std::sys::sgx::tls: fix TLS destructor pointer provenance

[phlip9]

Storing the TLS entry destructor as an AtomicUsize loses the pointer provenance and makes miri cry :'(

Instead, store an AtomicPtr<()>. I also find the SAFETY proof more convincing when the transmute from *mut () to Option<unsafe extern "C" fn(*mut u8)> is exactly mirrored.

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=c6da201f269c5f3115039177a43bcdd4

   Compiling playground v0.0.1 (/playground)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.70s
     Running `/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri runner target/miri/x86_64-unknown-linux-gnu/debug/playground`
error: Undefined Behavior: pointer not dereferenceable: pointer must point to some allocation, but got 0x26b51[noalloc] which is a dangling pointer (it has no provenance)
  --> src/tls-bad.rs:73:30
   |
73 |                     unsafe { dtor(value) }
   |                              ^^^^^^^^^^^ Undefined Behavior occurred here
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
   = note: stack backtrace:
           0: <tls::ActiveTls<'_> as std::ops::Drop>::drop
               at src/tls-bad.rs:73:30: 73:41
           1: std::ptr::drop_glue::<tls::ActiveTls<'_>> - shim(Some(tls::ActiveTls<'_>))
               at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:848:1: 850:25
           2: main
               at src/main.rs:50:1: 50:2

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to 1 previous error

[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @clarfonthey (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

Please see the contribution instructions and our LLM policy for more information.

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @ChrisDenton, libs
• @ChrisDenton, libs expanded to 13 candidates
• Random selection from ChrisDenton, JohnTitor, Mark-Simulacrum, clarfonthey, nia-e

[maxdexh]

Wow, i recently changed something surrounding this code and i just assumed this was an AtomicPtr, because the alternative makes no sense!

@rustbot label T-opsem

[clarfonthey]

Seems reasonable to me.

@bors try jobs=dist-various-2

(since this job checks x86_64-fortanix-unknown-sgx)

[rust-bors]

☀️ Try build successful (CI)
Build commit: 20692fd (20692fdf9e77d0edd88cab7130074a6576c0ab02)
Base parent: 6bb812b (6bb812b714cce8e08864b7b569303f25ab08eaad)

[clarfonthey]

@bors r+ rollup

Thank you!

[rust-bors]

📌 Commit a132402 has been approved by clarfonthey

It is now in the queue for this repository.