Reland D61583 [ELF] Error on relocations to STT_SECTION symbols if the sections were discarded

This restores r361830 "[ELF] Error on relocations to STT_SECTION symbols if the sections were discarded"
and dependent commits (r362218, r362497) which were reverted by r364321, with a fix of a --gdb-index issue.

.rela.debug_ranges contains relocations of range list entries:

    // start address of a range list entry
    // old: 0; after r361830: 0
    00000000000033a0 R_X86_64_64 .text._ZN2v88internal7Isolate7factoryEv + 0
    // end address of a range list entry
    // old: 0xe; after r361830: 0
    00000000000033a8 R_X86_64_64 .text._ZN2v88internal7Isolate7factoryEv + e

If both start and end addresses of a range list entry resolve to 0,
DWARFDebugRangeList::isEndOfListEntry() will return true, then the
.debug_range decoding loop will terminate prematurely:

    while (true) {
      decode StartAddress
      decode EndAddress
      if (Entry.isEndOfListEntry()) // prematurely
        break;
      Entries.push_back(Entry);
    }

In lld/ELF/SyntheticSections.cpp, readAddressAreas() will read
incomplete address ranges and the resulting .gdb_index will be
incomplete. For files that gdb hasn't loaded their debug info, gdb uses
.gdb_index to map addresses to CUs. The absent entries make gdb fail to
symbolize some addresses.

To address this issue, we simply allow relocations to undefined symbols
in DWARF.cpp:findAux() and let RelocationResolver resolve them.

This patch should fix:

[1] http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190603/659848.html
[2] https://bugs.chromium.org/p/chromium/issues/detail?id=978067

llvm-svn: 364391

1 files changed, 17 insertions, 3 deletions

diff --git a/lld/ELF/InputSection.cpp b/lld/ELF/InputSection.cpp
index 3e8d093e6d2..d077c017ca7 100644
--- a/lld/ELF/InputSection.cpp
+++ b/lld/ELF/InputSection.cpp
@@ -412,7 +412,8 @@ void InputSection::copyRelocations(uint8_t *Buf, ArrayRef<RelTy> Rels) {
 
   for (const RelTy &Rel : Rels) {
     RelType Type = Rel.getType(Config->IsMips64EL);
-    Symbol &Sym = getFile<ELFT>()->getRelocTargetSym(Rel);
+    const ObjFile<ELFT> *File = getFile<ELFT>();
+    Symbol &Sym = File->getRelocTargetSym(Rel);
 
     auto *P = reinterpret_cast<typename ELFT::Rela *>(Buf);
     Buf += sizeof(RelTy);
@@ -435,10 +436,23 @@ void InputSection::copyRelocations(uint8_t *Buf, ArrayRef<RelTy> Rels) {
       // .eh_frame is horribly special and can reference discarded sections. To
       // avoid having to parse and recreate .eh_frame, we just replace any
       // relocation in it pointing to discarded sections with R_*_NONE, which
-      // hopefully creates a frame that is ignored at runtime.
+      // hopefully creates a frame that is ignored at runtime. Also, don't warn
+      // on .gcc_except_table and debug sections.
+      //
+      // See the comment in maybeReportUndefined for PPC64 .toc .
       auto *D = dyn_cast<Defined>(&Sym);
       if (!D) {
-        error("STT_SECTION symbol should be defined");
+        if (!Sec->Name.startswith(".debug") &&
+            !Sec->Name.startswith(".zdebug") && Sec->Name != ".eh_frame" &&
+            Sec->Name != ".gcc_except_table" && Sec->Name != ".toc") {
+          uint32_t SecIdx = cast<Undefined>(Sym).DiscardedSecIdx;
+          Elf_Shdr_Impl<ELFT> Sec =
+              CHECK(File->getObj().sections(), File)[SecIdx];
+          warn("relocation refers to a discarded section: " +
+               CHECK(File->getObj().getSectionName(&Sec), File) +
+               "\n>>> referenced by " + getObjMsg(P->r_offset));
+        }
+        P->setSymbolAndType(0, 0, false);
         continue;
       }
       SectionBase *Section = D->Section->Repl;