ELF: Do not create relative relocations for undefined symbols.

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.

Differential Revision: http://reviews.llvm.org/D19044

llvm-svn: 266161

Author: pcc

lld/ELF/Writer.cpp

@@ -573,6 +573,10 @@ void Writer<ELFT>::scanRelocs(InputSectionBase<ELFT> &C, ArrayRef<RelTy> Rels) {
           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;
         AddDyn({DynType, Out<ELFT>::Got, Body.getGotOffset<ELFT>(),

lld/test/ELF/pie-weak.s

@@ -0,0 +1,16 @@
+# 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 --check-prefix=RELOCS %s
+# RUN: llvm-objdump -d %t | FileCheck --check-prefix=DISASM %s
+
+# RELOCS:      Relocations [
+# RELOCS-NEXT: ]
+
+.weak foo
+
+.globl _start
+_start:
+# DISASM: _start:
+# DISASM-NEXT: 1000: 48 8b 05 69 10 00 00 movq 4201(%rip), %rax
+#                                              ^ .got - (.text + 7)
+mov foo@gotpcrel(%rip), %rax