COFF: Use (name, output characteristics) as a key when grouping input sections into output sections.

[pcc]

Summary

This is what link.exe does and lets us avoid needing to worry about
merging output characteristics while adding input sections to output
sections.

With this change we can't process /merge in the same way as before
because sections with different output characteristics can still
be merged into one another. So this change moves the processing of
/merge to just before we assign addresses. In the case where there
are multiple output sections with the same name, link.exe only merges
the first section with the source name into the first section with
the target name, and we do the same.

This isn't quite enough though because link.exe has a special case for
.CRT in 32-bit mode: it processes sections whose output characteristics
are DATA | R | W as though the output characteristics were DATA | R
(so that they get merged into things like constructor lists in the
expected way). Chromium has a few such sections, and it turns out
that those sections were causing the problem that resulted in r318699
(merge .xdata into .rdata) being reverted: because of the previous
permission merging semantics, the .CRT sections were causing the entire
.rdata section to become writable, which caused the SEH runtime to
crash because it apparently requires .xdata to be read-only. This
change also implements the same special case.

This should unblock being able to merge .xdata into .rdata by default,
as well as .bss into .data, both of which will be done in followups.

Depends on D45799

[rnk]

because of the previous
permission merging semantics, the .CRT sections were causing the entire
.rdata section to become writable, which caused the SEH runtime to
crash because it apparently requires .xdata to be read-only. This
change also implements the same special case.

Wow. So, 32-bit Chromium linked with LLD had a writable .rdata. We definitely don't want to ship that. Thanks for investigating this!

lld/COFF/Writer.cpp

720

Do we need to remove From somehow? What are the semantics of /merge:.foo=.bar /merge:.baz=.foo? Move all input sections in .foo to .bar, and then move all input sections from .baz to .foo? Or, everything in .bar?

[ruiu]

lld/COFF/Writer.cpp

1138–1141

Does this mean that we now have multiple sections that have the same name? Is it OK?

[rnk]

@zturner: We need to add logic to this section merging code to make S_SECTION / S_COFFGROUP records.

[zturner]

In D45801#1072314, @rnk wrote:

@zturner: We need to add logic to this section merging code to make S_SECTION / S_COFFGROUP records.

We already emit the S_SECTION symbols (see the function addLinkerModuleSectionSymbol), but you're right, we don't emit the S_COFFGROUP symbols yet.

[pcc]

In D45801#1072340, @zturner wrote:

In D45801#1072314, @rnk wrote:

@zturner: We need to add logic to this section merging code to make S_SECTION / S_COFFGROUP records.

We already emit the S_SECTION symbols (see the function addLinkerModuleSectionSymbol), but you're right, we don't emit the S_COFFGROUP symbols yet.

What is an S_COFFGROUP? From looking at our test cases it appears to be a group of input sections with the same name (including the $ suffix), right? Can't we infer them in the PDB writer by looking at input section names?

lld/COFF/Writer.cpp

720

We don't need to remove From because it will be taken care of by removeEmptySections.

The semantics for your example are that everything gets moved to .bar, but before this change we were just taking one step (so .baz is renamed to .foo and the input .foo sections are merged into .bar). I didn't intend to change this as part of this change, but it turns out that we happen to do the right thing in your example because .baz is alphabetically less than .foo. Probably the best thing to do is to implement multi-step merging in this change as well.

1138–1141

It seems to be fine, it's what link.exe does and the Windows loader doesn't have a problem with it.

[pcc]

• Implement multi-step merging

[smeenai]

lld/COFF/Writer.cpp

458

If I'm reading this correctly link.exe only has this special case in 32-bit mode, but we're doing it for all architectures?

Also, when you say 32-bit, do you mean x86 specifically, or 32-bit ARM as well?

[pcc]

lld/COFF/Writer.cpp

458

I can only reproduce it with IMAGE_FILE_MACHINE_I386. Do you think it's worth limiting based on machine?

[pcc]

• Fix cycle detection and add a test

[smeenai]

lld/COFF/Writer.cpp

458

I don't think it's worth limiting based on machine, but it's worth mentioning in the comment that LLD is just doing the special-case for all architectures. I spent a bit of time hunting around for if this was somehow limited to 32-bit from an enclosing scope, for example (since the comment to me implied that we only wanted the special case on 32-bit as well).

