fix(debug): handle mapping edges, stray traps and dying debuggees

Assisted-by: GLM 5.3

debug/watchpoint_freebsd_amd64.go

@@ -75,17 +75,27 @@ func (s *Session) setDbRegs(dr *dbreg) error {
 // archStopTrace classifies a TRAP_TRACE stop.  On amd64 the kernel
 // delivers both the completed single-step and the debug-register hit
 // through T_TRCTRAP with TRAP_TRACE (sys/amd64/amd64/trap.c), and DR6's
-// B0-B3 bits name the watchpoint that fired.
+// B0-B3 bits name the watchpoint that fired.  The bits are sticky ("the
+// processor never clears DR6", Intel SDM vol 3, "Debug Registers"), so
+// they are acknowledged here: cleared once read, or the next hit on a
+// different slot would still see this slot's bit set and report this
+// slot's address again.
 func archStopTrace(s *Session, siAddr uint64) (StopReason, uint64) {
 	dr, err := s.getDbRegs()
 	if err != nil {
 		return StopSingleStep, 0
 	}
-	if status := dr.Dr[drStatus]; status&0xF != 0 {
-		for slot := range 4 {
-			if status&(1<<slot) != 0 && dr.Dr[slot] != 0 {
-				return StopWatchpoint, dr.Dr[slot]
-			}
+	status := dr.Dr[drStatus] & 0xF
+	if status == 0 {
+		return StopSingleStep, 0
+	}
+	// Best effort: the write-back only fails for a dead debuggee, for
+	// which no further watchpoint can fire anyway.
+	dr.Dr[drStatus] &^= 0xF
+	_ = s.setDbRegs(dr)
+	for slot := range 4 {
+		if status&(1<<slot) != 0 && dr.Dr[slot] != 0 {
+			return StopWatchpoint, dr.Dr[slot]
 		}
 	}
 	return StopSingleStep, 0