[lld] [PAC][lld][ELF] Use PAC instructions in PLT header with `-z pac-plt` (PR #116334)

[Peter Smith via llvm-commits]

smithp35 wrote:

> > What is the compatibility story about pacibsp and does GNU ld (which supports the option as well) decides to do something similar?
> 
> Tagging @smithp35
> 
> My understanding is that pac-plt changes never made it to glibc, right?

What I know:
The glibc PAC support never landed (https://bugzilla.redhat.com/show_bug.cgi?id=1764581) with original discussion https://public-inbox.org/libc-alpha/5ec73433-43c3-5e7a-62cb-a3b203cf41c5@arm.com/

My summary is that there were some concerns about backwards compatibility with previous glibc versions, and as RELRO was available on glibc platforms, pac-plt was deemed uneccessary.

My understanding is that GNU ld chose not to protect the lazy loader due to how RELRO is implemented in GNU ld
```
  . = DATA_SEGMENT_RELRO_END (SIZEOF (.got.plt) >= 24 ? 24 : 0, .);
  .got.plt        : { *(.got.plt) *(.igot.plt) }
```
The offset in DATA_SEGMENT_RELRO_END (ignored in LLD) places the first two .got.plt entries into RELRO so they don't need to be protected by pac-plt.

My understanding is that lld is unlikely to make its RELRO work in such a way that we can overlap the .got.plt.

So we're in the awkward situation that:
* -z pac-plt with lazy loading has a major gap on lld (lazy loading slot unprotected).
* Fixing lazy loading on LLD makes us incompatible with -z pac-plt on GNU ld. It isn't implemented in glibc, and I don't expect it will be.

I think that if we make the decision to change -z pac-plt we should at least raise a bugzilla ticket to let the GNU folk know that should they ever implement -z pac-plt in glibc then they should change their implementation to protect the lazy loading slot as the existing reason not to is fragile (relies on GNU ld's RELRO implementation). The original patch did protect it, https://sourceware.org/legacy-ml/binutils/2019-03/msg00025.html, however there was a follow up patch that removed it https://sourceware.org/legacy-ml/binutils/2019-04/msg00208.html

The alternative is to have -z pac-plt=option with a default to the GNU ld, but with an option to change.



https://github.com/llvm/llvm-project/pull/116334