[yaml2obj] - Allow overriding e_shentsize, e_shoff, e_shnum and e_shstrndx fields in the YAML.

[grimar]

Summary

This allows setting different values for e_shentsize, e_shoff, e_shnum
and e_shstrndx fields and is useful for producing broken inputs for various
test cases.

[MaskRay]

test/tools/yaml2obj/elf-header-sh-fields.yaml

2

that we can

tools/yaml2obj/yaml2elf.cpp

212

(uint16_t)(*Doc.Header.SHEntSize)

I think one of the pairs of () can be deleted..

[grimar]

• Addressed review comments.

[jhenderson]

test/tools/yaml2obj/elf-header-sh-fields.yaml

5

Get rid of "At"

10

For consistency with the other test cases, move this YAML after the DEFAULT checks.

44–45

FWIW, I think this is a bug in llvm-readelf. I reckon it should be able to dump the file header regardless of the values, unless it actually has to reference something else (e.g. for many sections).

48

Nit: it would be easier for me to read if -j0x3a has a space in it, i.e. -j 0x3a.

[grimar]

• Addressed review comments.

test/tools/yaml2obj/elf-header-sh-fields.yaml

44–45

Yes, probably.
One of problems is that even before executing the readelf's main logic,
llvm::Object::ELFObjectFile<ELFT> tries to scan the sections really early during creation of the object from buffer: https://github.com/llvm-mirror/llvm/blob/master/include/llvm/Object/ELFObjectFile.h#L939
It is not obvious for me why that is done in that way and it that place and not later. I'll try to investingate it.

[grimar]

Are you happy with the latest version, James?

[jhenderson]

Yes, LGTM.

include/llvm/ObjectYAML/ELFYAML.h

struct FileHeader {
   ELF_ELFCLASS Class;
   ELF_ELFDATA Data;
   ELF_ELFOSABI OSABI;
   llvm::yaml::Hex8 ABIVersion;
   ELF_ET Type;
   ELF_EM Machine;
   ELF_EF Flags;
   llvm::yaml::Hex64 Entry;
+
+  Optional<llvm::yaml::Hex16> SHEntSize;
+  Optional<llvm::yaml::Hex16> SHOffset;
+  Optional<llvm::yaml::Hex16> SHNum;
+  Optional<llvm::yaml::Hex16> SHStrNdx;
 };
 
 struct SectionName {
   StringRef Section;
 };
 
 struct ProgramHeader {
   ELF_PT Type;

lib/ObjectYAML/ELFYAML.cpp

void MappingTraits<ELFYAML::FileHeader>::mapping(IO &IO,
   IO.mapRequired("Class", FileHdr.Class);
   IO.mapRequired("Data", FileHdr.Data);
   IO.mapOptional("OSABI", FileHdr.OSABI, ELFYAML::ELF_ELFOSABI(0));
   IO.mapOptional("ABIVersion", FileHdr.ABIVersion, Hex8(0));
   IO.mapRequired("Type", FileHdr.Type);
   IO.mapRequired("Machine", FileHdr.Machine);
   IO.mapOptional("Flags", FileHdr.Flags, ELFYAML::ELF_EF(0));
   IO.mapOptional("Entry", FileHdr.Entry, Hex64(0));
+
+  IO.mapOptional("SHEntSize", FileHdr.SHEntSize);
+  IO.mapOptional("SHOffset", FileHdr.SHOffset);
+  IO.mapOptional("SHNum", FileHdr.SHNum);
+  IO.mapOptional("SHStrNdx", FileHdr.SHStrNdx);
 }
 
 void MappingTraits<ELFYAML::ProgramHeader>::mapping(
     IO &IO, ELFYAML::ProgramHeader &Phdr) {
   IO.mapRequired("Type", Phdr.Type);
   IO.mapOptional("Flags", Phdr.Flags, ELFYAML::ELF_PF(0));
   IO.mapOptional("Sections", Phdr.Sections);
   IO.mapOptional("VAddr", Phdr.VAddr, Hex64(0));

