[obj2yaml][yaml2obj] - Stop recognizing SHT_MIPS_ABIFLAGS on non-MIPS targets.

Currently we are always recognizing the `SHT_MIPS_ABIFLAGS` section,
even on non-MIPS targets.

The problem of doing this is briefly discussed in D88228 which does the same for `SHT_ARM_EXIDX`:

"The problem is that `SHT_ARM_EXIDX` shares the value with `SHT_X86_64_UNWIND (0x70000001U)`.
We might have other machine specific conflicts, e.g.
`SHT_ARM_ATTRIBUTES` vs `SHT_MSP430_ATTRIBUTES` vs `SHT_RISCV_ATTRIBUTES (0x70000003U)`."

I think we should only recognize target specific sections when the machine type
matches. I.e. `SHT_MIPS_*` should be recognized only on `MIPS`, `SHT_ARM_*`
only on `ARM` etc.

This patch stops recognizing `SHT_MIPS_ABIFLAGS` on `non-MIPS` targets.

Note: I had to update `ScalarEnumerationTraits<ELFYAML::MIPS_ISA>::enumeration`, because
otherwise test crashes, calling `llvm_unreachable`.

Differential revision: https://reviews.llvm.org/D88294

llvm/lib/ObjectYAML/ELFYAML.cpp

@@ -816,6 +816,7 @@ void ScalarEnumerationTraits<ELFYAML::MIPS_ISA>::enumeration(
   IO.enumCase(Value, "MIPS5", 5);
   IO.enumCase(Value, "MIPS32", 32);
   IO.enumCase(Value, "MIPS64", 64);
+  IO.enumFallback<Hex32>(Value);
 }
 
 void ScalarBitSetTraits<ELFYAML::MIPS_AFL_ASE>::bitset(
@@ -1306,6 +1307,14 @@ void MappingTraits<std::unique_ptr<ELFYAML::Chunk>>::mapping(
     IO.mapRequired("Type", Type);
   }
 
+  const auto &Obj = *static_cast<ELFYAML::Object *>(IO.getContext());
+  if (Obj.getMachine() == ELF::EM_MIPS && Type == ELF::SHT_MIPS_ABIFLAGS) {
+    if (!IO.outputting())
+      Section.reset(new ELFYAML::MipsABIFlags());
+    sectionMapping(IO, *cast<ELFYAML::MipsABIFlags>(Section.get()));
+    return;
+  }
+
   switch (Type) {
   case ELF::SHT_DYNAMIC:
     if (!IO.outputting())
@@ -1348,11 +1357,6 @@ void MappingTraits<std::unique_ptr<ELFYAML::Chunk>>::mapping(
       Section.reset(new ELFYAML::GnuHashSection());
     sectionMapping(IO, *cast<ELFYAML::GnuHashSection>(Section.get()));
     break;
-  case ELF::SHT_MIPS_ABIFLAGS:
-    if (!IO.outputting())
-      Section.reset(new ELFYAML::MipsABIFlags());
-    sectionMapping(IO, *cast<ELFYAML::MipsABIFlags>(Section.get()));
-    break;
   case ELF::SHT_GNU_verdef:
     if (!IO.outputting())
       Section.reset(new ELFYAML::VerdefSection());

llvm/test/tools/obj2yaml/ELF/mips-abi-flags.yaml

@@ -39,3 +39,35 @@ Sections:
     CPR2Size:     REG_NONE
     Flags1:       [ ODDSPREG ]
     Flags2:       0x0
+
+## Check how we dump the SHT_MIPS_ABIFLAGS (0x7000002a) section when
+## the machine type is not EM_MIPS. It is dumped as a regular
+## section of an unknown type.
+
+# RUN: yaml2obj %s --docnum=2 -DMACHINE=EM_NONE -o %t2.notmips
+# RUN: obj2yaml %t2.notmips | FileCheck %s --check-prefix=NOT-MIPS
+
+# RUN: yaml2obj %s --docnum=2 -DMACHINE=EM_MIPS -o %t2.mips
+# RUN: obj2yaml %t2.mips | FileCheck %s --check-prefix=MIPS
+
+# MIPS:      - Name: .MIPS.abiflags
+# MIPS-NEXT:   Type: SHT_MIPS_ABIFLAGS
+# MIPS-NEXT:   ISA:  0x00000000
+# MIPS-NEXT: ...
+
+# NOT-MIPS:      - Name:    .MIPS.abiflags
+# NOT-MIPS-NEXT:   Type:    0x7000002A
+# NOT-MIPS-NEXT:   Content: '000000000000000000000000000000000000000000000000'
+# NOT-MIPS-NEXT: ...
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2MSB
+  Type:    ET_REL
+  Machine: [[MACHINE]]
+Sections:
+  - Name:   .MIPS.abiflags
+    Type:   SHT_PROGBITS
+    ShType: 0x7000002a ## SHT_MIPS_ABIFLAGS.
+    Size:   0x18

llvm/test/tools/yaml2obj/ELF/mips-abi-flags.yaml

@@ -27,7 +27,7 @@ FileHeader:
   Class:   ELFCLASS64
   Data:    ELFDATA2MSB
   Type:    ET_REL
-  Machine: EM_MIPS
+  Machine: [[MACHINE=EM_MIPS]]
 Sections:
   - Name:         .MIPS.abiflags
     Type:         SHT_MIPS_ABIFLAGS
@@ -43,3 +43,10 @@ Sections:
     CPR2Size:     REG_NONE
     Flags1:       [ ODDSPREG ]
     Flags2:       0x0
+
+## Check we don't recognize the SHT_MIPS_ABIFLAGS section for non-MIPS targets.
+
+# RUN: not yaml2obj %s -DMACHINE=EM_NONE 2>&1 | FileCheck %s --check-prefix=ERR
+
+# ERR:      error: invalid hex32 number
+# ERR-NEXT: Type: SHT_MIPS_ABIFLAGS

llvm/tools/obj2yaml/elf2yaml.cpp

@@ -460,6 +460,10 @@ ELFDumper<ELFT>::dumpSections() {
 
   auto GetDumper = [this](unsigned Type)
       -> std::function<Expected<ELFYAML::Chunk *>(const Elf_Shdr *)> {
+    if (Obj.getHeader().e_machine == ELF::EM_MIPS &&
+        Type == ELF::SHT_MIPS_ABIFLAGS)
+      return [this](const Elf_Shdr *S) { return dumpMipsABIFlags(S); };
+
     switch (Type) {
     case ELF::SHT_DYNAMIC:
       return [this](const Elf_Shdr *S) { return dumpDynamicSection(S); };
@@ -472,8 +476,6 @@ ELFDumper<ELFT>::dumpSections() {
       return [this](const Elf_Shdr *S) { return dumpRelrSection(S); };
     case ELF::SHT_GROUP:
       return [this](const Elf_Shdr *S) { return dumpGroup(S); };
-    case ELF::SHT_MIPS_ABIFLAGS:
-      return [this](const Elf_Shdr *S) { return dumpMipsABIFlags(S); };
     case ELF::SHT_NOBITS:
       return [this](const Elf_Shdr *S) { return dumpNoBitsSection(S); };
     case ELF::SHT_NOTE: