[lld] [ELF] Enhance --no-allow-shlib-undefined to report non-exported definition (PR #70769)

[Fangrui Song via llvm-commits]

https://github.com/MaskRay updated https://github.com/llvm/llvm-project/pull/70769

>From 7176f8ab3e9137a61667c7e80a5fee822b8fd2dd Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Tue, 24 Oct 2023 14:15:23 -0700
Subject: [PATCH] [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
1981b1b6b92f7579a30c9ed32dbdf3bc749c1b40 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
1981b1b6b92f7579a30c9ed32dbdf3bc749c1b40
---
 lld/ELF/Writer.cpp                   | 10 ++++++++--
 lld/test/ELF/allow-shlib-undefined.s | 10 +++++++---
 2 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp
index 57e1aa06c6aa873..a84e4864ab0e5a5 100644
--- a/lld/ELF/Writer.cpp
+++ b/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) + "'");
+          }
+        }
       }
     }
   }
diff --git a/lld/test/ELF/allow-shlib-undefined.s b/lld/test/ELF/allow-shlib-undefined.s
index 5cab2160b10e0d1..56b44e144661c2d 100644
--- a/lld/test/ELF/allow-shlib-undefined.s
+++ b/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: