[lld][ELF] Demote symbols in discarded sections to Undefined.

lld/ELF/Driver.cpp

@@ -3061,6 +3061,37 @@ void LinkerDriver::link(opt::InputArgList &args) {
     script->addOrphanSections();
   }
 
+  // Demote symbols defined relative to input sections that are discarded by
+  // /DISCARD/ so that relocations referencing them will get reported.
+  if (script->seenDiscard) {
+    llvm::TimeTraceScope timeScope("Demote symbols in discarded sections");
+    parallelForEach(symtab.getSymbols(), [](Symbol *sym) {
+      Defined *d = dyn_cast<Defined>(sym);
+      if (!d)
+        return;
+      if (d->section && !d->section->isLive()) {
+        uint32_t secIdx = 0;
+        if (d->file) {
+          uint32_t idx = 0;
+          for (SectionBase *s : d->file->getSections()) {
+            if (s == d->section) {
+              secIdx = idx;
+              break;
+            }
+            idx++;
+          }
+        }
+        // If we don't change the binding from WEAK to GLOBAL here, the
+        // undefined symbol reporting will think this is undefined weak and
+        // not give a warning.
+        Undefined(d->file, sym->getName(),
+                  sym->isWeak() ? (uint8_t)STB_GLOBAL : sym->binding,
+                  sym->stOther, sym->type, secIdx)
+            .overwrite(*sym);
+      }
+    });
+  }
+
   {
     llvm::TimeTraceScope timeScope("Merge/finalize input sections");
 

lld/ELF/LinkerScript.cpp

@@ -613,6 +613,7 @@ void LinkerScript::processSectionCommands() {
         discard(*s);
       discardSynthetic(*osec);
       osec->commands.clear();
+      seenDiscard = true;
       return false;
     }
 

lld/ELF/LinkerScript.h

@@ -356,6 +356,7 @@ class LinkerScript final {
 
   bool hasSectionsCommand = false;
   bool seenDataAlign = false;
+  bool seenDiscard = false;
   bool seenRelroEnd = false;
   bool errorOnMissingSection = false;
   std::string backwardDotErr;

lld/ELF/Relocations.cpp

@@ -507,8 +507,7 @@ int64_t RelocationScanner::computeMipsAddend(const RelTy &rel, RelExpr expr,
 template <class ELFT>
 static std::string maybeReportDiscarded(Undefined &sym) {
   auto *file = dyn_cast_or_null<ObjFile<ELFT>>(sym.file);
-  if (!file || !sym.discardedSecIdx ||
-      file->getSections()[sym.discardedSecIdx] != &InputSection::discarded)
+  if (!file || !sym.discardedSecIdx)
     return "";
   ArrayRef<typename ELFT::Shdr> objSections =
       file->template getELFShdrs<ELFT>();
@@ -1575,7 +1574,8 @@ template <class ELFT> void elf::scanRelocations() {
         scanner.template scanSection<ELFT>(*sec);
       if (part.armExidx && part.armExidx->isLive())
         for (InputSection *sec : part.armExidx->exidxSections)
-          scanner.template scanSection<ELFT>(*sec);
+          if (sec->isLive())
+            scanner.template scanSection<ELFT>(*sec);
     }
   });
 }

lld/ELF/Symbols.cpp

@@ -321,7 +321,7 @@ void elf::maybeWarnUnorderableSymbol(const Symbol *sym) {
   //
   // Note, ld.bfd --symbol-ordering-file= does not warn on undefined symbols,
   // but we don't have to be compatible here.
-  if (sym->isUndefined() &&
+  if (sym->isUndefined() && !cast<Undefined>(sym)->discardedSecIdx &&
       config->unresolvedSymbols == UnresolvedPolicy::Ignore)
     return;
 
@@ -330,9 +330,12 @@ void elf::maybeWarnUnorderableSymbol(const Symbol *sym) {
 
   auto report = [&](StringRef s) { warn(toString(file) + s + sym->getName()); };
 
