Fix PREL31 relocation on ARM

[yuyichao]

Summary

This is a 31bits relative relocation instead of a 32bits absolute relocation.

[peter.smith]

Your solution is the same way that R_ARM_PREL31 is handled in lld, so this looks fine from a correctness point of view.

I'm not familiar with this area so I think you'll need approval from a maintainer, they may also insist on a test being written.

[yuyichao]

Your solution is the same way that R_ARM_PREL31 is handled in lld, so this looks fine from a correctness point of view.

Good to know.... Would have saved me some time....

they may also insist on a test being written.

The failure shows up as invalid unwind info. Any pointer to a similar test / how can I write one?

[peter.smith]

Apologies I'm not familiar with mcjit to know how to write a test for this. Speculating:

• To get a prel31 relocation that will fail there will need to be a negative offset. If the .arm.exidx section is after the executable code or any link to a .ARM.extab section in memory/file the offsets will be negative will show up problems.
• If there is anyway to test the top-bit of the relocated entry you should be able to detect it.

[yuyichao]

lib/ExecutionEngine/RuntimeDyld/RuntimeDyldELF.cpp

468

I think I made a stupid mistake here... Should be 0x80000000 instead of 0x8000000 (7 zeros instead of 6 ......) Will update later.

[yuyichao]

Fix bit31 mask....

[yuyichao]

Add test.

[peter.smith]

This looks good to me from an ARM perspective. I'm not a contributor in this area so you may want to wait a bit to see if anyone objects on style grounds.

[yuyichao]

Ping. Does this still need more review or is this OK to merge? I'll also need someone else to push for me since I don't have commit access.

[rengolin]

There's nothing controversial about this patch, LGTM.

lib/ExecutionEngine/RuntimeDyld/RuntimeDyldELF.cpp

 
   case ELF::R_ARM_NONE:
     break;
+    // Write a 31bit signed offset
   case ELF::R_ARM_PREL31:
+    *TargetPtr &= 0x8000000;
+    *TargetPtr |= (Value - FinalAddress) & ~0x8000000;
+    break;
   case ELF::R_ARM_TARGET1:
   case ELF::R_ARM_ABS32:
     *TargetPtr = Value;