[ELF2] Don't create RelativeReloc for weak undef symbols

[hfinkel]

Summary

When we have a R_PPC64_ADDR64 for a weak undef symbol, which thus resolves to 0, and we're creating a shared library, we need to make sure that it stays 0 (because code that conditionally calls the weak function tests for this). Unfortunately, we were creating a R_PPC64_RELATIVE for these relocation targets, making the address of the undefined weak symbol equal to the base address of the shared library (which is non-zero). It seems like, in general, we should not be creating RelativeReloc relocs for undef weak symbols.

[• rafael]

The test I think is correct. Give me a minute to see if there is a
better place in the code to put the check.

[• rafael]

The test is OK, but would the attached patch fix the problem for you?

I think it is a better spot to put the test.

• t.diff691 BDownload

[hfinkel]

(as suggested by Rafael)

[hfinkel]

r250558, thanks!

ELF/Writer.cpp

if (Body) {
       if (NeedsGot) {
         if (Body->isInGot())
           continue;
         Out<ELFT>::Got->addEntry(Body);
       }
     }
 
     bool CBP = canBePreempted(Body, NeedsGot);
-    if (!CBP && (!Config->Shared || Target->isRelRelative(Type)))
+    if (!CBP && (!Config->Shared || Target->isRelRelative(Type) ||
+                 (Body && Body->isWeak() && Body->isUndefined())))
       continue;
     if (CBP)
       Body->setUsedInDynamicReloc();
     Out<ELFT>::RelaDyn->addReloc({C, RI});
   }
 }
 
 template <class ELFT>

test/elf2/ppc64-weak-undef-call-shared.s

+# RUN: llvm-mc -filetype=obj -triple=powerpc64-unknown-linux %s -o %t.o
+# RUN: ld.lld2 -shared %t.o -o %t.so
+# RUN: llvm-readobj -t -r -dyn-symbols %t.so | FileCheck %s
+# REQUIRES: ppc
+
+.section        ".toc","aw"
+.quad weakfunc
+// CHECK-NOT: R_PPC64_RELATIVE
+
+.weak weakfunc
+