test/tools/yaml2obj/elf-header-sh-fields.yaml

+## In this test case we check that we can override the default values for
+## e_shentsize, e_shoff, e_shnum and e_shstrndx fields in the YAML.
+
+## First we check the default values.
+
+# RUN: yaml2obj --docnum=1 %s -o %t1
+# RUN: llvm-readelf --file-headers %t1 | FileCheck %s --check-prefix=DEFAULT
+
+# DEFAULT:   Start of section headers:          64 (bytes into file)
+# DEFAULT:   Size of section headers:           64 (bytes)
+# DEFAULT:   Number of section headers:         4
+# DEFAULT:   Section header string table index: 3
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_X86_64
+
+## Override 3 fields: e_shoff, e_shnum and e_shstrndx. Check the output.
+
+# RUN: yaml2obj --docnum=2 %s -o %t2
+# RUN: llvm-readelf --file-headers %t2 | FileCheck %s --check-prefix=CUSTOM
+
+# CUSTOM: Start of section headers:          2 (bytes into file)
+# CUSTOM: Size of section headers:           64 (bytes)
+# CUSTOM: Number of section headers:         3
+# CUSTOM: Section header string table index: 4
+
+--- !ELF
+FileHeader:
+  Class:     ELFCLASS64
+  Data:      ELFDATA2LSB
+  Type:      ET_REL
+  Machine:   EM_X86_64
+  SHEntSize: 64
+  SHOffset:  2
+  SHNum:     3
+  SHStrNdx:  4
+
+## Finally, we use the same YAML as above, but set e_shentsize to 1.
+## Check the result using raw output from 'od' because llvm-readelf
+## is unable to dump such headers.
+
+# RUN: yaml2obj --docnum=3 %s -o %t3
+# RUN: od -t x1 -v -j 0x3a -N 1 %t3 | FileCheck %s --check-prefix=NEWSIZE
+# RUN: od -t x1 -v -j 0x3a -N 1 %t2 | FileCheck %s --check-prefix=OLDSIZE
+# NEWSIZE: 0000072 01
+# OLDSIZE: 0000072 40
+
+--- !ELF
+FileHeader:
+  Class:     ELFCLASS64
+  Data:      ELFDATA2LSB
+  Type:      ET_REL
+  Machine:   EM_X86_64
+  SHEntSize: 1
+  SHOffset:  2
+  SHNum:     3
+  SHStrNdx:  4

tools/obj2yaml/elf2yaml.cpp

ELFDumper<ELFT>::getUniquedSymbolName(const Elf_Sym *Sym, StringRef StrTable,
   }
 
   return Name;
 }
 
 template <class ELFT> ErrorOr<ELFYAML::Object *> ELFDumper<ELFT>::dump() {
   auto Y = make_unique<ELFYAML::Object>();
 
-  // Dump header
+  // Dump header. We do not dump SHEntSize, SHOffset, SHNum and SHStrNdx field.
+  // When not explicitly set, the values are set by yaml2obj automatically
+  // and there is no need to dump them here.
   Y->Header.Class = ELFYAML::ELF_ELFCLASS(Obj.getHeader()->getFileClass());
   Y->Header.Data = ELFYAML::ELF_ELFDATA(Obj.getHeader()->getDataEncoding());
   Y->Header.OSABI = Obj.getHeader()->e_ident[ELF::EI_OSABI];
   Y->Header.ABIVersion = Obj.getHeader()->e_ident[ELF::EI_ABIVERSION];
   Y->Header.Type = Obj.getHeader()->e_type;
   Y->Header.Machine = Obj.getHeader()->e_machine;
   Y->Header.Flags = Obj.getHeader()->e_flags;
   Y->Header.Entry = Obj.getHeader()->e_entry;

tools/yaml2obj/yaml2elf.cpp

void ELFState<ELFT>::initELFHeader(Elf_Ehdr &Header) {
   Header.e_machine = Doc.Header.Machine;
   Header.e_version = EV_CURRENT;
   Header.e_entry = Doc.Header.Entry;
   Header.e_phoff = sizeof(Header);
   Header.e_flags = Doc.Header.Flags;
   Header.e_ehsize = sizeof(Elf_Ehdr);
   Header.e_phentsize = sizeof(Elf_Phdr);
   Header.e_phnum = Doc.ProgramHeaders.size();
-  Header.e_shentsize = sizeof(Elf_Shdr);
+
+  Header.e_shentsize =
+      Doc.Header.SHEntSize ? (uint16_t)*Doc.Header.SHEntSize : sizeof(Elf_Shdr);
   // Immediately following the ELF header and program headers.
   Header.e_shoff =
-      sizeof(Header) + sizeof(Elf_Phdr) * Doc.ProgramHeaders.size();
-  Header.e_shnum = SN2I.size() + 1;
-  Header.e_shstrndx = SN2I.get(".shstrtab");
+      Doc.Header.SHOffset
+          ? (uint16_t)*Doc.Header.SHOffset
+          : sizeof(Header) + sizeof(Elf_Phdr) * Doc.ProgramHeaders.size();
+  Header.e_shnum =
+      Doc.Header.SHNum ? (uint16_t)*Doc.Header.SHNum : SN2I.size() + 1;
+  Header.e_shstrndx = Doc.Header.SHStrNdx ? (uint16_t)*Doc.Header.SHStrNdx
+                                          : SN2I.get(".shstrtab");
 }
 
 template <class ELFT>
 void ELFState<ELFT>::initProgramHeaders(std::vector<Elf_Phdr> &PHeaders) {
   for (const auto &YamlPhdr : Doc.ProgramHeaders) {
     Elf_Phdr Phdr;
     Phdr.p_type = YamlPhdr.Type;
     Phdr.p_flags = YamlPhdr.Flags;