[ELF2] Fix binaries so they actually run on FreeBSD.

Since FreeBSD 4.1, the kernel expects binaries to be marked with
ELFOSABI_FREEBSD in the ELF header to exec() them. LLD unconditionally 
sets OSABI to ELF_OSABINONE, and everything linked with it won't run 
on FreeBSD (unless explicitly rebranded).

Example:
% ./aarch64-hello
ELF binary type "0" not known.
zsh: exec format error: ./aarch64-hello

FreeBSD could be modified to accept ELF_OSABINONE, but that would break all
existing binaries, so the kernel needs to support both ABINONE and ABIFREEBSD.
I plan to push this change in FreeBSD one day, which, unfortunately, is 
not today. This code patches lld so it sets the header field correctly.

For completeness, the rationale of this change is explained in the FreeBSD 
commit message, and it's apparently to pleasee binutils maintainers at the time.
https://svnweb.freebsd.org/base?view=revision&revision=59342

Differential Revision:	 http://reviews.llvm.org/D13140

llvm-svn: 248554

lld/ELF/InputFiles.h

@@ -105,6 +105,9 @@ template <class ELFT> class ELFData {
   llvm::object::ELFFile<ELFT> &getObj() { return ELFObj; }
 
   uint16_t getEMachine() const { return getObj().getHeader()->e_machine; }
+  uint8_t getOSABI() const {
+    return getObj().getHeader()->e_ident[llvm::ELF::EI_OSABI];
+  }
 
   StringRef getStringTable() const { return StringTable; }
 

lld/ELF/Writer.cpp

@@ -500,14 +500,15 @@ template <class ELFT> void Writer<ELFT>::writeHeader() {
                                ? ELFDATA2LSB
                                : ELFDATA2MSB;
   EHdr->e_ident[EI_VERSION] = EV_CURRENT;
-  EHdr->e_ident[EI_OSABI] = ELFOSABI_NONE;
+
+  const SymbolTable &Symtab = SymTabSec.getSymTable();
+  auto &FirstObj = cast<ObjectFile<ELFT>>(*Symtab.getFirstELF());
+  EHdr->e_ident[EI_OSABI] = FirstObj.getOSABI();
 
   // FIXME: Generalize the segment construction similar to how we create
   // output sections.
-  const SymbolTable &Symtab = SymTabSec.getSymTable();
 
   EHdr->e_type = Config->Shared ? ET_DYN : ET_EXEC;
-  auto &FirstObj = cast<ObjectFile<ELFT>>(*Symtab.getFirstELF());
   EHdr->e_machine = FirstObj.getEMachine();
   EHdr->e_version = EV_CURRENT;
   SymbolBody *Entry = Symtab.getEntrySym();

lld/test/elf2/basic-freebsd.s

@@ -0,0 +1,25 @@
+# Verify that OSABI is set to the correct value.
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-freebsd %s -o %t
+# RUN: lld -flavor gnu2 %t -o %t2
+# RUN: llvm-readobj -file-headers %t2 | FileCheck %s
+# REQUIRES: x86
+
+.globl _start
+_start:
+  mov $1, %rax
+  mov $42, %rdi
+  syscall
+
+# CHECK: ElfHeader {
+# CHECK-NEXT:   Ident {
+# CHECK-NEXT:     Magic: (7F 45 4C 46)
+# CHECK-NEXT:     Class: 64-bit (0x2)
+# CHECK-NEXT:     DataEncoding: LittleEndian (0x1)
+# CHECK-NEXT:     FileVersion: 1
+# CHECK-NEXT:     OS/ABI: FreeBSD (0x9)
+# CHECK-NEXT:     ABIVersion: 0
+# CHECK-NEXT:     Unused: (00 00 00 00 00 00 00)
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Type: Executable (0x2)
+# CHECK-NEXT:   Machine: EM_X86_64 (0x3E)