[ELF] Check if LinkSec is nullptr when initializing SHF_LINK_ORDER sections

[MaskRay]

Summary

This protects lld from a null pointer dereference when a faulty input file has such invalid sh_link fields.

[ruiu]

ELF/InputFiles.cpp

481–484

It feels like setting to a nullptr is a little bit tricky. I'd probably keep the original code for this line, and add another null check after InputSectionBase *LinkSec = this->Sections[Sec.sh_link];

[MaskRay]

Beautify

[MaskRay]

ELF/InputFiles.cpp

481–484

Duplicate fatal calls seem verbose

if (Sec.sh_link >= this->Sections.size())
  fatal(toString(this) +
        ": invalid sh_link index: " + Twine(Sec.sh_link));
InputSectionBase *LinkSec = this->Sections[Sec.sh_link];
if (!LinkSec)
  fatal(toString(this) +
        ": invalid sh_link index: " + Twine(Sec.sh_link));

How about the current change?

[ruiu]

LGTM

Looks better!

ELF/InputFiles.cpp

case SHT_NULL:
       break;
     default:
       this->Sections[I] = createInputSection(Sec);
     }
 
     // .ARM.exidx sections have a reverse dependency on the InputSection they
     // have a SHF_LINK_ORDER dependency, this is identified by the sh_link.
     if (Sec.sh_flags & SHF_LINK_ORDER) {
-      if (Sec.sh_link >= this->Sections.size())
+      InputSectionBase *LinkSec = nullptr;
+      if (Sec.sh_link < this->Sections.size())
+        LinkSec = this->Sections[Sec.sh_link];
+      if (!LinkSec)
         fatal(toString(this) +
               ": invalid sh_link index: " + Twine(Sec.sh_link));
 
-      InputSectionBase *LinkSec = this->Sections[Sec.sh_link];
       InputSection *IS = cast<InputSection>(this->Sections[I]);
       LinkSec->DependentSections.push_back(IS);
       if (!isa<InputSection>(LinkSec))
         error("a section " + IS->Name +
               " with SHF_LINK_ORDER should not refer a non-regular "
               "section: " +
               toString(LinkSec));
     }

test/ELF/invalid/linkorder-invalid-sec2.test

+# REQUIRES: x86
+# RUN: yaml2obj %s -o %t.o
+# RUN: not ld.lld %t.o -o /dev/null 2>&1 | FileCheck %s
+# CHECK: invalid sh_link index: 0
+
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_X86_64
+Sections:
+  - Name:          .linkorder
+    Type:          SHT_PROGBITS
+    Flags:         [ SHF_ALLOC, SHF_EXECINSTR, SHF_LINK_ORDER ]
+    Link:          0