ELF/ARM: Ignore R_ARM_V4BX for ARMv4 but allow linking

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.

llvm-svn: 235880

1 files changed, 5 insertions, 0 deletions

diff --git a/lld/lib/ReaderWriter/ELF/ELFFile.cpp b/lld/lib/ReaderWriter/ELF/ELFFile.cpp
index ab2ec605687..57c25038eda 100644
--- a/lld/lib/ReaderWriter/ELF/ELFFile.cpp
+++ b/lld/lib/ReaderWriter/ELF/ELFFile.cpp
@@ -38,6 +38,11 @@ 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();