ELF: Do not create relative relocations for undefined symbols.

[pcc]

Summary

We need to ensure that the address of an undefined weak symbol evaluates to
zero. We were getting this right for non-PIC executables (where the symbol
can be evaluated directly) and for DSOs (where we emit a symbolic relocation
for these symbols, as they are preemptible). But we weren't getting it right
for PIEs. Probably the simplest way to ensure that these symbols evaluate
to zero is by not creating a relocation in .got for them.

[• rafael]

LGTM

test/ELF/pie-weak.s

5

please also check the content of .text by disassembling it.

[ruiu]

LGTM

ELF/Writer.cpp

if (Target->needsGot(Type, Body)) {
         continue;
 
       if (Preemptible || Config->Pic) {
         uint32_t DynType;
         if (Body.isTls())
           DynType = Target->TlsGotRel;
         else if (Preemptible)
           DynType = Target->GotRel;
+        else if (Body.isUndefined())
+          // Weak undefined symbols evaluate to zero, so don't create
+          // relocations for them.
+          continue;
         else
           DynType = Target->RelativeRel;
         Out<ELFT>::RelaDyn->addReloc({DynType, Out<ELFT>::Got,
                                       Body.getGotOffset<ELFT>(), !Preemptible,
                                       &Body, 0});
       }
       continue;
     }

test/ELF/pie-weak.s

+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: ld.lld -pie %t.o -o %t
+# RUN: llvm-readobj -r %t | FileCheck %s
+
+# CHECK:      Relocations [
+# CHECK-NEXT: ]
+
+.weak foo
+
+.globl _start
+_start:
+mov foo@gotpcrel(%rip), %rax