[ELF] Set EF_ARM_EABI_VER5 for ARM

Without this flag set, an AArch64 Linux kernel won't try to load the executable
(even if a 32 bit arm kernel will run the binary just fine).

Patch by Martin Storsjö!

Differential revision: https://reviews.llvm.org/D24471

llvm-svn: 281394

lld/ELF/Writer.cpp

@@ -1262,7 +1262,12 @@ template <class ELFT> void Writer<ELFT>::writeHeader() {
   EHdr->e_shnum = OutputSections.size() + 1;
   EHdr->e_shstrndx = Out<ELFT>::ShStrTab->SectionIndex;
 
-  if (Config->EMachine == EM_MIPS)
+  if (Config->EMachine == EM_ARM)
+    // We don't currently use any features incompatible with EF_ARM_EABI_VER5,
+    // but we don't have any firm guarantees of conformance. Linux AArch64
+    // kernels (as of 2016) require an EABI version to be set.
+    EHdr->e_flags = EF_ARM_EABI_VER5;
+  else if (Config->EMachine == EM_MIPS)
     EHdr->e_flags = getMipsEFlags<ELFT>();
 
   if (!Config->Relocatable) {

lld/test/ELF/arm-eabi-version.s

@@ -0,0 +1,14 @@
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o
+// RUN: ld.lld -static %t.o -o %tout
+// RUN: llvm-readobj -file-headers %tout | FileCheck %s
+// REQUIRES: arm
+ .syntax unified
+ .text
+ .globl _start
+_start:
+ bx lr
+
+// CHECK:  Flags [
+// CHECK-NEXT:    0x1000000
+// CHECK-NEXT:    0x4000000
+// CHECK-NEXT:  ]