[pcc]

• Clarify comment

[pcc]

• Clarify comment further

[pcc]

Any more comments on this?

[ruiu]

No I don't. LGTM

lld/COFF/Writer.h

 
 // OutputSection represents a section in an output file. It's a
 // container of chunks. OutputSection and Chunk are 1:N relationship.
 // Chunks cannot belong to more than one OutputSections. The writer
 // creates multiple OutputSections and assign them unique,
 // non-overlapping file offsets and RVAs.
 class OutputSection {
 public:
-  OutputSection(llvm::StringRef N) : Name(N), Header({}) {}
+  OutputSection(llvm::StringRef N, uint32_t Chars) : Name(N) {
+    Header.Characteristics = Chars;
+  }
   void addChunk(Chunk *C);
+  void merge(OutputSection *Other);
   ArrayRef<Chunk *> getChunks() { return Chunks; }
   void addPermissions(uint32_t C);
   void setPermissions(uint32_t C);
   uint64_t getRVA() { return Header.VirtualAddress; }
   uint64_t getFileOff() { return Header.PointerToRawData; }
   void writeHeaderTo(uint8_t *Buf);
 
   // Returns the size of this section in an executable memory image.
public:
   // Set offset into the string table storing this section name.
   // Used only when the name is longer than 8 bytes.
   void setStringTableOff(uint32_t V) { StringTableOff = V; }
 
   // N.B. The section index is one based.
   uint32_t SectionIndex = 0;
 
   llvm::StringRef Name;
-  llvm::object::coff_section Header;
+  llvm::object::coff_section Header = {};
 
 private:
   uint32_t StringTableOff = 0;
   std::vector<Chunk *> Chunks;
 };
 
 }
 }
 
 #endif

lld/COFF/Writer.cpp

public:
   Writer() : Buffer(errorHandler().OutputBuffer) {}
   void run();
 
 private:
   void createSections();
   void createMiscChunks();
   void createImportTables();
   void createExportTable();
+  void mergeSections();
   void assignAddresses();
   void removeEmptySections();
   void createSymbolAndStringTable();
   void openFile(StringRef OutputPath);
   template <typename PEHeaderTy> void writeHeader();
   void createSEHTable();
   void createGuardCFTables();
   void markSymbolsForRVATable(ObjFile *File,
private:
   uint64_t FileSize;
   uint32_t PointerToSymbolTable = 0;
   uint64_t SizeOfImage;
   uint64_t SizeOfHeaders;
 
   OutputSection *TextSec;
   OutputSection *RdataSec;
   OutputSection *DataSec;
+  OutputSection *PdataSec;
   OutputSection *IdataSec;
   OutputSection *EdataSec;
   OutputSection *DidatSec;
   OutputSection *RsrcSec;
   OutputSection *RelocSec;
 
   // The first and last .pdata sections in the output file.
   //

 
 void writeResult() { Writer().run(); }
 
 void OutputSection::addChunk(Chunk *C) {
   Chunks.push_back(C);
   C->setOutputSection(this);
 }
 
-void OutputSection::addPermissions(uint32_t C) {
-  Header.Characteristics |= C & PermMask;
-}
-
 void OutputSection::setPermissions(uint32_t C) {
   Header.Characteristics = C & PermMask;
 }
 
+void OutputSection::merge(OutputSection *Other) {
+  for (Chunk *C : Other->Chunks)
+    C->setOutputSection(this);
+  Chunks.insert(Chunks.end(), Other->Chunks.begin(), Other->Chunks.end());
+  Other->Chunks.clear();
+}
+
 // Write the section header to a given buffer.
 void OutputSection::writeHeaderTo(uint8_t *Buf) {
   auto *Hdr = reinterpret_cast<coff_section *>(Buf);
   *Hdr = Header;
   if (StringTableOff) {
     // If name is too long, write offset into the string table as a name.
     sprintf(Hdr->Name, "/%d", StringTableOff);
   } else {

