[ELF] - Do not crash when do --gc-sections for non-allocatable metadata sections.

Currently, LLD marks all non-allocatable sections except SHF_REL[A] as Live
when doing GC.

This can be a reason of the crash when SHF_LINK_ORDER sections
are involved, because their parents can be dead.

We should do GC for them correctly. The patch implements it.

Differential revision: https://reviews.llvm.org/D46880

1 files changed, 10 insertions, 4 deletions

diff --git a/lld/ELF/MarkLive.cpp b/lld/ELF/MarkLive.cpp
index afdb69ea456..bacbf0eee06 100644
--- a/lld/ELF/MarkLive.cpp
+++ b/lld/ELF/MarkLive.cpp
@@ -275,13 +275,18 @@ template <class ELFT> void elf::markLive() {
 
   // The -gc-sections option works only for SHF_ALLOC sections
   // (sections that are memory-mapped at runtime). So we can
-  // unconditionally make non-SHF_ALLOC sections alive.
+  // unconditionally make non-SHF_ALLOC sections alive except
+  // SHF_LINK_ORDER and SHT_REL/SHT_RELA sections.
   //
-  // Non SHF_ALLOC sections are not removed even if they are
+  // Usually, SHF_ALLOC sections are not removed even if they are
   // unreachable through relocations because reachability is not
   // a good signal whether they are garbage or not (e.g. there is
   // usually no section referring to a .comment section, but we
-  // want to keep it.)
+  // want to keep it.).
+  //
+  // Note on SHF_LINK_ORDER: Such sections contain metadata and they
+  // have a reverse dependency on the InputSection they are linked with.
+  // We are able to garbage collect them.
   //
   // Note on SHF_REL{,A}: Such sections reach here only when -r
   // or -emit-reloc were given. And they are subject of garbage
@@ -289,8 +294,9 @@ template <class ELFT> void elf::markLive() {
   // remove its relocation section.
   for (InputSectionBase *Sec : InputSections) {
     bool IsAlloc = (Sec->Flags & SHF_ALLOC);
+    bool IsLinkOrder = (Sec->Flags & SHF_LINK_ORDER);
     bool IsRel = (Sec->Type == SHT_REL || Sec->Type == SHT_RELA);
-    if (!IsAlloc && !IsRel)
+    if (!IsAlloc && !IsLinkOrder && !IsRel)
       Sec->Live = true;
   }