[ELF] Enhance --no-allow-shlib-undefined to report non-exported definition

For a DSO with all DT_NEEDED entries accounted for, if it contains an
undefined non-weak symbol that shares a name with a non-exported
definition (hidden visibility or localized by a version script), and
there is no DSO definition, we should also report an error. Because the
definition is not exported, it cannot resolve the DSO reference at
runtime.

GNU ld introduced this error-checking in [April
2003](https://sourceware.org/pipermail/binutils/2003-April/026568.html).
The feature is available for executable links but not for -shared, and it is
orthogonal to --no-allow-shlib-undefined. We make the feature part of
--no-allow-shlib-undefined and work with -shared when
--no-allow-shlib-undefined is specified.

A subset of this error-checking is covered by commit
1981b1b for --gc-sections discarded
sections. This patch covers non-discarded sections as well.

Internally, I have identified 2 bugs (which would fail with
LD_BIND_NOW=1) covered by commit
1981b1b

Author: MaskRay

lld/ELF/Writer.cpp

@@ -2023,10 +2023,16 @@ template <class ELFT> void Writer<ELFT>::finalizeSections() {
             });
         if (!allNeededIsKnown)
           continue;
-        for (Symbol *sym : file->requiredSymbols)
-          if (sym->isUndefined() && !sym->isWeak())
+        for (Symbol *sym : file->requiredSymbols) {
+          if (sym->isUndefined() && !sym->isWeak()) {
             diagnose("undefined reference due to --no-allow-shlib-undefined: " +
                      toString(*sym) + "\n>>> referenced by " + toString(file));
+          } else if (sym->isDefined() && sym->computeBinding() == STB_LOCAL) {
+            diagnose("non-exported symbol '" + toString(*sym) + "' in '" +
+                     toString(sym->file) + "' is referenced by DSO '" +
+                     toString(file) + "'");
+          }
+        }
       }
     }
   }

lld/test/ELF/allow-shlib-undefined.s

@@ -30,11 +30,11 @@
 # RUN: not ld.lld main.o a.so empty.so b.so -o /dev/null 2>&1 | FileCheck %s --check-prefix=CHECK2
 
 ## Test some cases when a relocatable object file provides a non-exported definition.
-# RUN: ld.lld main.o a.so def-hidden.o --fatal-warnings -o /dev/null
-# RUN: ld.lld main.o a.so def-hidden.o -shared --no-allow-shlib-undefined --fatal-warnings -o /dev/null
+# RUN: not ld.lld main.o a.so def-hidden.o -o /dev/null 2>&1 | FileCheck %s --check-prefix=NONEXPORTED
+# RUN: not ld.lld main.o a.so def-hidden.o -shared --no-allow-shlib-undefined -o /dev/null 2>&1 | FileCheck %s --check-prefix=NONEXPORTED
 # RUN: ld.lld main.o a.so def-hidden.o --allow-shlib-undefined --fatal-warnings -o /dev/null
 ## Test a relocatable object file definition that is converted to STB_LOCAL.
-# RUN: ld.lld main.o a.so def-hidden.o --version-script=local.ver --fatal-warnings -o /dev/null
+# RUN: not ld.lld main.o a.so def-hidden.o --version-script=local.ver -o /dev/null 2>&1 | FileCheck %s --check-prefix=NONEXPORTED
 
 ## The section containing the definition is discarded, and we report an error.
 # RUN: not ld.lld --gc-sections main.o a.so def-hidden.o -o /dev/null 2>&1 | FileCheck %s
@@ -57,6 +57,10 @@
 # WARN:        warning: undefined reference due to --no-allow-shlib-undefined: x1
 # WARN-NEXT:   >>> referenced by a.so
 
+# NONEXPORTED-NOT: error:
+# NONEXPORTED:     error: non-exported symbol 'x1' in 'def-hidden.o' is referenced by DSO 'a.so'
+# NONEXPORTED-NOT: {{.}}
+
 #--- main.s
 .globl _start
 _start: