed, 2 insertions, 13 deletions

diff --git a/sys/kern/kern_shutdown.c b/sys/kern/kern_shutdown.c
index ee6faa222206..cf9411420304 100644
--- a/sys/kern/kern_shutdown.c
+++ b/sys/kern/kern_shutdown.c
@@ -239,8 +239,8 @@ MTX_SYSINIT(dumper_configs, &dumpconf_list_lk, "dumper config list", MTX_DEF);
 static TAILQ_HEAD(dumpconflist, dumperinfo) dumper_configs =
     TAILQ_HEAD_INITIALIZER(dumper_configs);
 
-/* Context information for dump-debuggers. */
-static struct pcb dumppcb;		/* Registers. */
+/* Context information for dump-debuggers, saved by the dump_savectx() macro. */
+struct pcb dumppcb;			/* Registers. */
 lwpid_t dumptid;			/* Thread ID. */
 
 static struct cdevsw reroot_cdevsw = {
@@ -381,17 +381,6 @@ 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)
 {