[yaml2obj/obj2yaml] - Do not trigger llvm_unreachable when dumping/parsing relocations and e_machine is unsupported.

[grimar]

Summary

Currently when e_machine is set to something that is not supported by YAML lib,
then tools fail with llvm_unreachable. Though this situation is valid.

In this patch I suggest to allow them to handle relocations in this case.
It can be used to dump and create objects for broken or unsupported targets.

[MaskRay]

Though this situation is valid.

I would argue 0x1234 is an invalid case, but valid e_machine numbers are "unsupported" cases. Anyhow this is nice for testing purposes.

test/tools/obj2yaml/relocation-unsupported-machine.yaml

1

machine type -> the machine type
Or
e_machine

I find that the ELF spec describes e_machine as:

This member's value specifies the required architecture for an individual file.

"require architecture", not "machine type" as I thought.. So for the term used here, I have a slight preference for the literal e_machine now.

test/tools/yaml2obj/relocation-unsupported-machine.yaml

1

an unknown e_machine

lib/ObjectYAML/ELFYAML.cpp

#include "llvm/BinaryFormat/ELFRelocs/AMDGPU.def"
     break;
   case ELF::EM_BPF:
 #include "llvm/BinaryFormat/ELFRelocs/BPF.def"
     break;
   case ELF::EM_PPC64:
 #include "llvm/BinaryFormat/ELFRelocs/PowerPC64.def"
     break;
   default:
-    llvm_unreachable("Unsupported architecture");
+    // Nothing to do.
+    break;
   }
 #undef ELF_RELOC
   IO.enumFallback<Hex32>(Value);
 }
 
 void ScalarEnumerationTraits<ELFYAML::ELF_DYNTAG>::enumeration(
     IO &IO, ELFYAML::ELF_DYNTAG &Value) {
   const auto *Object = static_cast<ELFYAML::Object *>(IO.getContext());

test/tools/obj2yaml/relocation-unsupported-machine.yaml

+## Show that obj2yaml is able to dump relocations when machine type is unknown.
+
+# RUN: yaml2obj %s -o %t1
+# RUN: obj2yaml %t1 | FileCheck %s
+
+# CHECK:      --- !ELF
+# CHECK-NEXT: FileHeader:
+# CHECK-NEXT:   Class:   ELFCLASS64
+# CHECK-NEXT:   Data:    ELFDATA2MSB
+# CHECK-NEXT:   Type:    ET_REL
+# CHECK-NEXT:   Machine: 0x1234
+# CHECK-NEXT: Sections:
+# CHECK-NEXT:   - Name:    .rela.text
+# CHECK-NEXT:     Type:    SHT_RELA
+# CHECK-NEXT:     Link:    .symtab
+# CHECK-NEXT:     EntSize: 0x0000000000000018
+# CHECK-NEXT:     Relocations:
+# CHECK-NEXT:       - Offset: 0x0000000000000001
+# CHECK-NEXT:         Symbol: ''
+# CHECK-NEXT:         Type:   0x00000002
+# CHECK-NEXT:       - Offset: 0x0000000000000003
+# CHECK-NEXT:         Symbol: ''
+# CHECK-NEXT:         Type:   0x00000004
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2MSB
+  Type:    ET_REL
+  Machine: 0x1234
+Sections:
+  - Name: .rela.text
+    Type: SHT_RELA
+    Relocations:
+      - Offset: 0x1
+        Type:   0x2
+      - Offset: 0x3
+        Type:   0x4

test/tools/yaml2obj/relocation-unsupported-machine.yaml

+## Show that yaml2obj is able to write relocations for an unknown/unsupported
+## machine type properly.
+
+# RUN: yaml2obj %s -o %t1
+# RUN: llvm-readelf %t1 --relocations | FileCheck %s
+
+# CHECK: Relocation section '.rela.text' at offset 0x40 contains 2 entries:
+# CHECK:      Offset            Info             Type    Symbol's Value  Symbol's Name + Addend
+# CHECK-NEXT: 0000000000000001  0000000000000002 Unknown                                 0
+# CHECK-NEXT: 0000000000000003  0000000000000004 Unknown                                 0
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2MSB
+  Type:    ET_REL
+  Machine: 0x1234
+Sections:
+  - Name: .rela.text
+    Type: SHT_RELA
+    Relocations:
+      - Offset: 0x1
+        Type:   0x2
+      - Offset: 0x3
+        Type:   0x4