std::ptr::copy_nonoverlapping failing on windows

[rani-sharim]

I have a rust program that is compiled as a dll. This dll is then loaded by a 3rd party app.

Part of the process is creating "sessions" for the 3rd party app. For that it gives you a void pointer and basically tells you "write whatever you want in here, we won't try to read it, just give it back with each subsequent call". So for simplicity, I figured out I'd just have a uint32 session id and write it to the pointer, and read it back when given (code below).

This workes fine on 1.77.0, but crashes the 3rd party app on 1.79.0, one what seems like a memory violation.

Writing the session id:

// Session type is "pub type Session = *mut ::std::os::raw::c_void;"
// pSess is defined as "pSess: *mut bindings::Session"

let session_id = session_manager.create_session()?;

debug!("Session Id: {session_id}");

let le_bytes = session_id.to_le_bytes();

unsafe {
    std::ptr::copy_nonoverlapping(le_bytes.as_ptr(), pSess as *mut u8,  le_bytes.len()); 
}

Reading back the session id:

fn decode_session_id(pSess: &bindings::Session) -> Result<SessionId, Error> {
    if pSess.is_null() {
        return Err(anyhow::Error::msg(format!("invalid session id: null")));
    }
   
    let mut session_id_buf = [0u8; size_of::<SessionId>()];
   
    debug!("decode_session_id: before copy"); // <-- I can see this log

    unsafe {
        std::ptr::copy_nonoverlapping(pSess, session_id_buf.as_mut_ptr() as *mut bindings::Session, session_id_buf.len());
    }

    debug!("decode_session_id: after copy"); // <-- when failing, never gets to this

Version it worked on

It most recently worked on: 1.78.0

Version with regression

rustc --version --verbose:

rustc 1.79.0 (129f3b996 2024-06-10)
binary: rustc
commit-hash: 129f3b9964af4d4a709d1383930ade12dfe7c081
commit-date: 2024-06-10
host: x86_64-pc-windows-msvc
release: 1.79.0
LLVM version: 18.1.7

[saethlin]

Based on the current contents of the issue description, my best guess is that this is UB in your code, not a compiler bug. We have had a few similar reports recently. If you can share enough code to reproduce the crash on 1.79, we can figure out what's going on here.

[ChrisDenton]

std::ptr::copy_nonoverlapping(pSess, session_id_buf.as_mut_ptr() as *mut bindings::Session, session_id_buf.len());

This line looks wrong. The count is the number of items, not the byte size.

[rani-sharim]

@ChrisDenton Yes, that seems to be it. Thanks!