runtime: better trace for fault due to nil pointer call

R=r
CC=golang-dev
http://codereview.appspot.com/854048

1 files changed, 11 insertions, 3 deletions

diff --git a/src/pkg/runtime/freebsd/386/signal.c b/src/pkg/runtime/freebsd/386/signal.c
index ec8ac3a7d..be2f4ce6f 100644
--- a/src/pkg/runtime/freebsd/386/signal.c
+++ b/src/pkg/runtime/freebsd/386/signal.c
@@ -64,10 +64,18 @@ sighandler(int32 sig, Siginfo* info, void* context)
 		gp->sigcode0 = info->si_code;
 		gp->sigcode1 = (uintptr)info->si_addr;
 
-		sp = (uintptr*)r->mc_esp;
-		*--sp = r->mc_eip;
+		// Only push sigpanic if r->mc_eip != 0.
+		// If r->mc_eip == 0, probably panicked because of a
+		// call to a nil func.  Not pushing that onto sp will
+		// make the trace look like a call to sigpanic instead.
+		// (Otherwise the trace will end at sigpanic and we
+		// won't get to see who faulted.)
+		if(r->mc_eip != 0) {
+			sp = (uintptr*)r->mc_esp;
+			*--sp = r->mc_eip;
+			r->mc_esp = (uintptr)sp;
+		}
 		r->mc_eip = (uintptr)sigpanic;
-		r->mc_esp = (uintptr)sp;
 		return;
 	}