 // The main function of the writer.
 void Writer::run() {
   ScopedTimer T1(CodeLayoutTimer);
 
   createSections();
   createMiscChunks();
   createImportTables();
   createExportTable();
+  mergeSections();
   assignAddresses();
   removeEmptySections();
   setSectionPermissions();
   createSymbolAndStringTable();
 
   if (FileSize > UINT32_MAX)
     fatal("image size (" + Twine(FileSize) + ") " +
         "exceeds maximum allowable size (" + Twine(UINT32_MAX) + ")");
void Writer::createSections() {
   const uint32_t DATA = IMAGE_SCN_CNT_INITIALIZED_DATA;
   const uint32_t BSS = IMAGE_SCN_CNT_UNINITIALIZED_DATA;
   const uint32_t CODE = IMAGE_SCN_CNT_CODE;
   const uint32_t DISCARDABLE = IMAGE_SCN_MEM_DISCARDABLE;
   const uint32_t R = IMAGE_SCN_MEM_READ;
   const uint32_t W = IMAGE_SCN_MEM_WRITE;
   const uint32_t X = IMAGE_SCN_MEM_EXECUTE;
 
-  SmallDenseMap<StringRef, OutputSection *> Sections;
-  auto CreateSection = [&](StringRef Name, uint32_t Perms) {
-    auto I = Config->Merge.find(Name);
-    if (I != Config->Merge.end())
-      Name = I->second;
-    OutputSection *&Sec = Sections[Name];
+  SmallDenseMap<std::pair<StringRef, uint32_t>, OutputSection *> Sections;
+  auto CreateSection = [&](StringRef Name, uint32_t OutChars) {
+    OutputSection *&Sec = Sections[{Name, OutChars}];
     if (!Sec) {
-      Sec = make<OutputSection>(Name);
+      Sec = make<OutputSection>(Name, OutChars);
       OutputSections.push_back(Sec);
     }
-    Sec->addPermissions(Perms);
     return Sec;
   };
 
   // Try to match the section order used by link.exe.
   TextSec = CreateSection(".text", CODE | R | X);
   CreateSection(".bss", BSS | R | W);
   RdataSec = CreateSection(".rdata", DATA | R);
   DataSec = CreateSection(".data", DATA | R | W);
-  CreateSection(".pdata", DATA | R);
+  PdataSec = CreateSection(".pdata", DATA | R);
   IdataSec = CreateSection(".idata", DATA | R);
   EdataSec = CreateSection(".edata", DATA | R);
   DidatSec = CreateSection(".didat", DATA | R);
   RsrcSec = CreateSection(".rsrc", DATA | R);
   RelocSec = CreateSection(".reloc", DATA | DISCARDABLE | R);
 
-  // Then bin chunks by name.
-  std::map<StringRef, std::vector<Chunk *>> Map;
+  // Then bin chunks by name and output characteristics.
+  std::map<std::pair<StringRef, uint32_t>, std::vector<Chunk *>> Map;
   for (Chunk *C : Symtab->getChunks()) {
     auto *SC = dyn_cast<SectionChunk>(C);
     if (SC && !SC->isLive()) {
       if (Config->Verbose)
         SC->printDiscardedMessage();
       continue;
     }
-    Map[C->getSectionName()].push_back(C);
+    Map[{C->getSectionName(), C->getOutputCharacteristics()}].push_back(C);
   }
 
   // Process an /order option.
   if (!Config->Order.empty())
     for (auto &Pair : Map)
       sortBySectionOrder(Pair.second);
 
   // Then create an OutputSection for each section.
   // '$' and all following characters in input section names are
   // discarded when determining output section. So, .text$foo
   // contributes to .text, for example. See PE/COFF spec 3.2.
   for (auto Pair : Map) {
-    StringRef Name = getOutputSectionName(Pair.first);
-    if (Name == ".pdata") {
-      if (!FirstPdata)
-        FirstPdata = Pair.second.front();
-      LastPdata = Pair.second.back();
-    }
-    OutputSection *Sec = CreateSection(Name, 0);
+    StringRef Name = getOutputSectionName(Pair.first.first);
+    uint32_t OutChars = Pair.first.second;
+
+    // In link.exe, there is a special case for the I386 target where .CRT
+    // sections are treated as if they have output characteristics DATA | R if
+    // their characteristics are DATA | R | W. This implements the same special
+    // case for all architectures.
+    if (Name == ".CRT")
+      OutChars = DATA | R;
+
+    OutputSection *Sec = CreateSection(Name, OutChars);
     std::vector<Chunk *> &Chunks = Pair.second;
-    for (Chunk *C : Chunks) {
+    for (Chunk *C : Chunks)
       Sec->addChunk(C);
-      Sec->addPermissions(C->getOutputCharacteristics());
-    }
   }
 
