[llvm] Fix symbol name offset in parsing chained-fixup entry function

[fengzhichu]

I could not find any symbol name when using objdump to parse chained-fixup entries. Then I found this bug that NameOffset had a wrong value because NameOffset and WeakImport varibles were exchanged and calculation for NameOffset was also wrong.

The definition of dyld_chained_import_addend64 is in MachO.h.

struct dyld_chained_import_addend64 {
  uint64_t lib_ordinal : 16;
  uint64_t weak_import : 1;
  uint64_t reserved : 15;
  uint64_t name_offset : 32;
  uint64_t addend;
};

https://github.com/llvm/llvm-project/blame/main/llvm/include/llvm/BinaryFormat/MachO.h#L1109-L1115

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[llvmbot]

@llvm/pr-subscribers-llvm-binary-utilities

Author: Hummer (fengzhichu)

Changes

I could not find any symbol name when using objdump to parse chained-fixup entries. Then I found this bug that NameOffset had a wrong value because NameOffset and WeakImport varibles were exchanged and calculation for NameOffset was also wrong.

The definition of dyld_chained_import_addend64 is in MachO.h.

struct dyld_chained_import_addend64 {
  uint64_t lib_ordinal : 16;
  uint64_t weak_import : 1;
  uint64_t reserved : 15;
  uint64_t name_offset : 32;
  uint64_t addend;
};

https://github.com/llvm/llvm-project/blame/main/llvm/include/llvm/BinaryFormat/MachO.h#L1109-L1115

---

Full diff: https://github.com/llvm/llvm-project/pull/83564.diff

1 Files Affected:

• (modified) llvm/lib/Object/MachOObjectFile.cpp (+2-2)

diff --git a/llvm/lib/Object/MachOObjectFile.cpp b/llvm/lib/Object/MachOObjectFile.cpp
index 1cfd0a069463e9..fd310d36e65c2e 100644
--- a/llvm/lib/Object/MachOObjectFile.cpp
+++ b/llvm/lib/Object/MachOObjectFile.cpp
@@ -5231,8 +5231,8 @@ MachOObjectFile::getDyldChainedFixupTargets() const {
       auto RawValue = getArray<uint64_t, 2>(*this, ImportPtr);
 
       LibOrdinal = getEncodedOrdinal<uint16_t>(RawValue[0] & 0xFFFF);
-      NameOffset = (RawValue[0] >> 16) & 1;
-      WeakImport = RawValue[0] >> 17;
+      WeakImport = (RawValue[0] >> 16) & 1;
+      NameOffset = RawValue[0] >> 32;
       Addend = RawValue[1];
     } else {
       llvm_unreachable("Import format should have been checked");

[dwblaikie]

Test coverage?

[MaskRay]

https://maskray.me/blog/2021-08-08-toolchain-testing#i-dont-know-where-to-add-a-test

You can comment out code that your patch has modified, or make an intentional mistake, then run the whole testsuite to locate relevant tests. You can use git log -- file to learn the history of these tests.

The llvm-objdump Mach-O code generally has quite poor test coverage. So it's possible you cannot find the test immediately. You may need to study tests under llvm/test/Object and llvm/test/tools/llvm-objdump/MachO, and sometimes a refactoring before fixing an issue is useful.