[ELF] Demote symbols in /DISCARD/ discarded sections to Undefined (#69295)

When an input section is matched by /DISCARD/ in a linker script, GNU ld
reports errors for relocations referencing symbols defined in the section:

    `.aaa' referenced in section `.bbb' of a.o: defined in discarded section `.aaa' of a.o

Implement the error by demoting eligible symbols to `Undefined` and changing
STB_WEAK to STB_GLOBAL. As a side benefit, in relocatable links, relocations
referencing symbols defined relative to /DISCARD/ discarded sections no longer
set symbol/type to zeros.

It's arguable whether a weak reference to a discarded symbol should lead to
errors. GNU ld reports an error and our demoting approach reports an error as
well.

Close #58891

Co-authored-by: Bevin Hansson <[email protected]>

Author: MaskRay

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/MapFile.cpp

@@ -229,7 +229,7 @@ static void writeCref(raw_fd_ostream &os) {
       if (isa<SharedSymbol>(sym))
         map[sym].insert(file);
       if (auto *d = dyn_cast<Defined>(sym))
-        if (!d->isLocal() && (!d->section || d->section->isLive()))
+        if (!d->isLocal())
           map[d].insert(file);
     }
   }

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

@@ -316,12 +316,13 @@ void elf::maybeWarnUnorderableSymbol(const Symbol *sym) {
   if (!config->warnSymbolOrdering)
     return;
 
-  // If UnresolvedPolicy::Ignore is used, no "undefined symbol" error/warning
-  // is emitted. It makes sense to not warn on undefined symbols.
+  // If UnresolvedPolicy::Ignore is used, no "undefined symbol" error/warning is
+  // emitted. It makes sense to not warn on undefined symbols (excluding those
+  // demoted by demoteSymbols).
   //
   // 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 +331,12 @@ 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

@@ -251,26 +251,59 @@ void elf::addReservedSymbols() {
   ElfSym::edata2 = add("_edata", -1);
 }
 
+static void demoteDefined(Defined &sym, DenseMap<SectionBase *, size_t> &map) {
+  if (map.empty())
+    for (auto [i, sec] : llvm::enumerate(sym.file->getSections()))
+      map.try_emplace(sec, i);
+  // Change WEAK to GLOBAL so that if a scanned relocation references sym,
+  // maybeReportUndefined will report an error.
+  uint8_t binding = sym.isWeak() ? uint8_t(STB_GLOBAL) : sym.binding;
+  Undefined(sym.file, sym.getName(), binding, sym.stOther, sym.type,
+            /*discardedSecIdx=*/map.lookup(sym.section))
+      .overwrite(sym);
+}
+
 // If all references to a DSO happen to be weak, the DSO is not added to
 // DT_NEEDED. If that happens, replace ShardSymbol with Undefined to avoid
 // dangling references to an unneeded DSO. Use a weak binding to avoid
 // --no-allow-shlib-undefined diagnostics. Similarly, demote lazy symbols.
+//
+// In addition, demote symbols defined in discarded sections, so that
+// references to /DISCARD/ discarded symbols will lead to errors.
 static void demoteSymbolsAndComputeIsPreemptible() {
   llvm::TimeTraceScope timeScope("Demote symbols");
+  DenseMap<InputFile *, DenseMap<SectionBase *, size_t>> sectionIndexMap;
   for (Symbol *sym : symtab.getSymbols()) {
-    auto *s = dyn_cast<SharedSymbol>(sym);
-    if (sym->isLazy() || (s && !cast<SharedFile>(s->file)->isNeeded)) {
-      uint8_t binding = sym->isLazy() ? sym->binding : uint8_t(STB_WEAK);
-      Undefined(nullptr, sym->getName(), binding, sym->stOther, sym->type)
-          .overwrite(*sym);
-      sym->versionId = VER_NDX_GLOBAL;
+    if (auto *d = dyn_cast<Defined>(sym)) {
+      if (d->section && !d->section->isLive())
+        demoteDefined(*d, sectionIndexMap[d->file]);
+    } else {
+      auto *s = dyn_cast<SharedSymbol>(sym);
+      if (sym->isLazy() || (s && !cast<SharedFile>(s->file)->isNeeded)) {
+        uint8_t binding = sym->isLazy() ? sym->binding : uint8_t(STB_WEAK);
+        Undefined(nullptr, sym->getName(), binding, sym->stOther, sym->type)
+            .overwrite(*sym);
+        sym->versionId = VER_NDX_GLOBAL;
+      }
     }
 
     if (config->hasDynSymTab)
       sym->isPreemptible = computeIsPreemptible(*sym);
   }
 }
 
+static void demoteLocalSymbolsInDiscardedSections() {
+  llvm::TimeTraceScope timeScope("Demote local symbols");
+  parallelForEach(ctx.objectFiles, [&](ELFFileBase *file) {
+    DenseMap<SectionBase *, size_t> sectionIndexMap;
+    for (Symbol *sym : file->getLocalSymbols()) {
+      Defined *d = dyn_cast<Defined>(sym);
+      if (d && d->section && !d->section->isLive())
+        demoteDefined(*d, sectionIndexMap);
+    }
+  });
+}
+
 // Fully static executables don't support MTE globals at this point in time, as
 // we currently rely on:
 //   - A dynamic loader to process relocations, and
@@ -1958,6 +1991,12 @@ template <class ELFT> void Writer<ELFT>::finalizeSections() {
   }
 
   demoteSymbolsAndComputeIsPreemptible();
+  // Also demote local symbols defined relative to discarded input sections so
+  // that relocations referencing them will lead to errors. To avoid unneeded
+  // work, we only do this when /DISCARD/ is seen, but this demotation also
+  // applies to --gc-sections discarded sections.
+  if (script->seenDiscard)
+    demoteLocalSymbolsInDiscardedSections();
 