   // Finally, move some output sections to the end.
   auto SectionOrder = [&](OutputSection *S) {
     // .reloc should come last of all since it refers to RVAs of data in the
     // previous sections.
     if (S == RelocSec)
       return 3;
void Writer::createSymbolAndStringTable() {
   // We position the symbol table to be adjacent to the end of the last section.
   uint64_t FileOff = FileSize;
   PointerToSymbolTable = FileOff;
   FileOff += OutputSymtab.size() * sizeof(coff_symbol16);
   FileOff += 4 + Strtab.size();
   FileSize = alignTo(FileOff, SectorSize);
 }
 
+void Writer::mergeSections() {
+  if (!PdataSec->getChunks().empty()) {
+    FirstPdata = PdataSec->getChunks().front();
+    LastPdata = PdataSec->getChunks().back();
+  }
+
+  for (auto &P : Config->Merge) {
+    StringRef ToName = P.second;
+    if (P.first == ToName)
+      continue;
+    StringSet<> Names;
+    while (1) {
+      if (!Names.insert(ToName).second)
+        fatal("/merge: cycle found for section '" + P.first + "'");
+      auto I = Config->Merge.find(ToName);
+      if (I == Config->Merge.end())
+        break;
+      ToName = I->second;
+    }
+    OutputSection *From = findSection(P.first);
+    OutputSection *To = findSection(ToName);
+    if (!From)
+      continue;
+    if (!To) {
+      From->Name = ToName;
+      continue;
+    }
+    To->merge(From);
+  }
+}
+
 // Visits all sections to assign incremental, non-overlapping RVAs and
 // file offsets.
 void Writer::assignAddresses() {
   SizeOfHeaders = DOSStubSize + sizeof(PEMagic) + sizeof(coff_file_header) +
                   sizeof(data_directory) * NumberfOfDataDirectory +
                   sizeof(coff_section) * OutputSections.size();
   SizeOfHeaders +=
       Config->is64() ? sizeof(pe32plus_header) : sizeof(pe32_header);

 }
 
 // Handles /section options to allow users to overwrite
 // section attributes.
 void Writer::setSectionPermissions() {
   for (auto &P : Config->Section) {
     StringRef Name = P.first;
     uint32_t Perm = P.second;
-    if (auto *Sec = findSection(Name))
+    for (OutputSection *Sec : OutputSections)
+      if (Sec->Name == Name)
         Sec->setPermissions(Perm);
   }
 }
 
 // Write section contents to a mmap'ed file.
 void Writer::writeSections() {
   // Record the number of sections to apply section index relocations
   // against absolute symbols. See applySecIdx in Chunks.cpp..
   DefinedAbsolute::NumOutputSections = OutputSections.size();
 

lld/test/COFF/crt-chars.test

+# RUN: yaml2obj %s > %t.obj
+# RUN: lld-link /out:%t.dll /entry:__ImageBase /dll %t.obj
+# RUN: llvm-readobj -sections -section-data %t.dll | FileCheck %s
+
+# CHECK: Name: .CRT
+# CHECK: Characteristics [
+# CHECK-NEXT: IMAGE_SCN_CNT_INITIALIZED_DATA
+# CHECK-NEXT: IMAGE_SCN_MEM_READ
+# CHECK-NEXT: ]
+# CHECK-NEXT: SectionData (
+# CHECK-NEXT: 010203
+# CHECK-NEXT: )
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:
+  - Name:            .CRT$XCZ
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    SectionData:     03
+  - Name:            .CRT$XCU
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       1
+    SectionData:     02
+  - Name:            .CRT$XCA
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    SectionData:     01
+symbols:
+...

lld/test/COFF/merge.test

 # RUN: yaml2obj < %s > %t.obj
 # RUN: lld-link /out:%t.exe /entry:main /subsystem:console /force \
 # RUN:   /merge:.foo=.abc /merge:.bar=.def %t.obj /debug
 # RUN: llvm-readobj -sections %t.exe | FileCheck %s
 
