miri: ub: type validation failed: encountered an unaligned reference (required 8 byte alignment but found 2)

[matthiaskrgr]

I tried this code:

// run-pass
#![allow(dead_code)]
#[repr(packed)]
pub struct Good {
    data: &'static u32,
    data2: [&'static u32; 2],
    aligned: [u8; 32],
}

// kill this test when that turns to a hard error
#[allow(unaligned_references)]
fn main() {
    let good = Good { data: &0, data2: [&0, &0], aligned: [0; 32] };

    let _ = &good.data; // ok
    let _ = &good.data2[0]; // ok

    let _ = &good.data;
    let _ = &good.data2[0];
    let _ = &*good.data; // ok, behind a pointer
    let _ = &good.aligned; // ok, has align 1
    let _ = &good.aligned[2]; // ok, has align 1
}

error: Undefined Behavior: type validation failed: encountered an unaligned reference (required 8 byte alignment but found 2)
  --> src/main.rs:15:13
   |
15 |     let _ = &good.data; // ok
   |             ^^^^^^^^^^ type validation failed: encountered an unaligned reference (required 8 byte alignment but found 2)
   |
   = 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

rustc 1.62.0-nightly (ec77f2524 2022-04-17)
binary: rustc
commit-hash: ec77f252434a532fdb5699ae4f21a3072d211edd
commit-date: 2022-04-17
host: x86_64-unknown-linux-gnu
release: 1.62.0-nightly
LLVM version: 14.0.0

[PatchMixolydic]

This might be intentional. The original file is a test which seems to be related to #27060 (which has moved to #82523), which tracks the deprecation of unsound references to repr(packed) fields. Given the presence of #[allow(unaligned_references)], this appears to be a regression test to ensure that this code doesn't break before unaligned_references becomes a hard error.

[RalfJung]

Indeed this is not a bug. You allowed the unaligned_references lint which would have told you that there is a soundness issue in this code. :)

warning: reference to packed field is unaligned
  [--> src/main.rs:16:13
](https://play.rust-lang.org/#)   |
16 |     let _ = &good.data;
   |             ^^^^^^^^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, [see issue #82523 <https://github.com/rust-lang/rust/issues/82523>](https://github.com/rust-lang/rust/issues/82523)
   = note: fields of packed structs are not properly aligned, and creating a misaligned reference is undefined behavior (even if that reference is never dereferenced)
   = help: copy the field contents to a local variable, or replace the reference with a raw pointer and use `read_unaligned`/`write_unaligned` (loads and stores via `*p` must be properly aligned even when using raw pointers)