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

2 files changed, 20 insertions, 0 deletions

diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index ff1ba2e6635..c4f8ffc25c5 100644
--- a/lld/ELF/Writer.cpp
+++ b/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>(),
diff --git a/lld/test/ELF/pie-weak.s b/lld/test/ELF/pie-weak.s
new file mode 100644
index 00000000000..eff9553cf80
--- /dev/null
+++ b/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