+# RUN: lld-link /out:%t.exe /entry:main /subsystem:console /force \
+# RUN:   /merge:.foo=.bar /merge:.bar=.abc %t.obj /debug
+# RUN: llvm-readobj -sections %t.exe | FileCheck --check-prefix=CHECK2 %s
+
 # RUN: not lld-link /out:%t.exe /entry:main /subsystem:console /force \
 # RUN:   /merge:.rsrc=.foo %t.obj /debug 2>&1 | FileCheck --check-prefix=NO-RSRC %s
 # RUN: not lld-link /out:%t.exe /entry:main /subsystem:console /force \
 # RUN:   /merge:.foo=.rsrc %t.obj /debug 2>&1 | FileCheck --check-prefix=NO-RSRC %s
 # RUN: not lld-link /out:%t.exe /entry:main /subsystem:console /force \
 # RUN:   /merge:.reloc=.foo %t.obj /debug 2>&1 | FileCheck --check-prefix=NO-RELOC %s
 # RUN: not lld-link /out:%t.exe /entry:main /subsystem:console /force \
 # RUN:   /merge:.foo=.reloc %t.obj /debug 2>&1 | FileCheck --check-prefix=NO-RELOC %s
+# RUN: not lld-link /out:%t.exe /entry:main /subsystem:console /force \
+# RUN:   /merge:.foo=.foo1 /merge:.foo1=.foo %t.obj /debug 2>&1 | FileCheck --check-prefix=NO-CYCLE %s
+# RUN: not lld-link /out:%t.exe /entry:main /subsystem:console /force \
+# RUN:   /merge:.foo=.foo1 /merge:.foo1=.foo2 /merge:.foo2=.foo1 %t.obj /debug 2>&1 | FileCheck --check-prefix=NO-CYCLE %s
 
 # CHECK: Name: .def
 # CHECK: Name: .abc
 
+# CHECK2-NOT: Name: .bar
+# CHECK2: Name: .abc
+# CHECK2-NOT: Name: .bar
+
 # NO-RSRC: /merge: cannot merge '.rsrc' with any section
 # NO-RELOC: /merge: cannot merge '.reloc' with any section
 
+# NO-CYCLE: /merge: cycle found for section '.foo'
+
 --- !COFF
 header:
   Machine:         IMAGE_FILE_MACHINE_AMD64
   Characteristics: []
 sections:
   - Name:            .foo
     Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
     Alignment:       4

