cmd/link, runtime: use a sentinel value for unreachable method

In the method table, the method's code pointer is stored as an
offset from the start of the text section. Currently, for an
unreachable method, the offset is left as 0, which resolves to
the start of the text section at run time. It is possible that
there is valid code there. If an unreachable method is ever
reached (due to a compiler or linker bug), the execution will
jump to a wrong location but may continue to run for a while,
until it fails with a seemingly unrelated error.

This CL changes it to use -1 for unreachable method instead. At
run time this will resolve to an invalid address, which makes it
fail immediately if it is ever reached.

Change-Id: Ied6ed7f1833c4f3b991fdf55d8810d70d307b2e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/257203
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/src/runtime/type.go b/src/runtime/type.go
index 52b6cb30b4..81455f3532 100644
--- a/src/runtime/type.go
+++ b/src/runtime/type.go
@@ -217,7 +217,9 @@ func (t *_type) nameOff(off nameOff) name {
 }
 
 func resolveTypeOff(ptrInModule unsafe.Pointer, off typeOff) *_type {
-	if off == 0 {
+	if off == 0 || off == -1 {
+		// -1 is the sentinel value for unreachable code.
+		// See cmd/link/internal/ld/data.go:relocsym.
 		return nil
 	}
 	base := uintptr(ptrInModule)
@@ -257,6 +259,11 @@ func (t *_type) typeOff(off typeOff) *_type {
 }
 
 func (t *_type) textOff(off textOff) unsafe.Pointer {
+	if off == -1 {
+		// -1 is the sentinel value for unreachable code.
+		// See cmd/link/internal/ld/data.go:relocsym.
+		return unsafe.Pointer(^uintptr(0))
+	}
 	base := uintptr(unsafe.Pointer(t))
 	var md *moduledata
 	for next := &firstmoduledata; next != nil; next = next.next {