[ELF] - Allow calls to a protected function in a shared library from non-PIC code.

[grimar]

Summary

GNU linkers allow to do that,
this should fix the https://llvm.org/bugs/show_bug.cgi?id=30960.

[grimar]

Will update in a minute.

[grimar]

• Added correct testcase.

[• rafael]

ELF/Relocations.cpp

449

I think this is wrong.

It is possible that we should accept the test for other reasons, but at this point in the code we are trying to preempt a symbol, and that is not possible with protected visibility.

It is possible the address is not significant, but we don't know that. There is no flag in ELF to inform us.

[grimar]

Solution is incorrect, more details available at PR page (https://bugs.llvm.org//show_bug.cgi?id=30960)

ELF/Relocations.cpp

static RelExpr adjustExpr(const elf::ObjectFile<ELFT> &File, SymbolBody &Body,
   // only memory. 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))) {
     StringRef Name = getSymbolName(File.getStringTable(), Body);
     error("can't create dynamic relocation " + getRelName(Type) + " against " +
           (Name.empty() ? "readonly segment" : "symbol " + Name));
     return Expr;
   }
-  if (Body.getVisibility() != STV_DEFAULT) {
+
+  // GNU linkers allows non PIC program calls to a protected
+  // function in a shared library. We do the same.
+  if (!Body.isFunc() && Body.getVisibility() != STV_DEFAULT) {
     error("cannot preempt symbol " + Body.getName());
     return Expr;
   }
   if (Body.isObject()) {
     // Produce a copy relocation.
     auto *B = cast<SharedSymbol<ELFT>>(&Body);
     if (!B->needsCopy())
       addCopyRelSymbol(B);

test/ELF/Inputs/protected-shared-func.s

+.text
+.protected bar
+.globl bar
+.type bar,@function
+bar:

test/ELF/protected-shared-func.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %p/Inputs/protected-shared-func.s -o %t2.o
+# RUN: ld.lld -shared %t2.o -o %t2.so
+# RUN: ld.lld %t.o %t2.so -o %t
+# RUN: llvm-readobj -r %t | FileCheck %s
+
+# CHECK:      Relocations [
+# CHECK-NEXT:   Section ({{.*}}) .rela.plt {
+# CHECK-NEXT:     0x13018 R_X86_64_JUMP_SLOT bar 0x0
+# CHECK-NEXT:   }
+# CHECK-NEXT: ]
+
+.globl _start
+_start:
+ callq bar