lld/test/COFF/output-chars.test

+# RUN: yaml2obj %s > %t.obj
+# RUN: lld-link /out:%t.dll /entry:__ImageBase /dll %t.obj
+# RUN: llvm-readobj -sections %t.dll | FileCheck %s
+# RUN: lld-link /out:%t.dll /entry:__ImageBase /dll %t.obj /section:.foo,rwe
+# RUN: llvm-readobj -sections %t.dll | FileCheck --check-prefix=SECTION %s
+# RUN: lld-link /out:%t.dll /entry:__ImageBase /dll %t.obj /merge:.foo=.bar
+# RUN: llvm-readobj -sections -section-data %t.dll | FileCheck --check-prefix=MERGE %s
+# RUN: lld-link /out:%t.dll /entry:__ImageBase /dll %t.obj /merge:.foo=.bar /section:.foo,rwe
+# RUN: llvm-readobj -sections %t.dll | FileCheck --check-prefix=MERGE-SECTION %s
+
+# CHECK: Name: .foo
+# CHECK: Characteristics [
+# CHECK-NEXT: IMAGE_SCN_CNT_INITIALIZED_DATA
+# CHECK-NEXT: IMAGE_SCN_MEM_READ
+# CHECK-NEXT: ]
+
+# CHECK: Name: .foo
+# CHECK: Characteristics [
+# CHECK-NEXT: IMAGE_SCN_CNT_INITIALIZED_DATA
+# CHECK-NEXT: IMAGE_SCN_MEM_READ
+# CHECK-NEXT: IMAGE_SCN_MEM_WRITE
+# CHECK-NEXT: ]
+
+# SECTION: Name: .foo
+# SECTION: Characteristics [
+# SECTION-NEXT: IMAGE_SCN_MEM_EXECUTE
+# SECTION-NEXT: IMAGE_SCN_MEM_READ
+# SECTION-NEXT: IMAGE_SCN_MEM_WRITE
+# SECTION-NEXT: ]
+
+# SECTION: Name: .foo
+# SECTION: Characteristics [
+# SECTION-NEXT: IMAGE_SCN_MEM_EXECUTE
+# SECTION-NEXT: IMAGE_SCN_MEM_READ
+# SECTION-NEXT: IMAGE_SCN_MEM_WRITE
+# SECTION-NEXT: ]
+
+# MERGE: Name: .bar
+# MERGE: Characteristics [
+# MERGE-NEXT: IMAGE_SCN_CNT_INITIALIZED_DATA
+# MERGE-NEXT: IMAGE_SCN_MEM_READ
+# MERGE-NEXT: ]
+# MERGE-NEXT: SectionData (
+# MERGE-NEXT: 0000: 0301
+
+# MERGE: Name: .bar
+# MERGE: Characteristics [
+# MERGE-NEXT: IMAGE_SCN_CNT_INITIALIZED_DATA
+# MERGE-NEXT: IMAGE_SCN_MEM_READ
+# MERGE-NEXT: IMAGE_SCN_MEM_WRITE
+# MERGE-NEXT: ]
+# MERGE-NEXT: SectionData (
+# MERGE-NEXT: 0000: 04
+
+# MERGE: Name: .foo
+# MERGE: Characteristics [
+# MERGE-NEXT: IMAGE_SCN_CNT_INITIALIZED_DATA
+# MERGE-NEXT: IMAGE_SCN_MEM_READ
+# MERGE-NEXT: IMAGE_SCN_MEM_WRITE
+# MERGE-NEXT: ]
+# MERGE-NEXT: SectionData (
+# MERGE-NEXT: 0000: 02
+
+# MERGE-SECTION: Name: .bar
+# MERGE-SECTION: Characteristics [
+# MERGE-SECTION-NEXT: IMAGE_SCN_CNT_INITIALIZED_DATA
+# MERGE-SECTION-NEXT: IMAGE_SCN_MEM_READ
+# MERGE-SECTION-NEXT: ]
+
+# MERGE-SECTION: Name: .bar
+# MERGE-SECTION: Characteristics [
+# MERGE-SECTION-NEXT: IMAGE_SCN_CNT_INITIALIZED_DATA
+# MERGE-SECTION-NEXT: IMAGE_SCN_MEM_READ
+# MERGE-SECTION-NEXT: IMAGE_SCN_MEM_WRITE
+# MERGE-SECTION-NEXT: ]
+
+# MERGE-SECTION: Name: .foo
+# MERGE-SECTION: Characteristics [
+# MERGE-SECTION-NEXT: IMAGE_SCN_MEM_EXECUTE
+# MERGE-SECTION-NEXT: IMAGE_SCN_MEM_READ
+# MERGE-SECTION-NEXT: IMAGE_SCN_MEM_WRITE
+# MERGE-SECTION-NEXT: ]
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:
+  - Name:            .foo
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    SectionData:     01
+  - Name:            .foo
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       1
+    SectionData:     02
+  - Name:            .bar
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    SectionData:     03
+  - Name:            .bar
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
+    Alignment:       1
+    SectionData:     04
+symbols:
+...

lld/test/COFF/unwind.test

 # RUN: yaml2obj < %s > %t.obj
 #
 # RUN: lld-link /out:%t.exe /entry:main %t.obj
 # RUN: llvm-readobj -file-headers %t.exe | FileCheck -check-prefix=HEADER %s
 # RUN: llvm-objdump -unwind-info %t.exe | FileCheck -check-prefix=UNWIND %s
 #
 # RUN: lld-link /merge:.pdata=.rdata /out:%t.exe /entry:main %t.obj
 # RUN: llvm-readobj -file-headers -sections %t.exe | FileCheck -check-prefix=HEADER-MERGE %s
 #
 # HEADER: ExceptionTableRVA: 0x3000
 #
 # FIXME: llvm-readobj currently does not understand files with .pdata merged
 # into .rdata. But we can at least check that the section headers look correct.
 #
-# HEADER-MERGE: ExceptionTableRVA: 0x2000
+# HEADER-MERGE: ExceptionTableRVA: 0x2004
 # HEADER-MERGE-NEXT: ExceptionTableSize: 0x30
 # HEADER-MERGE: Name: .rdata
 # HEADER-MERGE-NEXT: VirtualSize: 0x34
 # HEADER-MERGE-NEXT: VirtualAddress: 0x2000
 #
 # UNWIND: Function Table:
 # UNWIND:   Start Address: 0x1000
 # UNWIND:   End Address: 0x101b