[LLD] [COFF] Check the aux section definition size for IMAGE_COMDAT_S…

…ELECT_SAME_SIZE

Binutils generated sections seem to be padded to a multiple of 16 bytes,
but the aux section definition contains the original, unpadded section
length.

The size check used for IMAGE_COMDAT_SELECT_SAME_SIZE previously
only checked the size of the section itself. When checking the
currently processed object file against the previously chosen
comdat section, we easily have access to the aux section definition
of the currently processed section, but we have to iterate over the
symbols of the previously selected object file to find the section
definition of the previously picked section. (We don't want to
inflate SectionChunk to carry more data, for something that is only
needed in corner cases.) Only do this when the mingw flag is set.

This fixes statically linking clang-built C++ object files against
libstdc++ built with GCC, if the object files contain e.g. typeinfo.

Differential Revision: https://reviews.llvm.org/D86659

lld/COFF/InputFiles.cpp

@@ -472,8 +472,23 @@ Symbol *ObjFile::createUndefined(COFFSymbolRef sym) {
   return symtab->addUndefined(name, this, sym.isWeakExternal());
 }
 
-void ObjFile::handleComdatSelection(COFFSymbolRef sym, COMDATType &selection,
-                                    bool &prevailing, DefinedRegular *leader) {
+static const coff_aux_section_definition *findSectionDef(COFFObjectFile *obj,
+                                                         int32_t section) {
+  uint32_t numSymbols = obj->getNumberOfSymbols();
+  for (uint32_t i = 0; i < numSymbols; ++i) {
+    COFFSymbolRef sym = check(obj->getSymbol(i));
+    if (sym.getSectionNumber() != section)
+      continue;
+    if (const coff_aux_section_definition *def = sym.getSectionDefinition())
+      return def;
+  }
+  return nullptr;
+}
+
+void ObjFile::handleComdatSelection(
+    COFFSymbolRef sym, COMDATType &selection, bool &prevailing,
+    DefinedRegular *leader,
+    const llvm::object::coff_aux_section_definition *def) {
   if (prevailing)
     return;
   // There's already an existing comdat for this symbol: `Leader`.
@@ -540,8 +555,16 @@ void ObjFile::handleComdatSelection(COFFSymbolRef sym, COMDATType &selection,
     break;
 
   case IMAGE_COMDAT_SELECT_SAME_SIZE:
-    if (leaderChunk->getSize() != getSection(sym)->SizeOfRawData)
-      symtab->reportDuplicate(leader, this);
+    if (leaderChunk->getSize() != getSection(sym)->SizeOfRawData) {
+      if (!config->mingw) {
+        symtab->reportDuplicate(leader, this);
+      } else {
+        const coff_aux_section_definition *leaderDef = findSectionDef(
+            leaderChunk->file->getCOFFObj(), leaderChunk->getSectionNumber());
+        if (!leaderDef || leaderDef->Length != def->Length)
+          symtab->reportDuplicate(leader, this);
+      }
+    }
     break;
 
   case IMAGE_COMDAT_SELECT_EXACT_MATCH: {
@@ -657,7 +680,7 @@ Optional<Symbol *> ObjFile::createDefined(
     COMDATType selection = (COMDATType)def->Selection;
 
     if (leader->isCOMDAT)
-      handleComdatSelection(sym, selection, prevailing, leader);
+      handleComdatSelection(sym, selection, prevailing, leader, def);
 
     if (prevailing) {
       SectionChunk *c = readSection(sectionNumber, def, getName());

lld/COFF/InputFiles.h

@@ -255,9 +255,10 @@ class ObjFile : public InputFile {
   // match the existing symbol and its selection. If either old or new
   // symbol have selection IMAGE_COMDAT_SELECT_LARGEST, Sym might replace
   // the existing leader. In that case, Prevailing is set to true.
-  void handleComdatSelection(COFFSymbolRef sym,
-                             llvm::COFF::COMDATType &selection,
-                             bool &prevailing, DefinedRegular *leader);
+  void
+  handleComdatSelection(COFFSymbolRef sym, llvm::COFF::COMDATType &selection,
+                        bool &prevailing, DefinedRegular *leader,
+                        const llvm::object::coff_aux_section_definition *def);
 
   llvm::Optional<Symbol *>
   createDefined(COFFSymbolRef sym,

lld/test/COFF/Inputs/comdat-binutils.yaml

@@ -0,0 +1,30 @@
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [ IMAGE_FILE_RELOCS_STRIPPED, IMAGE_FILE_LINE_NUMS_STRIPPED ]
+sections:
+  - Name:            '.rdata$mysymbol'
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_LNK_COMDAT, IMAGE_SCN_MEM_READ ]
+    Alignment:       16
+    SectionData:     2A000000000000000000000000000000
+symbols:
+  - Name:            '.rdata$mysymbol'
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          1
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+      Selection:       IMAGE_COMDAT_SELECT_SAME_SIZE
+  - Name:            mysymbol
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...

lld/test/COFF/Inputs/comdat-llvm.yaml

@@ -0,0 +1,30 @@
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:
+  - Name:            '.rdata$mysymbol'
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_LNK_COMDAT, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    SectionData:     2A
+symbols:
+  - Name:            '.rdata$mysymbol'
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          1
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        3686517206
+      Number:          1
+      Selection:       IMAGE_COMDAT_SELECT_SAME_SIZE
+  - Name:            mysymbol
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...

lld/test/COFF/comdat-gcc-compatibility-size.test

@@ -0,0 +1,15 @@
+# RUN: yaml2obj %p/Inputs/comdat-llvm.yaml > %t.llvm.o
+# RUN: yaml2obj %p/Inputs/comdat-binutils.yaml > %t.binutils.o
+# RUN: lld-link -lldmingw -noentry -dll %t.llvm.o %t.binutils.o -out:%t.dll
+# RUN: lld-link -lldmingw -noentry -dll %t.binutils.o %t.llvm.o -out:%t.dll
+# RUN: not lld-link -noentry -dll %t.llvm.o %t.binutils.o -out:%t.dll
+# RUN: not lld-link -noentry -dll %t.binutils.o %t.llvm.o -out:%t.dll
+
+# The test object files have been generated by assembling the following
+# snippet using binutils and llvm.
+
+#        .section .rdata$mysymbol, "dr"
+#        .linkonce same_size
+#        .globl mysymbol
+#mysymbol:
+#        .byte 42