Expand comment and error message

ELF/Relocations.cpp

if (Config->Shared || (Config->Pic && !isRelExpr(Expr))) {
     error(S.getLocation(RelOff) + ": can't create dynamic relocation " +
           toString(Type) + " against " +
           (Body.getName().empty() ? "local symbol in readonly segment"
                                   : "symbol '" + toString(Body) + "'") +
           " defined in " + toString(Body.File));
     return Expr;
   }
   if (Body.getVisibility() != STV_DEFAULT) {
-    error(S.getLocation(RelOff) + ": cannot preempt symbol '" + toString(Body) +
-          "' defined in " + toString(Body.File));
+    // Default visibility is the only one that allows us to preempt the symbol
+    // with a dummy plt or copy relocation. If the symbol has any other
+    // visibility the .so can refer to it directly and the dynamic linker will
+    // not be able to do anything about it.
+    error(S.getLocation(RelOff) +
+          ": cannot preempt non-default visibility symbol '" + toString(Body) +
+          "' defined in " + toString(Body.File) +
+          ".\n The executable has to use a got or plt to refer to it.");
     return Expr;
   }
   if (Body.isObject()) {
     // Produce a copy relocation.
     auto *B = cast<SharedSymbol<ELFT>>(&Body);
     if (!B->needsCopy())
       addCopyRelSymbol(B);
     return Expr;

test/ELF/copy-errors.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.s -o %t2.o
 // RUN: ld.lld %t2.o -o %t2.so -shared
 // RUN: not ld.lld %t.o %t2.so -o %t 2>&1 | FileCheck %s
 
 .global _start
 _start:
 
 
 call bar
-// CHECK: {{.*}}.o:(.text+0x1): cannot preempt symbol 'bar' defined in {{.*}}.so
+// CHECK: {{.*}}.o:(.text+0x1): cannot preempt non-default visibility symbol 'bar' defined in {{.*}}.so
+// CHECK-NEXT: The executable has to use a got or plt to refer to it.
 
 call zed
 // CHECK: symbol 'zed' defined in {{.*}}.so is missing type