[Object/ELF] - Check index argument in getSymbol().

[grimar]

Summary

Without this check LLD crashes when SHT_GROUP section has invalid symbol index
because of next code:

template <class ELFT>
StringRef elf::ObjectFile<ELFT>::getShtGroupSignature(const Elf_Shdr &Sec) {
..
  const Elf_Sym *Sym = Obj.getSymbol(Symtab, Sec.sh_info);
..
}

If sh_info is too large, &Symbols[Index] just asserts.

No testcases provided because llvm-objdump/llvm-readelf does not use getSymbol() function.
Though if this be landed I will be happy to add testcase for lld showing the issue.

[grimar]

Ping.

[ruiu]

LGTM

[grimar]

Thanks, Rui !

ELF.h

uint32_t getExtendedSymbolTableIndex(const Elf_Sym *Sym,
                                        ArrayRef<Elf_Word> ShndxTable) const;
   const Elf_Ehdr *getHeader() const { return Header; }
   ErrorOr<const Elf_Shdr *> getSection(const Elf_Sym *Sym,
                                        const Elf_Shdr *SymTab,
                                        ArrayRef<Elf_Word> ShndxTable) const;
   ErrorOr<const Elf_Shdr *> getSection(uint32_t Index) const;
 
   const Elf_Sym *getSymbol(const Elf_Shdr *Sec, uint32_t Index) const {
-    return &symbols(Sec)[Index];
+    Elf_Sym_Range Symbols = symbols(Sec);
+    if (Index > Symbols.size())
+      report_fatal_error("Invalid symbol index");
+    return &Symbols[Index];
   }
 
   ErrorOr<StringRef> getSectionName(const Elf_Shdr *Section) const;
   template <typename T>
   ErrorOr<ArrayRef<T>> getSectionContentsAsArray(const Elf_Shdr *Sec) const;
   ErrorOr<ArrayRef<uint8_t> > getSectionContents(const Elf_Shdr *Sec) const;
 };