Support PC relative relocation on AArch64 and PPC64 in RelocationResolver

[yuyichao]

Summary

Ref https://github.com/JuliaLang/julia/issues/35460

When compiled with --code-model=large --relocation-model=static, LLVM emits PC relative relocations in the .eh_frame section. Loading it then causes a waring to be printed.

AFAICT, the actual relocation is not actually used by any existing tools which is why I'm not adding any test. This also seems to be the case for X86 ref https://reviews.llvm.org/D67779 . The only test done is confirming that this fixes the warning with llvm-dwarfdump.

[MaskRay]

Yes, .eh_frame may have such PC-relative relocations. Ideally R_PPC64_REL32 needs 0xffffffff. We need tests for these relocation types. If you don't mind I'll add them and commit the patch for you.

[yuyichao]

Is there a difference between R_PPC64_REL32 vs R_AARCH64_PREL32 vs R_X86_64_PC32? I didn't add mask because the x86 one doesn't have it.

I know for some relocations, some bits should be masked off. However, I'm under the impression that for these relocation if the value doesn't fit it would just be an error. Is that the case?

[MaskRay]

In D83800#2159287, @yuyichao wrote:

Is there a difference between R_PPC64_REL32 vs R_AARCH64_PREL32 vs R_X86_64_PC32? I didn't add mask because the x86 one doesn't have it.

I know for some relocations, some bits should be masked off. However, I'm under the impression that for these relocation if the value doesn't fit it would just be an error. Is that the case?

Ideally the interface should report an error. Unfortunately it does not do that (yet). This is OK because RelocationResolver is only used by llvm-dwarfdump and LLD (and also llvm-xray but that use case can be avoided) and their use cases don't really error reporting for relocations.

[yuyichao]

OK, so I guess at least R_X86_64_PC32 should be fixed too. If you don't mind, feel free to commit an updated version of this. I can also update this later next week after I'm free again, though, as I said, I have a hard time constructing an IR test for this....

This is OK because RelocationResolver is only used by llvm-dwarfdump and LLD

And I still hope that there are fewer relocation resolvers in LLVM but I digress...

[MaskRay]

Closed by b922004ea29d54534c4f09b9cfa655bf5f3360f0 & 3073a3aa1ef1ce8c9cac9b97a8e5905dd8779e16

I left a comment in that Julia bug as well... Thanks!

[yuyichao]

Seems that this wasn't automatically closed by the two commits so I'm closing manually...

llvm/lib/Object/RelocationResolver.cpp

default:
     llvm_unreachable("Invalid relocation type");
   }
 }
 
 static bool supportsAArch64(uint64_t Type) {
   switch (Type) {
   case ELF::R_AARCH64_ABS32:
   case ELF::R_AARCH64_ABS64:
+  case ELF::R_AARCH64_PREL32:
+  case ELF::R_AARCH64_PREL64:
     return true;
   default:
     return false;
   }
 }
 
 static uint64_t resolveAArch64(RelocationRef R, uint64_t S, uint64_t A) {
   switch (R.getType()) {
   case ELF::R_AARCH64_ABS32:
     return (S + getELFAddend(R)) & 0xFFFFFFFF;
   case ELF::R_AARCH64_ABS64:
     return S + getELFAddend(R);
+  case ELF::R_AARCH64_PREL32:
+  case ELF::R_AARCH64_PREL64:
+    return S + getELFAddend(R) - R.getOffset();
   default:
     llvm_unreachable("Invalid relocation type");
   }
 }
 
 static bool supportsBPF(uint64_t Type) {
   switch (Type) {
   case ELF::R_BPF_64_32:
default:
     llvm_unreachable("Invalid relocation type");
   }
 }
 
 static bool supportsPPC64(uint64_t Type) {
   switch (Type) {
   case ELF::R_PPC64_ADDR32:
   case ELF::R_PPC64_ADDR64:
+  case ELF::R_PPC64_REL32:
+  case ELF::R_PPC64_REL64:
     return true;
   default:
     return false;
   }
 }
 
 static uint64_t resolvePPC64(RelocationRef R, uint64_t S, uint64_t A) {
   switch (R.getType()) {
   case ELF::R_PPC64_ADDR32:
     return (S + getELFAddend(R)) & 0xFFFFFFFF;
   case ELF::R_PPC64_ADDR64:
     return S + getELFAddend(R);
+  case ELF::R_PPC64_REL32:
+  case ELF::R_PPC64_REL64:
+    return S + getELFAddend(R) - R.getOffset();
   default:
     llvm_unreachable("Invalid relocation type");
   }
 }
 
 static bool supportsSystemZ(uint64_t Type) {
   switch (Type) {
   case ELF::R_390_32: