[libcxx-commits] [compiler-rt] [libcxx] [libcxxabi] [libunwind] [runtimes][PAC] Harden unwinding when possible (PR #143230)

[Oliver Hunt via libcxx-commits]

ojhunt wrote:

> @ojhunt I closed all the threads which are no longer relevant or are duplicates. 

👍

> ## To be done in scope of this PR
> ### Trivial formatting issues
> These require almost zero effort to apply. Please consider fixing these first so we do not need to worry about them anymore.

Done

> 
> ### Non-trivial styling-related issues
> 1. Refactor preprocessor checks against ptrauth. See thread https://github.com/llvm/llvm-project/pull/143230/files#r2369029642 and commit [644405b](https://github.com/llvm/llvm-project/commit/644405b56cfa59dd3787119182df087dff6e756c)
> 2. Avoid use of `__ptrauth_restricted_intptr` which is not present in mainline. See thread https://github.com/llvm/llvm-project/pull/143230/files#r2369036612 and commit [a2390e1](https://github.com/llvm/llvm-project/commit/a2390e1e285023af78d27d768540bb8f30efea76). I do get the point that it's needed in downstream, but I suppose that it's better to avoid exposing downstream-specific stuff to mainline. I suppose that you can use a simple downstream patch over mainline libunwind which would resolve the build issue for you.

That would mean that apple clang will not be able to build the runtimes. That's not a real option.

> 3. Do smth with `[[maybe_unused]]` attribute which only serves as a workaround for a spurious compiler bug. See thread https://github.com/llvm/llvm-project/pull/143230/files#r2368805350 and commit [4eaa8c7](https://github.com/llvm/llvm-project/commit/4eaa8c7dff48bfc96b071ab0e219369839e3758c)

I've moved this behind a matching #define of the code that calls them, I'm trying to work out if there is a real reason for the functions being separated from their usage as much as they are.

> ### An issue causing a runtime crash
> Do not mix pac-ret and ptrauth_returns. See thread https://github.com/llvm/llvm-project/pull/143230/files#r2369419226 and commit [ced8b99](https://github.com/llvm/llvm-project/commit/ced8b99373c9b0756f1718999996f44a74bdf46d).

Fixed with an arm64e guard.

> ### Other non-trivial issues
> Verify FP is handled correctly https://github.com/llvm/llvm-project/pull/143230/files#r2369428305

It is, I've fixed the misleading - we were experimenting with signed frame pointers for a while.

> ## To be done as a follow-up
> 1. [[PAC] Update pointer authentication docs #96528 (comment)](https://github.com/llvm/llvm-project/issues/96528#issuecomment-3320192622)
> 2. [[PAC][libunwind] Refactor way of describing internal signing schemas #160101](https://github.com/llvm/llvm-project/issues/160101)
> 3. [[PAC][libunwind] Unify signed return address handling logic #160110](https://github.com/llvm/llvm-project/issues/160110)
> 4. [[PAC][libunwind] Investigate if compatibility with v8.2-a could be enhanced #160114](https://github.com/llvm/llvm-project/issues/160114)

Pinged Ahmed on this one, I don't know the full impact of that change

> 5. [[PAC][libunwind] Enhance comments and error messages related to libunwind hardening #160117](https://github.com/llvm/llvm-project/issues/160117)

My preference would be to remove those from release builds entirely and replace LIBUNWIND_ABORT() with explicit immediate traps.

> 6. [[PAC][libunwind] Move signing schemas which are part of public ABI to ptrauth.h header #160119](https://github.com/llvm/llvm-project/issues/160119)

This is possibly difficult. The functional ABI implication of most of these is purely between libcxx and libcxxabi, and our platform model does not support updating/changing those separately, which means we think we might be able to change/improve some of the aspects in future. Moving them to `ptrauth.h` functionally removes that option even if there was no technical reason to.

> 7. [[PAC][ibunwind] Support signed personality pointer on Linux targets #160120](https://github.com/llvm/llvm-project/issues/160120)

this is the "personality" discriminated one right? I think I already incorporated it into the latest revision.

https://github.com/llvm/llvm-project/pull/143230