-  if (sym->isUndefined())
-    report(": unable to order undefined symbol: ");
-  else if (sym->isShared())
+  if (sym->isUndefined()) {
+    if (cast<Undefined>(sym)->discardedSecIdx)
+      report(": unable to order discarded symbol: ");
+    else
+      report(": unable to order undefined symbol: ");
+  } else if (sym->isShared())
     report(": unable to order shared symbol: ");
   else if (d && !d->section)
     report(": unable to order absolute symbol: ");

lld/ELF/Writer.cpp

@@ -722,6 +722,27 @@ template <class ELFT> void Writer<ELFT>::copyLocalSymbols() {
       // No reason to keep local undefined symbol in symtab.
       if (!dr)
         continue;
+
+      // Demote locals which did not end up in any partition. This is similar
+      // to what we do in Driver.cpp, but that only works on globals.
+      if (script->seenDiscard && dr->section && !dr->section->isLive()) {
+        uint32_t secIdx = 0;
+        if (dr->file) {
+          uint32_t idx = 0;
+          for (SectionBase *s : dr->file->getSections()) {
+            if (s == dr->section) {
+              secIdx = idx;
+              break;
+            }
+            idx++;
+          }
+        }
+        Undefined(dr->file, b->getName(), b->binding, b->stOther, b->type,
+                  secIdx)
+            .overwrite(*b);
+        continue;
+      }
+
       if (includeInSymtab(*b) && shouldKeepInSymtab(*dr))
         in.symTab->addSymbol(b);
     }

lld/test/ELF/linkerscript/discard-section.s

@@ -4,9 +4,32 @@
 # RUN: rm -rf %t && split-file %s %t && cd %t
 # RUN: llvm-mc -filetype=obj -triple=x86_64 a.s -o a.o
 # RUN: llvm-mc -filetype=obj -triple=x86_64 b.s -o b.o
-# RUN: ld.lld -T a.lds a.o b.o -z undefs -o /dev/null 2>&1 | count 0
-# RUN: ld.lld -T a.lds a.o b.o -o /dev/null 2>&1 | count 0
-# RUN: ld.lld -r -T a.lds a.o b.o -o /dev/null 2>&1 | count 0
+# RUN: not ld.lld --threads=1 -T a.lds a.o b.o -z undefs -o /dev/null 2>&1 | FileCheck %s --check-prefix=SECTION --implicit-check-not=error:
+# RUN: not ld.lld --threads=1 -T a.lds a.o b.o -o /dev/null 2>&1 | FileCheck %s --check-prefixes=SECTION,SYMBOL --implicit-check-not=error:
+# RUN: ld.lld -r -T a.lds a.o b.o -o /dev/null 2>&1 | FileCheck %s --check-prefix=WARNING --implicit-check-not=error:
+
+# SECTION: error: relocation refers to a discarded section: .aaa
+# SECTION-NEXT: >>> defined in a.o
+# SECTION-NEXT: >>> referenced by a.o:(.bbb+0x0)
+
+# SYMBOL: error: relocation refers to a symbol in a discarded section: global
+# SYMBOL-NEXT: >>> defined in a.o
+# SYMBOL-NEXT: >>> referenced by b.o:(.data+0x0)
+
+# SYMBOL: error: relocation refers to a symbol in a discarded section: weak
+# SYMBOL-NEXT: >>> defined in a.o
+# SYMBOL-NEXT: >>> referenced by b.o:(.data+0x8)
+
+# SYMBOL: error: relocation refers to a symbol in a discarded section: weakref1
+# SYMBOL-NEXT: >>> defined in a.o
+# SYMBOL-NEXT: >>> referenced by b.o:(.data+0x10)
+
+# SYMBOL: error: relocation refers to a symbol in a discarded section: weakref2
+# SYMBOL-NEXT: >>> defined in a.o
+# SYMBOL-NEXT: >>> referenced by b.o:(.data+0x18)
+
+# WARNING: warning: relocation refers to a discarded section: .aaa
+# WARNING-NEXT: >>> referenced by a.o:(.rela.bbb+0x0)
 
 #--- a.s
 .globl _start