[ELF] - Fix for: error "invalid section index: xxx" when linking FreeBSD kernel.

We found that GNU assembler 2.17.50 [FreeBSD] 2007-07-03
could generate broken objects. STT_SECTION symbols can be
associated with SHT_REL[A]/SHT_SYMTAB/SHT_STRTAB sections.

This is PR28868, patch fixes handling of such files.

Differential revision: https://reviews.llvm.org/D23201

llvm-svn: 278550

lld/ELF/InputFiles.cpp

@@ -343,10 +343,15 @@ elf::ObjectFile<ELFT>::getSection(const Elf_Sym &Sym) const {
   uint32_t Index = this->getSectionIndex(Sym);
   if (Index == 0)
     return nullptr;
-  if (Index >= Sections.size() || !Sections[Index])
+  if (Index >= Sections.size())
     fatal(getFilename(this) + ": invalid section index: " + Twine(Index));
   InputSectionBase<ELFT> *S = Sections[Index];
-  if (S == &InputSectionBase<ELFT>::Discarded)
+  // We found that GNU assembler 2.17.50 [FreeBSD] 2007-07-03
+  // could generate broken objects. STT_SECTION symbols can be
+  // associated with SHT_REL[A]/SHT_SYMTAB/SHT_STRTAB sections.
+  // In this case it is fine for section to be null here as we
+  // do not allocate sections of these types.
+  if (!S || S == &InputSectionBase<ELFT>::Discarded)
     return S;
   return S->Repl;
 }

lld/test/ELF/section-symbols.test

@@ -0,0 +1,35 @@
+# RUN: yaml2obj %s -o %t
+# RUN: ld.lld -shared %t -o %tout
+
+# Verify that lld can handle STT_SECTION symbols associated
+# with SHT_REL[A]/SHT_SYMTAB/SHT_STRTAB sections.
+
+!ELF
+FileHeader:      
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  OSABI:           ELFOSABI_FREEBSD
+  Type:            ET_REL
+  Machine:         EM_X86_64
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    AddressAlign:    0x0000000000000010
+    Content:         "00000000"
+  - Name:            .rela.text
+    Type:            SHT_RELA
+    Link:            .symtab
+    AddressAlign:    0x0000000000000008
+    Info:            .text
+    Relocations:
+Symbols:
+  Local:
+    - Type:            STT_SECTION
+      Section:         .rela.text
+    - Type:            STT_SECTION
+      Section:         .shstrtab
+    - Type:            STT_SECTION
+      Section:         .symtab
+    - Type:            STT_SECTION
+      Section:         .strtab