[llvm] [MC] Set SHF_EXCLUDE for AArch64 (and other) build attributes sections (PR #125824)

[Peter Smith via llvm-commits]

https://github.com/smithp35 commented:

I think SHF_EXCLUDE is too strong. I think the underlying problem is that LLD support hasn't been added yet. It is being worked on but there's no public patch as yet.

The original (Arm) specification for BuildAttributes https://github.com/ARM-software/abi-aa/blob/main/addenda32/addenda32.rst#313the-scope-of-build-attributes

`This standard places no requirements on the presence or absence of build attributes in executable files.`

This was added on behalf of the GNU project, which wanted to retain a "merged" build attributes section in the ELF output file.

In the original Arm LLD port I followed Arm's proprietary linker and did not output a Build Attributes section,  only to find that eglibc (Ubuntu 14.04 LTS) dlopen was using it and wouldn't work unless it was present.

ld -r relocatable links also need to output a merged build attributes section as the relocatable object can be used as an input for further links.

Setting SHF_EXCLUDE could have some unintended consequences with interoperation with GNU ld.

I think the best way forward for now is to make a placeholder LLD patch that just discards the AArch64 Build Attributes section. When the LLD support is implemented it can remove that.

I'll submit a PR for that and will post a link here.

https://github.com/llvm/llvm-project/pull/125824