[LLD] [COFF] Fix linking MSVC generated implib header objects

ecb5ea6 tried to fix cases
when LLD links what seems to be import library header objects
from MSVC. However, the fix seems incorrect; the review at
https://reviews.llvm.org/D133627 concluded that if this (treating
this kind of symbol as a common symbol) is what link.exe does,
it's fine.

However, this is most probably not what link.exe does. The
symbol mentioned in the commit message of
ecb5ea6 would be a common symbol
with a size of around 3 GB; this is not what might have been
intended.

That commit tried to avoid running into the error ".idata$4 should
not refer to special section 0"; that issue is fixed for a similar
style of section symbols in
4a4a8a1.

Therefore, revert ecb5ea6 and
extend the fix from 4a4a8a1 to
also work for the section symbols in MSVC generated import libraries.

The main detail about them, is that for symbols of type
IMAGE_SYM_CLASS_SECTION, the Value field is not an offset, but
it is an optional set of flags, corresponding to the Characteristics
of the section header (although it may be empty).

Author: mstorsjo

lld/COFF/InputFiles.cpp

@@ -458,9 +458,16 @@ Symbol *ObjFile::createRegular(COFFSymbolRef sym) {
       return nullptr;
     return symtab.addUndefined(name, this, false);
   }
-  if (sc)
+  if (sc) {
+    const coff_symbol_generic *symGen = sym.getGeneric();
+    if (sym.isSection()) {
+      auto *customSymGen = make<coff_symbol_generic>(*symGen);
+      customSymGen->Value = 0;
+      symGen = customSymGen;
+    }
     return make<DefinedRegular>(this, /*Name*/ "", /*IsCOMDAT*/ false,
-                                /*IsExternal*/ false, sym.getGeneric(), sc);
+                                /*IsExternal*/ false, symGen, sc);
+  }
   return nullptr;
 }
 
@@ -755,15 +762,23 @@ std::optional<Symbol *> ObjFile::createDefined(
     memset(hdr, 0, sizeof(*hdr));
     strncpy(hdr->Name, name.data(),
             std::min(name.size(), (size_t)COFF::NameSize));
-    // We have no idea what characteristics should be assumed here; pick
-    // a default. This matches what is used for .idata sections in the regular
-    // object files in import libraries.
-    hdr->Characteristics = IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_MEM_READ |
-                           IMAGE_SCN_MEM_WRITE | IMAGE_SCN_ALIGN_4BYTES;
+    // The Value field in a section symbol may contain the characteristics,
+    // or it may be zero, where we make something up (that matches what is
+    // used in .idata sections in the regular object files in import libraries).
+    if (sym.getValue())
+      hdr->Characteristics = sym.getValue() | IMAGE_SCN_ALIGN_4BYTES;
+    else
+      hdr->Characteristics = IMAGE_SCN_CNT_INITIALIZED_DATA |
+                             IMAGE_SCN_MEM_READ | IMAGE_SCN_MEM_WRITE |
+                             IMAGE_SCN_ALIGN_4BYTES;
     auto *sc = make<SectionChunk>(this, hdr);
     chunks.push_back(sc);
+
+    coff_symbol_generic *symGen = make<coff_symbol_generic>(*sym.getGeneric());
+    // Ignore the Value offset of these symbols, as it may be a bitmask.
+    symGen->Value = 0;
     return make<DefinedRegular>(this, /*name=*/"", /*isCOMDAT=*/false,
-                                /*isExternal=*/false, sym.getGeneric(), sc);
+                                /*isExternal=*/false, symGen, sc);
   }
 
   if (llvm::COFF::isReservedSectionNumber(sectionNumber))

lld/test/COFF/empty-section-decl.yaml

@@ -6,7 +6,7 @@
 # RUN: FileCheck %s --check-prefix=MAP < %t.map
 
 # CHECK:      Contents of section .itest:
-# CHECK-NEXT:  180001000 0c100080 01000000 00000000 01000000
+# CHECK-NEXT:  180001000 0c100000 0c100000 00000000 01000000
 
 # MAP: 00001000 0000000a     4         {{.*}}:(.itest$2)
 # MAP: 00001000 00000000     0                 .itest$2
@@ -28,7 +28,10 @@ sections:
     Relocations:
       - VirtualAddress:  0
         SymbolName:      '.itest$4'
-        Type:            IMAGE_REL_AMD64_ADDR64
+        Type:            IMAGE_REL_AMD64_ADDR32NB
+      - VirtualAddress:  4
+        SymbolName:      '.itest$6'
+        Type:            IMAGE_REL_AMD64_ADDR32NB
   - Name:            '.itest$6'
     Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
     Alignment:       2
@@ -42,13 +45,13 @@ symbols:
     ComplexType:     IMAGE_SYM_DTYPE_NULL
     StorageClass:    IMAGE_SYM_CLASS_SECTION
   - Name:            '.itest$6'
-    Value:           0
+    Value:           3221225536
     SectionNumber:   2
     SimpleType:      IMAGE_SYM_TYPE_NULL
     ComplexType:     IMAGE_SYM_DTYPE_NULL
-    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    StorageClass:    IMAGE_SYM_CLASS_SECTION
   - Name:            '.itest$4'
-    Value:           0
+    Value:           3221225536
     SectionNumber:   0
     SimpleType:      IMAGE_SYM_TYPE_NULL
     ComplexType:     IMAGE_SYM_DTYPE_NULL

llvm/include/llvm/Object/COFF.h

@@ -383,8 +383,8 @@ class COFFSymbolRef {
   }
 
   bool isCommon() const {
-    return (isExternal() || isSection()) &&
-           getSectionNumber() == COFF::IMAGE_SYM_UNDEFINED && getValue() != 0;
+    return isExternal() && getSectionNumber() == COFF::IMAGE_SYM_UNDEFINED &&
+           getValue() != 0;
   }
 
   bool isUndefined() const {
@@ -393,8 +393,7 @@ class COFFSymbolRef {
   }
 
   bool isEmptySectionDeclaration() const {
-    return isSection() && getSectionNumber() == COFF::IMAGE_SYM_UNDEFINED &&
-           getValue() == 0;
+    return isSection() && getSectionNumber() == COFF::IMAGE_SYM_UNDEFINED;
   }
 
   bool isWeakExternal() const {