misaligned pointer dereference

[oherrala]

I'm seeing this in our GitHub Actions failing. Tests worked with Rust 1.69.0 using stable-i686-pc-windows-msvc toolchain, but seems to fail with Rust 1.70.0 with same toolchain.

thread '<redacted>' panicked at 'panic in a function that cannot unwind', library\core\src\panicking.rs:126:5
thread '<redacted>' panicked at 'misaligned pointer dereference: address must be a multiple of 0x8 but is 0x189f604', C:\Users\runneradmin\.cargo\registry\src\index.crates.io-1cd66030c949c28d\ipconfig-0.3.1\src\adapter.rs:293:23

Affected function:

ipconfig/src/adapter.rs

Lines 289 to 301 in f6d918f

	unsafe fn get_gateways(mut gateway_ptr: PIP_ADAPTER_GATEWAY_ADDRESS_LH) -> Result<Vec<IpAddr>> {
	let mut gateways = vec![];
	while !gateway_ptr.is_null() {
	let gateway = &*gateway_ptr;
	let ipaddr = socket_address_to_ipaddr(&gateway.Address);
	gateways.push(ipaddr);
	gateway_ptr = gateway.Next;
	}
	Ok(gateways)
	}

[oherrala]

Rust 1.70.0 changelog has this line which might be relevant:

• Insert alignment checks for pointer dereferences when debug assertions are enabled rust-lang/rust#98112 This catches undefined behavior at runtime, and may cause existing code to fail.

[saethlin]

Ouch, yeah we don't test Windows targets in crater and I don't test them personally.

It looks to me like Windows is returning a pointer that is not sufficiently aligned. The standard library has used addr_of! to navigate such issues before, perhaps that would be viable fix here? https://github.com/rust-lang/rust/blob/f91b634643de14ab5156b0c084d80bd6845fb0ae/library/std/src/sys/windows/fs.rs#L766-L778

        let (name, is_directory, next_entry) = unsafe {
            let info = buffer.as_ptr().cast::<c::FILE_ID_BOTH_DIR_INFO>();
            // While this is guaranteed to be aligned in documentation for
            // https://docs.microsoft.com/en-us/windows/win32/api/winbase/ns-winbase-file_id_both_dir_info
            // it does not seem that reality is so kind, and assuming this
            // caused crashes in some cases (https://github.com/rust-lang/rust/issues/104530)
            // presumably, this can be blamed on buggy filesystem drivers, but who knows.
            let next_entry = ptr::addr_of!((*info).NextEntryOffset).read_unaligned() as usize;
            let length = ptr::addr_of!((*info).FileNameLength).read_unaligned() as usize;
            let attrs = ptr::addr_of!((*info).FileAttributes).read_unaligned();
            let name = from_maybe_unaligned(
                ptr::addr_of!((*info).FileName).cast::<u16>(),
                length / size_of::<u16>(),
            );

[liranringel]

Thanks for the report!
I fixed it, and I will publish a new version soon.