[ELF] Restore GC handling of LINK_ORDER, C-named sections

[eugenis]

Summary

start_xxx symbol keeps section xxx alive only if it is not
SHF_LINK_ORDER. Such sections can be used for user metadata, when
start_xxx is used to iterate over section contents at runtime, and
the liveness is determined solely by the linked (associated) section.

This was earlier implemented in r294592, and broken in r296723.

[ruiu]

This seems correct, but I'm not totally convinced that this is the right thing to do, because I could probably argue the other way. How did you notice this, and why do you want this behavior?

[grimar]

This testcase passes and without applying the patch.

test/ELF/gc-sections-metadata-startstop.s

9

what is zz ? there is no such section in testcase.

[eugenis]

Fix the test.

[eugenis]

In D30964#701308, @ruiu wrote:

This seems correct, but I'm not totally convinced that this is the right thing to do, because I could probably argue the other way. How did you notice this, and why do you want this behavior?

See D30121 and related commits.
ASan is using this to emit metadata for global variables. Metadata sections have LINK_ORDER pointing to the corresponding global section, with the intention that the presence of metadata does not affect the liveness of globals in any way, and at runtime start_asan_globals .. stop_asan_globals can be used to enumerate metadata for the surviving globals.

[eugenis]

Extend the test case with a check for __start_yy.

[ruiu]

LGTM

ELF/MarkLive.cpp

 template <class ELFT> static bool isReserved(InputSectionBase *Sec) {
   switch (Sec->Type) {
   case SHT_FINI_ARRAY:
   case SHT_INIT_ARRAY:
   case SHT_NOTE:
   case SHT_PREINIT_ARRAY:
     return true;
   default:
-    if (Sec->Flags & SHF_LINK_ORDER)
-      return false;
-
     if (!(Sec->Flags & SHF_ALLOC))
       return true;
 
     StringRef S = Sec->Name;
     return S.startswith(".ctors") || S.startswith(".dtors") ||
            S.startswith(".init") || S.startswith(".fini") ||
            S.startswith(".jcr");
   }
template <class ELFT> void elf::markLive() {
   // Preserve special sections and those which are specified in linker
   // script KEEP command.
   for (InputSectionBase *Sec : InputSections) {
     // .eh_frame is always marked as live now, but also it can reference to
     // sections that contain personality. We preserve all non-text sections
     // referred by .eh_frame here.
     if (auto *EH = dyn_cast_or_null<EhInputSection>(Sec))
       scanEhFrameSection<ELFT>(*EH, Enqueue);
+    if (Sec->Flags & SHF_LINK_ORDER)
+      continue;
     if (isReserved<ELFT>(Sec) || Script<ELFT>::X->shouldKeep(Sec))
       Enqueue({Sec, 0});
     else if (isValidCIdentifier(Sec->Name)) {
       CNamedSections[Saver.save("__start_" + Sec->Name)].push_back(Sec);
       CNamedSections[Saver.save("__end_" + Sec->Name)].push_back(Sec);
     }
   }
 

test/ELF/gc-sections-metadata-startstop.s

+# LINK_ORDER cnamed sections are not kept alive by the __start_* reference.
+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld --gc-sections %t.o -o %t
+# RUN: llvm-objdump -section-headers %t | FileCheck  %s
+
+# CHECK-NOT: yy
+# CHECK: xx
+# CHECK-NOT: yy
+
+.weak __start_xx
+.weak __start_yy
+
+.global _start
+_start:
+.quad __start_xx
+.quad __start_yy
+
+.section xx,"a"
+.quad 0
+
+.section .foo,"a"
+.quad 0
+
+.section yy,"am",@progbits,.foo
+.quad 0
+