Modify kdb_trap() so that it re-calls the dbbe_trap function as long as

the debugger back-end has changed.  This means that switching from ddb
to gdb no longer requires a "step" which can be dangerous on an
already-crashed kernel.

Also add a capability to get from the gdb back-end back to ddb, by
typing ^C in the console window.

While here, simplify kdb_sysctl_available() by using
sbuf_new_for_sysctl(), and use strlcpy() instead of strncpy() since the
strlcpy semantic is desired.

MFC after:	1 month

2 files changed, 23 insertions, 0 deletions

diff --git a/sys/gdb/gdb_main.c b/sys/gdb/gdb_main.c
index 233330e..8f3c271 100644
--- a/sys/gdb/gdb_main.c
+++ b/sys/gdb/gdb_main.c
@@ -95,7 +95,17 @@ gdb_init(void)
 static int
 gdb_trap(int type, int code)
 {
+	jmp_buf jb;
 	struct thread *thr_iter;
+	void *prev_jb;
+
+	prev_jb = kdb_jmpbuf(jb);
+	if (setjmp(jb) != 0) {
+		printf("%s bailing, hopefully back to ddb!\n", __func__);
+		gdb_listening = 0;
+		(void)kdb_jmpbuf(prev_jb);
+		return (1);
+	}
 
 	gdb_listening = 0;
 	/*
@@ -291,5 +301,6 @@ gdb_trap(int type, int code)
 			break;
 		}
 	}
+	(void)kdb_jmpbuf(prev_jb);
 	return (0);
 }
diff --git a/sys/gdb/gdb_packet.c b/sys/gdb/gdb_packet.c
index d3e5a0a..a62cc8d 100644
--- a/sys/gdb/gdb_packet.c
+++ b/sys/gdb/gdb_packet.c
@@ -31,6 +31,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/systm.h>
 #include <sys/ctype.h>
 #include <sys/kdb.h>
+#include <sys/ttydefaults.h>
 
 #include <machine/gdb_machdep.h>
 #include <machine/kdb.h>
@@ -60,6 +61,17 @@ gdb_getc(void)
 	do
 		c = gdb_cur->gdb_getc();
 	while (c == -1);
+
+	if (c == CTRL('C')) {
+		printf("Received ^C; trying to switch back to ddb.\n");
+
+		if (kdb_dbbe_select("ddb") != 0)
+			printf("The ddb backend could not be selected.\n");
+		else {
+			printf("using longjmp, hope it works!\n");
+			kdb_reenter();
+		}
+	}
 	return (c);
 }