[ELF] Allow R_PLT_PC (R_PC) to a hidden undefined weak symbol

[MaskRay]

Summary

This essentially reverts b841e119d77ed0502e3a2e710f26a899bef28b3c.

Such code construct can be used in the following way:

// glibc/stdlib/exit.c
// clang -fuse-ld=lld => succeeded
// clang -fuse-ld=lld -fpie -pie => relocation R_PLT_PC cannot refer to absolute symbol
__attribute__((weak, visibility("hidden"))) extern void __call_tls_dtors();
void __run_exit_handlers() {
  if (__call_tls_dtors)
      __call_tls_dtors();
}

Since we allow R_PLT_PC in -no-pie mode, it makes sense to allow it in
-pie mode as well.

[pcc]

LGTM

[merge_guards_bot]

Unit tests: fail. 61975 tests passed, 1 failed and 783 were skipped.

failed: LLVM.Bindings/Go/go.test

clang-tidy: unknown.

clang-format: pass.

Build artifacts: diff.json, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

[smeenai]

rGb841e119d77ed0502e3a2e710f26a899bef28b3c is in the 10.0 branch. Should this be cherry picked to 10.0? CC @hans

[MaskRay]

In D72943#1832079, @smeenai wrote:

rGb841e119d77ed0502e3a2e710f26a899bef28b3c is in the 10.0 branch. Should this be cherry picked to 10.0? CC @hans

This commit is in release/10.x

git branch origin/release/10.x --contains 9aacec8331d9bc1f80046e5954be1f5e0e0acd42 => success

[smeenai]

In D72943#1832114, @MaskRay wrote:

In D72943#1832079, @smeenai wrote:

rGb841e119d77ed0502e3a2e710f26a899bef28b3c is in the 10.0 branch. Should this be cherry picked to 10.0? CC @hans

This commit is in release/10.x

git branch origin/release/10.x --contains 9aacec8331d9bc1f80046e5954be1f5e0e0acd42 => success

Ah, you cherry-picked to 10.0 already. Thanks.

lld/ELF/Relocations.cpp

if (absVal && !relE)
     return true;
   if (!absVal && relE)
     return true;
   if (!absVal && !relE)
     return target->usesOnlyLowPageBits(type);
 
   assert(absVal && relE);
 
+  // Allow R_PLT_PC (optimized to R_PC here) to a hidden undefined weak symbol
+  // in PIC mode. This is a little strange, but it allows us to link function
+  // calls to such symbols (e.g. glibc/stdlib/exit.c:__run_exit_handlers).
+  // Normally such a call will be guarded with a comparison, which will load a
+  // zero from the GOT.
+  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;

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

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 // RUN: ld.lld %t.o -o %t.so -shared
 // RUN: llvm-readobj -r -S --section-data %t.so | FileCheck %s
+// RUN: ld.lld %t.o -o %t -pie
+// RUN: llvm-readobj -r -S --section-data %t | FileCheck %s
+
+/// This is usually guarded with a comparison. Don't report an error.
+call g
 
 .data
 .weak g
 .hidden g
 .quad g
 
 // CHECK:      Name: .data
 // CHECK-NEXT: Type: SHT_PROGBITS