runtime: fail silently if we unwind over sigpanic into C code

If we're running C code and the code panics, the runtime will inject a
call to sigpanic into the C code just like it would into Go code.
However, the return PC from this sigpanic will be in C code. We used
to silently abort the traceback if we didn't recognize a return PC, so
this went by quietly. Now we're much louder because in general this is
a bad thing. However, in this one particular case, it's fine, so if
we're in cgo and are looking at the return PC of sigpanic, silence the
debug output.

Fixes #23576.

Change-Id: I03d0c14d4e4d25b29b1f5804f5e9ccc4f742f876
Reviewed-on: https://go-review.googlesource.com/90896
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 9 insertions, 1 deletions

diff --git a/src/runtime/traceback.go b/src/runtime/traceback.go
index 0d5b06a1f0..2261942ab4 100644
--- a/src/runtime/traceback.go
+++ b/src/runtime/traceback.go
@@ -286,7 +286,15 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 				// In that context it is okay to stop early.
 				// But if callback is set, we're doing a garbage collection and must
 				// get everything, so crash loudly.
-				if callback != nil || printing {
+				doPrint := printing
+				if doPrint && gp.m.incgo && f.entry == sigpanicPC {
+					// We can inject sigpanic
+					// calls directly into C code,
+					// in which case we'll see a C
+					// return PC. Don't complain.
+					doPrint = false
+				}
+				if callback != nil || doPrint {
 					print("runtime: unexpected return pc for ", funcname(f), " called from ", hex(frame.lr), "\n")
 					tracebackHexdump(gp.stack, &frame, lrPtr)
 				}