[ELF] Suppress --no-allow-shlib-undefined diagnostic when a SharedSymbol is overridden by a hidden visibility Defined which is later discarded

Commit 1981b1b unexpectedly strengthened
--no-allow-shlib-undefined to catch a kind of ODR violation.
More precisely, when all three conditions are met, the new
`--no-allow-shlib-undefined` code reports an error.

* There is a DSO undef that can be satisfied by a definition from another DSO.
* The `SharedSymbol` is overridden by a non-exported (usually of hidden visibility) definition in a relocatable object file (`Defined`).
* The section containing the `Defined` is garbage-collected (it is not part of `.dynsym` and is not marked as live).

Technically, the hidden Defined in the executable can be intentional: it can be
meant to remain non-exported and not interact with any dynamic symbols of the same
name that might exist in other DSOs. To allow for such use cases, allocate a new bit in
Symbol and relax the --no-allow-shlib-undefined check to before commit 1981b1b.

Author: MaskRay

lld/ELF/InputFiles.cpp

@@ -1545,6 +1545,7 @@ template <class ELFT> void SharedFile::parse() {
       auto *s = symtab.addSymbol(
           SharedSymbol{*this, name, sym.getBinding(), sym.st_other,
                        sym.getType(), sym.st_value, sym.st_size, alignment});
+      s->dsoDefined = true;
       if (s->file == this)
         s->versionId = ver;
     }
@@ -1562,6 +1563,7 @@ template <class ELFT> void SharedFile::parse() {
     auto *s = symtab.addSymbol(
         SharedSymbol{*this, saver().save(name), sym.getBinding(), sym.st_other,
                      sym.getType(), sym.st_value, sym.st_size, alignment});
+    s->dsoDefined = true;
     if (s->file == this)
       s->versionId = idx;
   }

lld/ELF/Symbols.h

@@ -298,12 +298,13 @@ class Symbol {
   // of the symbol.
   uint8_t scriptDefined : 1;
 
+  // True if defined in a DSO. There may also be a definition in a relocatable
+  // object file.
+  uint8_t dsoDefined : 1;
+
   // True if defined in a DSO as protected visibility.
   uint8_t dsoProtected : 1;
 
-  // True if targeted by a range extension thunk.
-  uint8_t thunkAccessed : 1;
-
   // Temporary flags used to communicate which symbol entries need PLT and GOT
   // entries during postScanRelocations();
   std::atomic<uint16_t> flags;
@@ -320,6 +321,9 @@ class Symbol {
   uint16_t versionId;
   uint8_t versionScriptAssigned : 1;
 
+  // True if targeted by a range extension thunk.
+  uint8_t thunkAccessed : 1;
+
   void setFlags(uint16_t bits) {
     flags.fetch_or(bits, std::memory_order_relaxed);
   }

lld/ELF/Writer.cpp

@@ -2016,6 +2016,11 @@ template <class ELFT> void Writer<ELFT>::finalizeSections() {
       // to catch more cases. That is too much for us. Our approach resembles
       // the one used in ld.gold, achieves a good balance to be useful but not
       // too smart.
+      //
+      // If a DSO reference is resolved by a SharedSymbol, but the SharedSymbol
+      // is overridden by a hidden visibility Defined (which is later discarded
+      // due to GC), don't report the diagnostic. However, this may indicate an
+      // unintended SharedSymbol.
       for (SharedFile *file : ctx.sharedFiles) {
         bool allNeededIsKnown =
             llvm::all_of(file->dtNeeded, [&](StringRef needed) {
@@ -2024,6 +2029,8 @@ template <class ELFT> void Writer<ELFT>::finalizeSections() {
         if (!allNeededIsKnown)
           continue;
         for (Symbol *sym : file->requiredSymbols) {
+          if (sym->dsoDefined)
+            continue;
           if (sym->isUndefined() && !sym->isWeak()) {
             diagnose("undefined reference due to --no-allow-shlib-undefined: " +
                      toString(*sym) + "\n>>> referenced by " + toString(file));

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

@@ -40,7 +40,7 @@
 # RUN: not ld.lld --gc-sections main.o a.so def-hidden.o -o /dev/null 2>&1 | FileCheck %s
 ## The definition def.so is ignored.
 # RUN: ld.lld -shared def.o -o def.so
-# RUN: not ld.lld --gc-sections main.o a.so def.so def-hidden.o -o /dev/null 2>&1 | FileCheck %s
+# RUN: ld.lld --gc-sections main.o a.so def.so def-hidden.o --fatal-warnings -o /dev/null
 
 # CHECK-NOT:   error:
 # CHECK:       error: undefined reference due to --no-allow-shlib-undefined: x1{{$}}