[llvm-objcopy] - Do not drop the OS/ABI and ABIVersion fields of ELF header

[grimar]

Summary

This is https://bugs.llvm.org/show_bug.cgi?id=40005,

Patch teaches llvm-objcopy to preserve OS/ABI and ABIVersion fields of ELF header.
(Currently, it drops them to zero).

[grimar]

It requires D55884 to be landed first for the test case.

[grimar]

Will re-upload the correct diff.

[grimar]

• The diff is correct now.

[rupprecht]

tools/llvm-objcopy/ELF/Object.cpp

706

Not that it makes any difference in functionality, but I think using ELFOSABI_NONE explicitly (as it was in the place it's moved from) would be better

[jakehehrlich]

LGTM, I'd like James's approval and my only comment is about a feature we can add after this if its needed. James is also out on leave so go ahead and land this.

tools/llvm-objcopy/ELF/Object.cpp

703

I'd like @jhenderson to comment if their use case might ever require them to specify what the OSABI and/or ABIVersion might need to be set via a flag for the BianryELFBuilder case.

[arichardson]

LGTM. I recently tried to use llvm-objcopy for building CheriBSD (a fork of FreeBSD for the CHERI CPU) and ran into asserts due to missing ELFOSABI_FREEBSD.

[grimar]

Thanks, everyone for looking!

tools/llvm-objcopy/ELF/Object.cpp

706

Will do.

[jhenderson]

I'd like @jhenderson to comment if their use case might ever require them to specify what the OSABI and/or ABIVersion might need to be set via a flag for the BinaryELFBuilder case.

@jakehehrlich - At the moment, no we don't have a use case for that. If we ever do, I'll either file a bug or create a review.

@grimar - thanks for the fix!

test/tools/llvm-objcopy/ELF/copy-osabi.test

+# RUN: yaml2obj %s > %t
+# RUN: llvm-objcopy %t %t2
+# RUN: llvm-readobj -file-headers %t2 | FileCheck %s
+
+## Check that llvm-objcopy preserves the OSABI and ABIVersion fields.
+# CHECK: OS/ABI: FreeBSD (0x9)
+# CHECK: ABIVersion: 1
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  OSABI:           ELFOSABI_FREEBSD
+  ABIVersion:      1
+  Type:            ET_REL
+  Machine:         EM_AARCH64

tools/llvm-objcopy/ELF/Object.h

public:
   // not present in any segment. This could be a problem during file layout,
   // because other segments may get assigned an offset where either of the
   // two should reside, which will effectively corrupt the resulting binary.
   // Other than that we use these segments to track program header offsets
   // when they may not follow the ELF header.
   Segment ElfHdrSegment;
   Segment ProgramHdrSegment;
 
+  uint8_t OSABI;
+  uint8_t ABIVersion;
   uint64_t Entry;
   uint64_t SHOffset;
   uint32_t Type;
   uint32_t Machine;
   uint32_t Version;
   uint32_t Flags;
 
   StringTableSection *SectionNames = nullptr;

tools/llvm-objcopy/ELF/Object.cpp

 static bool compareSegmentsByPAddr(const Segment *A, const Segment *B) {
   if (A->PAddr < B->PAddr)
     return true;
   if (A->PAddr > B->PAddr)
     return false;
   return A->Index < B->Index;
 }
 
 template <class ELFT> void BinaryELFBuilder<ELFT>::initFileHeader() {
   Obj->Flags = 0x0;
   Obj->Type = ET_REL;
+  Obj->OSABI = 0;
+  Obj->ABIVersion = 0;
   Obj->Entry = 0x0;
   Obj->Machine = EMachine;
   Obj->Version = 1;
 }
 
 template <class ELFT> void BinaryELFBuilder<ELFT>::initHeaderSegment() {
   Obj->ElfHdrSegment.Index = 0;
 }
if (auto RelSec = dyn_cast<RelocationSection>(&Section)) {
       initGroupSection(GroupSec);
     }
   }
 }
 
 template <class ELFT> void ELFBuilder<ELFT>::build() {
   const auto &Ehdr = *ElfFile.getHeader();
 
+  Obj.OSABI = Ehdr.e_ident[EI_OSABI];
+  Obj.ABIVersion = Ehdr.e_ident[EI_ABIVERSION];
   Obj.Type = Ehdr.e_type;
   Obj.Machine = Ehdr.e_machine;
   Obj.Version = Ehdr.e_version;
   Obj.Entry = Ehdr.e_entry;
   Obj.Flags = Ehdr.e_flags;
 
   readSectionHeaders();
   readProgramHeaders();
template <class ELFT> void ELFWriter<ELFT>::writeEhdr() {
   Ehdr.e_ident[EI_MAG0] = 0x7f;
   Ehdr.e_ident[EI_MAG1] = 'E';
   Ehdr.e_ident[EI_MAG2] = 'L';
   Ehdr.e_ident[EI_MAG3] = 'F';
   Ehdr.e_ident[EI_CLASS] = ELFT::Is64Bits ? ELFCLASS64 : ELFCLASS32;
   Ehdr.e_ident[EI_DATA] =
       ELFT::TargetEndianness == support::big ? ELFDATA2MSB : ELFDATA2LSB;
   Ehdr.e_ident[EI_VERSION] = EV_CURRENT;
-  Ehdr.e_ident[EI_OSABI] = ELFOSABI_NONE;
-  Ehdr.e_ident[EI_ABIVERSION] = 0;
+  Ehdr.e_ident[EI_OSABI] = Obj.OSABI;
+  Ehdr.e_ident[EI_ABIVERSION] = Obj.ABIVersion;
 
   Ehdr.e_type = Obj.Type;
   Ehdr.e_machine = Obj.Machine;
   Ehdr.e_version = Obj.Version;
   Ehdr.e_entry = Obj.Entry;
   // We have to use the fully-qualified name llvm::size
   // since some compilers complain on ambiguous resolution.
   Ehdr.e_phnum = llvm::size(Obj.segments());