ELF/ARM: Ignore R_ARM_V4BX for ARMv4 but allow linking

[zatrazz]

Summary

This patch allow the ARM relocation R_ARM_V4BX to be processed by lld,
although it is not really handled in the static relocation code. The
relocation is in the form:

Relocation section '.rel.text' at offset 0x428 contains 4 entries:
Offset Info Type Sym.Value Sym. Name
00000014 00000028 R_ARM_V4BX

Meaning it does have a direct target, but rather references to an absolute
section *ABS* (in this exemple to the .text segment itself). It makes the
target Atom after file parse to not have a associated pointer and thus
generating a derrefence NULL point in ELFFile<ELFT>::findAtom. Current
approach is just ignore and return nullptr in such cases.

The problem relies that default GCC configuration
for arm-linux-gnueabi{hf} emits the relocation for the asm:

.syntax unified
.arm

.p2align 2
.type fn, %function
fn:

ldr r3, .LGOT
ldr r2, .LGOT+4

.LPIC:

add r3, pc, r3
ldr r2, [r3, r2]
cmp r2, #0
bxeq lr
b __start__

.LGOT:
.word _GLOBAL_OFFSET_TABLE_-(.LPIC+8)

.word start(GOT)

But only with the option -march=armv4 (which is the default GCC configuration).
For arm5 and forward the relocation is not created. This a special relocation
(defined miscellaneous for ARM) that instruct the linker to replace the bx
instruction into a mov. GNU linker has some options related to which substitution
it can create for such cases.

With this patch I can dynamically link an application against a GLIBC
arm-linux-gnueabi system configured with default GCC.

[ruiu]

LGTM

lib/ReaderWriter/ELF/ARM/ARMRelocationHandler.cpp

case R_ARM_ALU_PC_G1_NC:
     break;
   case R_ARM_LDR_PC_G2:
     relocR_ARM_LDR_PC_G2(loc, reloc, target, addend);
     break;
   case R_ARM_JUMP_SLOT:
   case R_ARM_IRELATIVE:
     // Runtime only relocations. Ignore here.
     break;
+  case R_ARM_V4BX:
+    // TODO implement
+    break;
   default:
     return make_unhandled_reloc_error();
   }
 
   return std::error_code();
 }

lib/ReaderWriter/ELF/ELFFile.cpp

std::error_code ELFFile<ELFT>::isCompatible(const MemoryBuffer &mb,
   return elf::isCompatible<ELFT>(mb, ctx);
 }
 
 template <typename ELFT>
 Atom *ELFFile<ELFT>::findAtom(const Elf_Sym *sourceSym,
                               const Elf_Sym *targetSym) {
   // Return the atom for targetSym if we can do so.
   Atom *target = _symbolToAtomMapping.lookup(targetSym);
+  if (!target)
+    // Some realocations (R_ARM_V4BX) do not have a defined
+    // target.  For this cases make it points to itself.
+    target = _symbolToAtomMapping.lookup(sourceSym);
+
   if (target->definition() != Atom::definitionRegular)
     return target;
   Atom::Scope scope = llvm::cast<DefinedAtom>(target)->scope();
   if (scope == DefinedAtom::scopeTranslationUnit)
     return target;
   if (!redirectReferenceUsingUndefAtom(sourceSym, targetSym))
     return target;
 

test/elf/ARM/rel-v4bx.test

+# REQUIRES: arm
+
+# Check handling of R_ARM_V4BX relocation.
+# RUN: yaml2obj -format=elf %s > %t-o.o
+# RUN: lld -flavor gnu -target arm -m armelf_linux_eabi \
+# RUN: --noinhibit-exec %t-o.o -o %t
+# RUN: llvm-objdump -d -triple=armv4t %t | FileCheck %s
+
+# CHECK: Disassembly of section .text:
+# CHECK: main:
+# The following instruction creates a RM_ARM_V4BX relocation that
+# allows the linker to replace the bx instruction to a ARMv4
+# compatible one.  Currently lld does not change it.
+# CHECK:   400114:       1e ff 2f 01     bxeq    lr
+
+---
+FileHeader:      
+  Class:           ELFCLASS32
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_ARM
+  Flags:           [ EF_ARM_EABI_VER5 ]
+Sections:        
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    AddressAlign:    0x0000000000000004
+    Content:         1EFF2F01
+  - Name:            .rel.text
+    Type:            SHT_REL
+    Flags:           [ SHF_INFO_LINK ]
+    Link:            .symtab
+    AddressAlign:    0x0000000000000004
+    Info:            .text
+    Relocations:     
+      - Offset:          0x0000000000000000
+        Symbol:          ''
+        Type:            R_ARM_V4BX
+  - Name:            .data
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+  - Name:            .bss
+    Type:            SHT_NOBITS
+    Flags:           [ SHF_WRITE, SHF_ALLOC ]
+    AddressAlign:    0x0000000000000001
+    Content:         ''
+  - Name:            .ARM.attributes
+    Type:            SHT_ARM_ATTRIBUTES
+    AddressAlign:    0x0000000000000001
+    Content:         4115000000616561626900010B000000060208010901
+Symbols:         
+  Local:           
+    - Name:            .text
+      Type:            STT_SECTION
+      Section:         .text
+    - Name:            .data
+      Type:            STT_SECTION
+      Section:         .data
+    - Name:            .bss
+      Type:            STT_SECTION
+      Section:         .bss
+    - Name:            '$a'
+      Section:         .text
+    - Name:            .ARM.attributes
+      Type:            STT_SECTION
+      Section:         .ARM.attributes
+  Global:          
+    - Name:            main
+      Type:            STT_FUNC
+      Section:         .text