cmd/link: revise previous __DWARF segment protection fix

Tweak the previous fix for issue 32673 (in CL 182958) to work around
problems with c-shared build mode that crop up on some of the builders
(10.11, 10.12).  We now consistently set vmaddr and vmsize to zero
for the DWARF segment regardless of build mode.

Updates #32673

Change-Id: Id1fc213590ad00c28352925e2d754d760e022b5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/183237Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/link/internal/ld/macho_combine_dwarf.go

@@ -245,7 +245,7 @@ func machoCombineDwarf(ctxt *Link, exef *os.File, exem *macho.File, dsym, outexe
 		}
 	}
 	// Do the final update of the DWARF segment's load command.
-	return machoUpdateDwarfHeader(&reader, ctxt.BuildMode, compressedSects)
+	return machoUpdateDwarfHeader(&reader, compressedSects, dwarfsize)
 }
 
 // machoCompressSections tries to compress the DWARF segments in dwarfm,
@@ -390,7 +390,7 @@ func machoUpdateSections(r loadCmdReader, seg, sect reflect.Value, deltaOffset,
 }
 
 // machoUpdateDwarfHeader updates the DWARF segment load command.
-func machoUpdateDwarfHeader(r *loadCmdReader, buildmode BuildMode, compressedSects []*macho.Section) error {
+func machoUpdateDwarfHeader(r *loadCmdReader, compressedSects []*macho.Section, dwarfsize uint64) error {
 	var seg, sect interface{}
 	cmd, err := r.Next()
 	if err != nil {
@@ -408,8 +408,6 @@ func machoUpdateDwarfHeader(r *loadCmdReader, buildmode BuildMode, compressedSec
 	}
 	segv := reflect.ValueOf(seg).Elem()
 	segv.FieldByName("Offset").SetUint(uint64(dwarfstart))
-	segv.FieldByName("Addr").SetUint(uint64(dwarfaddr))
-	segv.FieldByName("Prot").SetUint(0)
 
 	if compressedSects != nil {
 		var segSize uint64
@@ -417,23 +415,27 @@ func machoUpdateDwarfHeader(r *loadCmdReader, buildmode BuildMode, compressedSec
 			segSize += newSect.Size
 		}
 		segv.FieldByName("Filesz").SetUint(segSize)
-		segv.FieldByName("Memsz").SetUint(uint64(Rnd(int64(segSize), 1<<pageAlign)))
+	} else {
+		segv.FieldByName("Filesz").SetUint(dwarfsize)
 	}
 
 	deltaOffset := uint64(dwarfstart) - realdwarf.Offset
 	deltaAddr := uint64(dwarfaddr) - realdwarf.Addr
 
-	// If we set Memsz to 0 (and might as well set Addr too),
-	// then the xnu kernel will bail out halfway through load_segment
-	// and not apply further sanity checks that we might fail in the future.
-	// We don't need the DWARF information actually available in memory.
-	// But if we do this for buildmode=c-shared then the user-space
-	// dynamic loader complains about memsz < filesz. Sigh.
-	if buildmode != BuildModeCShared {
+	// We want the DWARF segment to be considered non-loadable, so
+	// force vmaddr and vmsize to zero. In addition, set the initial
+	// protection to zero so as to make the dynamic loader happy,
+	// since otherwise it may complain that that the vm size and file
+	// size don't match for the segment. See issues 21647 and 32673
+	// for more context. Also useful to refer to the Apple dynamic
+	// loader source, specifically ImageLoaderMachO::sniffLoadCommands
+	// in ImageLoaderMachO.cpp (various versions can be found online, see
+	// https://opensource.apple.com/source/dyld/dyld-519.2.2/src/ImageLoaderMachO.cpp.auto.html
+	// as one example).
 	segv.FieldByName("Addr").SetUint(0)
 	segv.FieldByName("Memsz").SetUint(0)
+	segv.FieldByName("Prot").SetUint(0)
 	deltaAddr = 0
-	}
 
 	if err := r.WriteAt(0, seg); err != nil {
 		return err