portable_atomic_util::Arc::into_raw() returns unaligned reference to the inner type

[notgull]

Repro:

#[cfg(feature = "has_bug")]
use portable_atomic_util::Arc;
#[cfg(not(feature = "has_bug"))]
use std::sync::Arc;

#[repr(align(128))]
struct Aligned(u32);

fn main() {
    let x = Arc::new(Aligned(128));
    println!("x: {:p}", &x);
    let ptr = Arc::into_raw(x);
    println!("ptr: {:p}", ptr);
    let x_ref = unsafe { &*ptr };
    println!("{:?}", &x_ref.0);
    drop(unsafe { Arc::from_raw(ptr) });
}

$ cargo miri run
Preparing a sysroot for Miri (target: x86_64-unknown-linux-musl)... done
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s
     Running `/home/jtnunley/.rustup/toolchains/nightly-x86_64-unknown-linux-musl/bin/cargo-miri runner /hard_data/cargo_target/miri/x86_64-unknown-linux-musl/debug/pu-test`
x: 0x246d8
ptr: 0x24400
128
$ cargo miri run --features has_bug
Preparing a sysroot for Miri (target: x86_64-unknown-linux-musl)... done
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s
     Running `/home/jtnunley/.rustup/toolchains/nightly-x86_64-unknown-linux-musl/bin/cargo-miri runner /hard_data/cargo_target/miri/x86_64-unknown-linux-musl/debug/pu-test`
x: 0x24860
ptr: 0x24490
error: Undefined Behavior: constructing invalid value: encountered an unaligned reference (required 128 byte alignment but found 16)
  --> src/main.rs:14:26
   |
14 |     let x_ref = unsafe { &*ptr };
   |                          ^^^^^ constructing invalid value: encountered an unaligned reference (required 128 byte alignment but found 16)
   |
   = 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: BACKTRACE:
   = note: inside `main` at src/main.rs:14:26: 14:31

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

error: aborting due to previous error

[taiki-e]

The problem is that the offset calculation does not take into account the padding after Header.
Perhaps mem::size_of::<Header>() should be cmp::max(mem::size_of::<Header>(), mem::align_of::<T>()).

portable-atomic/portable-atomic-util/src/arc.rs

Line 398 in 0a6a603

let new_ptr = strict::map_addr(raw_ptr, |addr| addr - mem::size_of::<Header>());

portable-atomic/portable-atomic-util/src/arc.rs

Line 366 in 0a6a603

let new_ptr = strict::map_addr(ptr, |addr| addr + mem::size_of::<Header>());

[notgull]

@taiki-e I will write a PR to fix this shortly.

[taiki-e]

Perhaps mem::size_of::<Header>() should be cmp::max(mem::size_of::<Header>(), mem::align_of::<T>()).

Ah, since align_of is incompatible with unsized type, it will actually be necessary to use align_of_val(&self.inner().value) (for as_ptr) and align_of_val(&*ptr) (for from_raw), instead of align_of::<T>().