Re-use `MultiHeader<()>` in subsequent `recvmmsg()` calls

[OliverNChalk]

Problem

Before call:

MultiHeaders { items: [mmsghdr { msg_hdr: msghdr { msg_name: 0x1, msg_namelen: 0, msg_iov: 0x5583b86edbe0, msg_iovlen: 1, msg_control: 0x5583b86edc30, msg_controllen: 32, msg_flags: 0 }, msg_len: 0 }], addresses: [core::mem::maybe_uninit::MaybeUninit<()>], _cmsg_buffers: Some([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]), msg_controllen: 32 }

After call:

MultiHeaders { items: [mmsghdr { msg_hdr: msghdr { msg_name: 0x1, msg_namelen: 28, msg_iov: 0x5583b86edbe0, msg_iovlen: 1, msg_control: 0x5583b86edc30, msg_controllen: 32, msg_flags: 0 }, msg_len: 1200 }], addresses: [core::mem::maybe_uninit::MaybeUninit<()>], _cmsg_buffers: Some([32, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 35, 0, 0, 0, 30, 33, 234, 102, 0, 0, 0, 0, 139, 247, 225, 58, 0, 0, 0, 0]), msg_controllen: 32 }

The main thing to call out is that the kernel set the msg_namelen value to 28. This makes re-using MultiHeader for subsequent reads impossible because the kernel will complain with err=Bad address (os error 14) due to the fact we have a msg_name: NULL and msg_namelen: 28 in the input.

I'd like to re-use MultiHeader for subsequent reads and have confirmed it works if I instead use MultiHeaders<UnixAddr>, as we now have space of the address to be written.

Potential Solution

Letting the caller set the msglen would allow us to re-zero it so the kernel doesn't think we have space available for an address when we don't.

Side Question

Do you know why msg_name renders like this: msg_hdr.msg_name: 0x1, I would have thought 0x0 for NULL?

[SteveLauC]

I'd like to re-use MultiHeader for subsequent reads

From git history, MultiHeaders was added to avoid the allocation in the loop within recvmmsg() in #1744, gentle ping on @pacak, could you please take a look at this issue?

[SteveLauC]

Do you know why msg_name renders like this: msg_hdr.msg_name: 0x1, I would have thought 0x0 for NULL?

I guess this is an optimization done by Rust, MultiHeader::preallocate() uses MaybeUninit::<S> under the hood, MaybeUninit<()> does not need to be writable, so Rust does not need to allocate memory, and thus MaybeUninit<()>.as_ptr() could use something like std::ptr::dangling():

$ cat src/main.rs 
#![feature(strict_provenance)]

fn main() {
    println!("{:p}", std::ptr::dangling() as *const nix::libc::c_void);
}

$ cargo +nightly r -q
0x1

[pacak]

According to documentation for recvmsg, msg_name is optional and it is valid for it to be NULL:

The msg_name field points to a caller-allocated buffer that is used to return the source address if the socket is unconnected. The caller should set msg_namelen to the size of this buffer before this call; upon return from a successful call, msg_namelen will contain the length of the returned address. If the application does not need to know the source address, msg_name can be specified as NULL.

We've been using it in production in this way for quite a while now while reusing reallocated headers with no problems on Linux.

The fact that msg_name is not NULL and kernel manages to write 28 bytes there even though it was told that the buffer is 0 bytes long is a bit worrying.

[pacak]

I guess API can/should be extended to be able to set and reset buffer length? My main use case was working with multicast streams so knowing source address is not necessary.

[pacak]

A data point, I'm using fbdac70c78975fe7c75b882337bf9f40a639f51a, about to upgrade. Both before and after I'm seeing msg_name: 0, msg_namelen: 0. Off to upgrade to more recent version.

[pacak]

0.25 works, 0.26 does not - producing msg_name: 1, going though the changelog.

[pacak]

https://github.com/nix-rust/nix/pull/2041 - regression originates here

[pacak]

Reverting this "fixes" this problem by initializing address with a null pointer, not sure if things fixed by #2041 are still working after that.

-    (*p).msg_name = (*address).as_mut_ptr() as *mut c_void;
+    (*p).msg_name = address as *mut c_void;

I guess the right fix would be to figure out how to make it work as expected. From what I can tell this code existed before my changes to recvmmsg... Poking around a bit more.

[pacak]

core::mem::maybe_uninit::MaybeUninit<()> is a zero sized type, vec of them occupies zero space, iterating over each member gets mutable pointer with addr 1 and size 0. recvmmsg is not aware of such tricks. So the fix would be to do something like this

This fixes the problem:

(*p).msg_name = if S::size() == 0 { ptr::nul_mut() } else { (*address).as_mut_ptr() as *mut c_void }

Proper fix should include some tests for both sized and unsized addresses. There might be similar patterns in the code.

[pacak]

After #2091 this rust-analyzer is no longer useful in the default state. What's the trick?

[OliverNChalk]

If you're using vscode you can:

    "rust-analyzer.cargo.features": "all",

in your global/workspace settings.json

[pacak]

I'm using neovim. This kind of setting doesn't belong to global editor settings - I'm working on other projects as well. I'll look into making a fix after I figure out the config - fairly low priority at the moment.

[OliverNChalk]

global/workspace

[pacak]

global/workspace

I made this that fixes the problem with R-A: https://github.com/nix-rust/nix/pull/2516

There will be one more to fix rustfmt and after that will make a fix for recvmmsg