Index: llvm/lib/ObjectYAML/ELFEmitter.cpp
===================================================================
--- llvm/lib/ObjectYAML/ELFEmitter.cpp
+++ llvm/lib/ObjectYAML/ELFEmitter.cpp
@@ -221,6 +221,8 @@
 
   void assignSectionAddress(Elf_Shdr &SHeader, ELFYAML::Section *YAMLSec);
 
+  BumpPtrAllocator StringAlloc;
+
 public:
   static bool writeELF(raw_ostream &OS, ELFYAML::Object &Doc,
                        yaml::ErrorHandler EH);
@@ -241,11 +243,6 @@
 ELFState<ELFT>::ELFState(ELFYAML::Object &D, yaml::ErrorHandler EH)
     : Doc(D), ErrHandler(EH) {
   std::vector<ELFYAML::Section *> Sections = Doc.getSections();
-  StringSet<> DocSections;
-  for (const ELFYAML::Section *Sec : Sections)
-    if (!Sec->Name.empty())
-      DocSections.insert(Sec->Name);
-
   // Insert SHT_NULL section implicitly when it is not defined in YAML.
   if (Sections.empty() || Sections.front()->Type != ELF::SHT_NULL)
     Doc.Chunks.insert(
@@ -253,6 +250,19 @@
         std::make_unique<ELFYAML::Section>(
             ELFYAML::Chunk::ChunkKind::RawContent, /*IsImplicit=*/true));
 
+  // We add a technical prefix for each unnamed section/fill. It does not affect
+  // the output, but allows us to map them by name in the code and
+  // report better error messages.
+  StringSet<> DocSections;
+  StringRef SuffixPrefix = " [unnamed, index #";
+  for (size_t I = 0; I < Doc.Chunks.size(); ++I) {
+    const std::unique_ptr<ELFYAML::Chunk> &C = Doc.Chunks[I];
+    if (C->Name.empty())
+      C->Name = StringRef(Twine(SuffixPrefix + Twine(I) + "]").str())
+                    .copy(StringAlloc);
+    DocSections.insert(C->Name);
+  }
+
   std::vector<StringRef> ImplicitSections;
   if (Doc.DynamicSymbols)
     ImplicitSections.insert(ImplicitSections.end(), {".dynsym", ".dynstr"});
@@ -415,7 +425,6 @@
   // valid SHN_UNDEF entry since SHT_NULL == 0.
   SHeaders.resize(Doc.getSections().size());
 
-  size_t SecNdx = -1;
   for (const std::unique_ptr<ELFYAML::Chunk> &D : Doc.Chunks) {
     if (auto S = dyn_cast<ELFYAML::Fill>(D.get())) {
       writeFill(*S, CBA);
@@ -423,15 +432,16 @@
       continue;
     }
 
-    ++SecNdx;
     ELFYAML::Section *Sec = cast<ELFYAML::Section>(D.get());
-    if (SecNdx == 0 && Sec->IsImplicit)
+    bool IsSpecialUndefSec = D == Doc.Chunks.front();
+    if (IsSpecialUndefSec && Sec->IsImplicit)
       continue;
 
     // We have a few sections like string or symbol tables that are usually
     // added implicitly to the end. However, if they are explicitly specified
     // in the YAML, we need to write them here. This ensures the file offset
     // remains correct.
+    size_t SecNdx = IsSpecialUndefSec ? 0 : SN2I.get(Sec->Name);
     Elf_Shdr &SHeader = SHeaders[SecNdx];
     if (initImplicitHeader(CBA, SHeader, Sec->Name,
                            Sec->IsImplicit ? nullptr : Sec))
@@ -452,7 +462,7 @@
     if (!Sec->Link.empty())
       SHeader.sh_link = toSectionIndex(Sec->Link, Sec->Name);
 
-    if (SecNdx == 0) {
+    if (IsSpecialUndefSec) {
       if (auto RawSec = dyn_cast<ELFYAML::RawContentSection>(Sec)) {
         // We do not write any content for special SHN_UNDEF section.
         if (RawSec->Size)
@@ -1412,9 +1422,6 @@
     if (IsSection)
       ++SecNdx;
 
-    if (C->Name.empty())
-      continue;
-
     if (!Seen.insert(C->Name).second)
       reportError("repeated section/fill name: '" + C->Name +
                   "' at YAML section/fill number " + Twine(I));
Index: llvm/test/tools/yaml2obj/ELF/custom-null-section.yaml
===================================================================
--- llvm/test/tools/yaml2obj/ELF/custom-null-section.yaml
+++ llvm/test/tools/yaml2obj/ELF/custom-null-section.yaml
@@ -128,7 +128,8 @@
 
 # RUN: not yaml2obj --docnum=6 %s -o %t6 2>&1 | FileCheck %s --check-prefix=CASE4
 
-# CASE4: error: unknown section referenced: '.foo' by YAML section ''
+# CASE4:      error: unknown section referenced: '.foo' by YAML section ' [unnamed, index #0]'
+# CASE4-NEXT: error: unknown section referenced: '.bar' by YAML section ' [unnamed, index #1]'
 
 --- !ELF
 FileHeader:
@@ -139,6 +140,8 @@
 Sections:
   - Type: SHT_NULL
     Link: .foo
+  - Type: SHT_NULL
+    Link: .bar
 
 ## Check that null section fields are set to zero, if they are unspecified.
 
Index: llvm/test/tools/yaml2obj/ELF/section-link.yaml
===================================================================
--- llvm/test/tools/yaml2obj/ELF/section-link.yaml
+++ llvm/test/tools/yaml2obj/ELF/section-link.yaml
@@ -28,8 +28,10 @@
 
 # RUN: not yaml2obj --docnum=2 %s 2>&1 | FileCheck %s --check-prefix=ERR
 
-# ERR: error: unknown section referenced: '.unknown1' by YAML section '.foo'
-# ERR: error: unknown section referenced: '.unknown2' by YAML section '.bar'
+# ERR:      error: unknown section referenced: '.unknown1' by YAML section '.foo'
+# ERR-NEXT: error: unknown section referenced: '.unknown2' by YAML section ' [unnamed, index #2]'
+# ERR-NEXT: error: unknown section referenced: '.unknown3' by YAML section '.bar'
+# ERR-NEXT: error: unknown section referenced: '.unknown4' by YAML section ' [unnamed, index #4]'
 
 --- !ELF
 FileHeader:
@@ -41,9 +43,13 @@
   - Name: .foo
     Type: SHT_PROGBITS
     Link: .unknown1
+  - Type: SHT_PROGBITS
+    Link: .unknown2
   - Name: .bar
     Type: SHT_PROGBITS
-    Link: .unknown2
+    Link: .unknown3
+  - Type: SHT_PROGBITS
+    Link: .unknown4
 
 ## Check we link SHT_GROUP to a symbol table by default if it exists.
 ## Also, check we can set an arbitrary value for sh_link.