[LLD] Emit dynamic relocations for references to script symbols in -pie links

[bd1976llvm]

Summary

https://reviews.llvm.org/D55423 caused LLD to stop emitting dynamic relocations for references to script symbols in -pie links.

This patch fixes that regression.

[peter.smith]

That looks ok to me, IIRC the main problem in D55423 was hitting the cannot refer to absolute symbol error, and this will still avoid that.

[grimar]

LGTM too (with 2 nits).

lld/test/ELF/linkerscript/symbol-pie.s

3

We normally put # REQUIRES: line first in the test cases. Please put the comment before the CHECK.

11

I would perhaps check the relocation too.

[MaskRay]

Errr. The commit description should be rewritten as:

-    https://reviews.llvm.org/D61298
+    Differential Revision:  https://reviews.llvm.org/D61298

to close the revision automatically. BTW, the test had trailing \r (which I have fixed)

[grimar]

Was committed as r359683

lld/ELF/Relocations.cpp

if (Sym.IsPreemptible)
     return false;
   if (!Config->Pic)
     return true;
 
   // The size of a non preemptible symbol is a constant.
   if (E == R_SIZE)
     return true;
 
-  // We set the final symbols values for linker script defined symbols later.
-  // They always can be computed as a link time constant.
-  if (Sym.ScriptDefined)
-    return true;
-
   // For the target and the relocation, we want to know if they are
   // absolute or relative.
   bool AbsVal = isAbsoluteValue(Sym);
   bool RelE = isRelExpr(E);
   if (AbsVal && !RelE)
     return true;
   if (!AbsVal && RelE)
     return true;
   if (!AbsVal && !RelE)
     return Target->usesOnlyLowPageBits(Type);
 
   // Relative relocation to an absolute value. This is normally unrepresentable,
   // but if the relocation refers to a weak undefined symbol, we allow it to
   // resolve to the image base. This is a little strange, but it allows us to
   // link function calls to such symbols. Normally such a call will be guarded
   // with a comparison, which will load a zero from the GOT.
   // Another special case is MIPS _gp_disp symbol which represents offset
   // between start of a function and '_gp' value and defined as absolute just
   // to simplify the code.
   assert(AbsVal && RelE);
   if (Sym.isUndefWeak())
     return true;
 
+  // We set the final symbols values for linker script defined symbols later.
+  // They always can be computed as a link time constant.
+  if (Sym.ScriptDefined)
+      return true;
+
   error("relocation " + toString(Type) + " cannot refer to absolute symbol: " +
         toString(Sym) + getLocation(S, Sym, RelOff));
   return true;
 }
 
 static RelExpr toPlt(RelExpr Expr) {
   switch (Expr) {
   case R_PPC_CALL:

lld/test/ELF/linkerscript/symbol-pie.s

+## Position independent executables require dynamic
+## relocations for references to non-absolute script
+## symbols so that their values are correct at runtime.
+
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: echo "foo = .;" > %t.script
+# RUN: ld.lld -pie -o %t --script %t.script %t.o
+# RUN: llvm-readelf -r %t | FileCheck %s
+
+# CHECK: Relocation section '.rela.dyn' at offset {{.*}} contains 1 entries:
+
+.data
+.quad foo