Commits on Aug 21, 2021

1. Stop emitting the dso_local LLVM attribute for external symbols und… …

   …er the static relocation model on macOS.
   
   This matches Clang's behavior:
   
   https://github.com/llvm/llvm-project/blob/973cb2c326be9f256da0897c4d2ef117dc22761d/clang/lib/CodeGen/CodeGenModule.cpp#L1038-L1040
   
   Even if `dso_local` were properly supported in this way on macOS, it seems
   incorrect to add this annotation as liberally as we did. The `dso_local`
   annotation is for symbols that ultimately end up in the same linkage unit, but
   we were adding this annotation even for `static` values inside `extern` blocks
   marked with `#[link(type="framework")]`, which should be considered dynamically
   linked.  Note that Clang likewise avoids emitting `dso_local` for `dllimport`
   symbols:
   
   https://github.com/llvm/llvm-project/blob/973cb2c326be9f256da0897c4d2ef117dc22761d/clang/lib/CodeGen/CodeGenModule.cpp#L1005-L1007
   
   This issue caused breakage in the `ring` crate, which links to a symbol defined
   in `Security.framework` that ultimately resolves to address `0x0`:
   
   https://github.com/briansmith/ring/blob/b94d61e044b42827fefd71d5f61e8c58a7659870/src/rand.rs#L390
   
   For this symbol, the use of `dso_local` causes LLVM to emit a relocation of
   type `X86_64_RELOC_SIGNED`, which is a 32-bit signed PC-relative offset. If the
   binary is large enough, `0x0` might be out of range, and the link will fail.
   Avoiding `dso_local` causes LLVM to use the GOT instead, emitting a relocation
   of type `X86_64_RELOC_GOT_LOAD`, which will properly handle the large offset
   and cause the link to succeed.
   
   As a side note, the static relocation model is effectively deprecated for
   security reasons on macOS, as it prohibits PIE. It's also completely
   unsupported on Apple Silicon, so I don't think it's worth going to the effort
   of properly supporting this model on that platform.

   

   pcwalton committed Aug 21, 2021
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