cmd/internal/obj/x86: align functions with trap instruction

Align functions with 0xCC (INT $3) - breakpoint instruction,
instead of 0x00, which can disassemble into valid instruction.

Change-Id: Ieda191886efc4aacb86f58bea1169fd1b3b57636
Reviewed-on: https://go-review.googlesource.com/16102
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Gregory Shimansky <gregory.shimansky@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/internal/obj/x86/asm6.go

@@ -1754,7 +1754,16 @@ func span6(ctxt *obj.Link, s *obj.LSym) {
 		c = naclpad(ctxt, s, c, -c&31)
 	}
 
-	c += -c & (FuncAlign - 1)
+	// Pad functions with trap instruction, to catch invalid jumps
+	if c&(FuncAlign-1) != 0 {
+		v = -c & (FuncAlign - 1)
+		obj.Symgrow(ctxt, s, int64(c)+int64(v))
+		for i := c; i < c+v; i++ {
+			// 0xCC is INT $3 - breakpoint instruction
+			s.P[i] = uint8(0xCC)
+		}
+		c += v
+	}
 	s.Size = int64(c)
 
 	if false { /* debug['a'] > 1 */