debug/macho: fix DWARF for section names longer than 16 chars

The Mach-O file format truncates section names to 16 characters
maximum, which makes some sections unrecognizable to debug/dwarf.
This CL works around this problem by re-expanding the truncated section
names.

This problem was originally reported as:
go-delve/delve#3797

Change-Id: I8c4a02493b8d5c3f63c831da43f6292124edf670
Reviewed-on: https://go-review.googlesource.com/c/go/+/608995
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

src/debug/macho/file.go

@@ -610,15 +610,33 @@ func (f *File) Section(name string) *Section {
 // DWARF returns the DWARF debug information for the Mach-O file.
 func (f *File) DWARF() (*dwarf.Data, error) {
 	dwarfSuffix := func(s *Section) string {
+		sectname := s.Name
+		var pfx int
 		switch {
-		case strings.HasPrefix(s.Name, "__debug_"):
-			return s.Name[8:]
-		case strings.HasPrefix(s.Name, "__zdebug_"):
-			return s.Name[9:]
+		case strings.HasPrefix(sectname, "__debug_"):
+			pfx = 8
+		case strings.HasPrefix(sectname, "__zdebug_"):
+			pfx = 9
 		default:
 			return ""
 		}
-
+		// Mach-O executables truncate section names to 16 characters, mangling some DWARF sections.
+		// As of DWARFv5 these are the only problematic section names (see DWARFv5 Appendix G).
+		for _, longname := range []string{
+			"__debug_str_offsets",
+			"__zdebug_line_str",
+			"__zdebug_loclists",
+			"__zdebug_pubnames",
+			"__zdebug_pubtypes",
+			"__zdebug_rnglists",
+			"__zdebug_str_offsets",
+		} {
+			if sectname == longname[:16] {
+				sectname = longname
+				break
+			}
+		}
+		return sectname[pfx:]
 	}
 	sectionData := func(s *Section) ([]byte, error) {
 		b, err := s.Data()