   // Change values of linker-script-defined symbols from placeholders (assigned
   // by declareSymbols) to actual definitions.

lld/test/ELF/gc-sections-tls.s

@@ -7,6 +7,11 @@
 
 # ERR: error: {{.*}}.o has an STT_TLS symbol but doesn't have an SHF_TLS section
 
+## TODO As a corner case, when /DISCARD/ is present, demoteLocalSymbolsInDiscardedSections
+## demotes tls and the error is not triggered.
+# RUN: echo 'SECTIONS { /DISCARD/ : {} }' > %t.lds
+# RUN: ld.lld %t.o --gc-sections -T %t.lds -o /dev/null
+
 ## If we happen to have a PT_TLS, we will resolve the relocation to
 ## an arbitrary value (current implementation uses a negative value).
 # RUN: echo '.section .tbss,"awT"; .globl root; root: .long 0' | \
@@ -17,6 +22,9 @@
 # CHECK:      Hex dump of section '.noalloc':
 # CHECK-NEXT: 0x00000000 {{[0-9a-f]+}} ffffffff
 
+.globl _start
+_start:
+
 .section .tbss,"awT",@nobits
 tls:
   .long 0

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

@@ -4,28 +4,55 @@
 # 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 a.ro 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=LOCAL --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=LOCAL,NONLOCAL --implicit-check-not=error:
+# RUN: ld.lld -r -T a.lds a.o b.o -o a.ro 2>&1 | FileCheck %s --check-prefix=WARNING --implicit-check-not=warning:
 # RUN: llvm-readelf -r -s a.ro | FileCheck %s --check-prefix=RELOC
 
+# LOCAL:      error: relocation refers to a discarded section: .aaa
+# LOCAL-NEXT: >>> defined in a.o
+# LOCAL-NEXT: >>> referenced by a.o:(.bbb+0x0)
+
+# NONLOCAL:      error: relocation refers to a symbol in a discarded section: global
+# NONLOCAL-NEXT: >>> defined in a.o
+# NONLOCAL-NEXT: >>> referenced by b.o:(.data+0x0)
+
+# NONLOCAL:      error: relocation refers to a symbol in a discarded section: weak
+# NONLOCAL-NEXT: >>> defined in a.o
+# NONLOCAL-NEXT: >>> referenced by b.o:(.data+0x8)
+
+# NONLOCAL:      error: relocation refers to a symbol in a discarded section: weakref1
+# NONLOCAL-NEXT: >>> defined in a.o
+# NONLOCAL-NEXT: >>> referenced by b.o:(.data+0x10)
+
+# NONLOCAL:      error: relocation refers to a symbol in a discarded section: weakref2
+# NONLOCAL-NEXT: >>> defined in a.o
+# NONLOCAL-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)
+
 # RELOC:      Relocation section '.rela.bbb' at offset {{.*}} contains 1 entries:
 # RELOC-NEXT:     Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
 # RELOC-NEXT: 0000000000000000  0000000000000000 R_X86_64_NONE                             0
 # RELOC-EMPTY:
 # RELOC-NEXT: Relocation section '.rela.data' at offset {{.*}} contains 4 entries:
 # RELOC-NEXT:     Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
-# RELOC-NEXT: 0000000000000000  0000000000000001 R_X86_64_64                               0
-# RELOC-NEXT: 0000000000000008  0000000000000001 R_X86_64_64                               0
-# RELOC-NEXT: 0000000000000010  0000000000000001 R_X86_64_64                               0
-# RELOC-NEXT: 0000000000000018  0000000000000001 R_X86_64_64                               0
+# RELOC-NEXT: 0000000000000000  0000000500000001 R_X86_64_64            0000000000000000 global + 0
+# RELOC-NEXT: 0000000000000008  0000000700000001 R_X86_64_64            0000000000000000 weak + 0
+# RELOC-NEXT: 0000000000000010  0000000600000001 R_X86_64_64            0000000000000000 weakref1 + 0
+# RELOC-NEXT: 0000000000000018  0000000800000001 R_X86_64_64            0000000000000000 weakref2 + 0
 
 # RELOC:      Num:    Value          Size Type    Bind   Vis      Ndx Name
 # RELOC-NEXT:   0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
 # RELOC-NEXT:   1: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 .text
 # RELOC-NEXT:   2: 0000000000000000     0 SECTION LOCAL  DEFAULT    2 .bbb
 # RELOC-NEXT:   3: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 .data
 # RELOC-NEXT:   4: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT    1 _start
+# RELOC-NEXT:   5: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT   UND global
+# RELOC-NEXT:   6: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT   UND weakref1
+# RELOC-NEXT:   7: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT   UND weak
+# RELOC-NEXT:   8: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT   UND weakref2
 # RELOC-EMPTY:
 
 #--- a.s