Index: llvm/trunk/test/tools/yaml2obj/explicit-dynsym-no-dynstr.yaml
===================================================================
--- llvm/trunk/test/tools/yaml2obj/explicit-dynsym-no-dynstr.yaml
+++ llvm/trunk/test/tools/yaml2obj/explicit-dynsym-no-dynstr.yaml
@@ -0,0 +1,22 @@
+## Check we do not crash/assert when .dynsym is specified
+## explicitly, but .dynstr is not present.
+
+# RUN: yaml2obj %s -o %t
+# RUN: not llvm-readelf --section-headers %t 2>&1 | FileCheck %s
+
+## TODO: Check that .dynsym has Link field set to 0.
+##       GNU readelf is able to dump sections headers,
+##       but llvm-readelf reports an error below too early.
+##       See https://bugs.llvm.org/show_bug.cgi?id=42215.
+
+# CHECK: error: invalid sh_type for string table, expected SHT_STRTAB
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_DYN
+  Machine: EM_X86_64
+Sections:
+  - Name: .dynsym
+    Type: SHT_SYMTAB
Index: llvm/trunk/tools/yaml2obj/yaml2elf.cpp
===================================================================
--- llvm/trunk/tools/yaml2obj/yaml2elf.cpp
+++ llvm/trunk/tools/yaml2obj/yaml2elf.cpp
@@ -367,7 +367,18 @@
   bool IsStatic = STType == SymtabType::Static;
   SHeader.sh_name = DotShStrtab.getOffset(IsStatic ? ".symtab" : ".dynsym");
   SHeader.sh_type = IsStatic ? ELF::SHT_SYMTAB : ELF::SHT_DYNSYM;
-  SHeader.sh_link = IsStatic ? SN2I.get(".strtab") : SN2I.get(".dynstr");
+
+  // When we describe the .dynsym section in the document explicitly, it is
+  // allowed to omit the "DynamicSymbols" tag. In this case .dynstr is not added
+  // implicitly and we should be able to leave the Link zeroed if .dynstr is not
+  // defined.
+  unsigned Link = 0;
+  if (IsStatic)
+    Link = SN2I.get(".strtab");
+  else
+    SN2I.lookup(".dynstr", Link);
+  SHeader.sh_link = Link;
+
   if (!IsStatic)
     SHeader.sh_flags |= ELF::SHF_ALLOC;