[ELF] - Allow relocation to a weak undefined symbol when -z notext is…

… given.

Previously we failed to resolve them when produced executables:
"relocation R_X86_64_32 cannot be used against shared object; recompile with -fPIC"

Patch fixes it so that we resolve them to 0 for executables. 
And for -shared case we still should produce the relocation.

This finishes fixing PR35720.

DIfferential revision: https://reviews.llvm.org/D41551

llvm-svn: 321473

lld/ELF/Relocations.cpp

@@ -585,6 +585,19 @@ template <class ELFT>
 static RelExpr adjustExpr(Symbol &Sym, RelExpr Expr, RelType Type,
                           InputSectionBase &S, uint64_t RelOff,
                           bool &IsConstant) {
+  // If a relocation can be applied at link-time, we don't need to
+  // create a dynamic relocation in the first place.
+  if (IsConstant)
+    return Expr;
+
+  // If the relocation is to a weak undef, and we are producing
+  // executable, give up on it and produce a non preemptible 0.
+  if (!Config->Shared && Sym.isUndefWeak()) {
+    Sym.IsPreemptible = false;
+    IsConstant = true;
+    return Expr;
+  }
+
   // If a section writable or if we are allowed to create dynamic relocations
   // against read-only sections (i.e. when "-z notext" is given), we can create
   // any dynamic relocation the dynamic linker knows how to handle.
@@ -596,22 +609,9 @@ static RelExpr adjustExpr(Symbol &Sym, RelExpr Expr, RelType Type,
     return Expr;
   }
 
-  // If a relocation can be applied at link-time, we don't need to
-  // create a dynamic relocation in the first place.
-  if (IsConstant)
-    return Expr;
-
   // If we got here we know that this relocation would require the dynamic
   // linker to write a value to read only memory.
 
-  // If the relocation is to a weak undef, give up on it and produce a
-  // non preemptible 0.
-  if (Sym.isUndefWeak()) {
-    Sym.IsPreemptible = false;
-    IsConstant = true;
-    return Expr;
-  }
-
   // We can hack around it if we are producing an executable and
   // the refered symbol can be preemepted to refer to the executable.
   if (Config->Shared || (Config->Pic && !isRelExpr(Expr))) {

lld/test/ELF/znotext-weak-undef.s

@@ -0,0 +1,16 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: not ld.lld -z notext -shared %t.o -o %t 2>&1 | FileCheck %s
+# CHECK: relocation R_X86_64_32 cannot be used against shared object; recompile with -fPIC
+
+# RUN: ld.lld -z notext %t.o -o %t
+# RUN: llvm-readobj -r %t | FileCheck %s --check-prefix=EXE
+# EXE:      Relocations [
+# EXE-NEXT: ]
+
+.text
+.global foo
+.weak foo
+
+_start:
+mov $foo,%eax