Add a new ReturnFlags type for flags returned from recvmsg

`RecvMsgReturn`'s `flags` field was previously `RecvFlags`, however
`recvmsg` returns a different set of flags than that. To address that,
add a new type, `ReturnFlags`, which contains the flags that are
returned from `recvmsg`.

Fixes #1287.

src/backend/libc/net/send_recv.rs

@@ -63,6 +63,7 @@ bitflags! {
     #[repr(transparent)]
     #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
     pub struct RecvFlags: u32 {
+        /// `MSG_CMSG_CLOEXEC`
         #[cfg(not(any(
             apple,
             solarish,
@@ -73,7 +74,6 @@ bitflags! {
             target_os = "nto",
             target_os = "vita",
         )))]
-        /// `MSG_CMSG_CLOEXEC`
         const CMSG_CLOEXEC = bitcast!(c::MSG_CMSG_CLOEXEC);
         /// `MSG_DONTWAIT`
         #[cfg(not(windows))]
@@ -104,3 +104,33 @@ bitflags! {
         const _ = !0;
     }
 }
+
+bitflags! {
+    /// `MSG_*` flags returned from [`recvmsg`], in the `flags` field of
+    /// [`RecvMsgReturn`]
+    ///
+    /// [`recvmsg`]: crate::net::recvmsg
+    /// [`RecvMsgReturn`]: crate::net::RecvMsgReturn
+    #[repr(transparent)]
+    #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
+    pub struct ReturnFlags: u32 {
+        /// `MSG_OOB`
+        const OOB = bitcast!(c::MSG_OOB);
+        /// `MSG_EOR`
+        const EOR = bitcast!(c::MSG_EOR);
+        /// `MSG_TRUNC`
+        const TRUNC = bitcast!(c::MSG_TRUNC);
+        /// `MSG_CTRUNC`
+        const CTRUNC = bitcast!(c::MSG_CTRUNC);
+
+        /// `MSG_CMSG_CLOEXEC`
+        #[cfg(linux_kernel)]
+        const CMSG_CLOEXEC = bitcast!(c::MSG_CMSG_CLOEXEC);
+        /// `MSG_ERRQUEUE`
+        #[cfg(linux_kernel)]
+        const ERRQUEUE = bitcast!(c::MSG_ERRQUEUE);
+
+        /// <https://docs.rs/bitflags/*/bitflags/#externally-defined-flags>
+        const _ = !0;
+    }
+}

src/backend/libc/net/syscalls.rs

@@ -30,7 +30,7 @@ use {
 #[cfg(not(any(target_os = "redox", target_os = "wasi")))]
 use {
     super::read_sockaddr::{initialize_family_to_unspec, maybe_read_sockaddr_os, read_sockaddr_os},
-    super::send_recv::{RecvFlags, SendFlags},
+    super::send_recv::{RecvFlags, ReturnFlags, SendFlags},
     super::write_sockaddr::{encode_sockaddr_v4, encode_sockaddr_v6},
     crate::net::{AddressFamily, Protocol, Shutdown, SocketFlags, SocketType},
     core::ptr::null_mut,
@@ -344,7 +344,7 @@ pub(crate) fn recvmsg(
             RecvMsgReturn {
                 bytes,
                 address: addr,
-                flags: RecvFlags::from_bits_retain(bitcast!(msghdr.msg_flags)),
+                flags: ReturnFlags::from_bits_retain(bitcast!(msghdr.msg_flags)),
             }
         })
     })

src/backend/linux_raw/c.rs

@@ -69,12 +69,12 @@ pub(crate) use linux_raw_sys::{
         IPV6_MULTICAST_LOOP, IPV6_RECVTCLASS, IPV6_TCLASS, IPV6_UNICAST_HOPS, IPV6_V6ONLY,
         IP_ADD_MEMBERSHIP, IP_ADD_SOURCE_MEMBERSHIP, IP_DROP_MEMBERSHIP, IP_DROP_SOURCE_MEMBERSHIP,
         IP_FREEBIND, IP_MULTICAST_LOOP, IP_MULTICAST_TTL, IP_RECVTOS, IP_TOS, IP_TTL,
-        MSG_CMSG_CLOEXEC, MSG_CONFIRM, MSG_DONTROUTE, MSG_DONTWAIT, MSG_EOR, MSG_ERRQUEUE,
-        MSG_MORE, MSG_NOSIGNAL, MSG_OOB, MSG_PEEK, MSG_TRUNC, MSG_WAITALL, SCM_CREDENTIALS,
-        SCM_RIGHTS, SHUT_RD, SHUT_RDWR, SHUT_WR, SOCK_DGRAM, SOCK_RAW, SOCK_RDM, SOCK_SEQPACKET,
-        SOCK_STREAM, SOL_SOCKET, SOL_XDP, SO_ACCEPTCONN, SO_BROADCAST, SO_COOKIE, SO_DOMAIN,
-        SO_ERROR, SO_INCOMING_CPU, SO_KEEPALIVE, SO_LINGER, SO_OOBINLINE, SO_ORIGINAL_DST,
-        SO_PASSCRED, SO_PROTOCOL, SO_RCVBUF, SO_RCVBUFFORCE, SO_RCVTIMEO_NEW,
+        MSG_CMSG_CLOEXEC, MSG_CONFIRM, MSG_CTRUNC, MSG_DONTROUTE, MSG_DONTWAIT, MSG_EOR,
+        MSG_ERRQUEUE, MSG_MORE, MSG_NOSIGNAL, MSG_OOB, MSG_PEEK, MSG_TRUNC, MSG_WAITALL,
+        SCM_CREDENTIALS, SCM_RIGHTS, SHUT_RD, SHUT_RDWR, SHUT_WR, SOCK_DGRAM, SOCK_RAW, SOCK_RDM,
+        SOCK_SEQPACKET, SOCK_STREAM, SOL_SOCKET, SOL_XDP, SO_ACCEPTCONN, SO_BROADCAST, SO_COOKIE,
+        SO_DOMAIN, SO_ERROR, SO_INCOMING_CPU, SO_KEEPALIVE, SO_LINGER, SO_OOBINLINE,
+        SO_ORIGINAL_DST, SO_PASSCRED, SO_PROTOCOL, SO_RCVBUF, SO_RCVBUFFORCE, SO_RCVTIMEO_NEW,
         SO_RCVTIMEO_NEW as SO_RCVTIMEO, SO_RCVTIMEO_OLD, SO_REUSEADDR, SO_REUSEPORT, SO_SNDBUF,
         SO_SNDTIMEO_NEW, SO_SNDTIMEO_NEW as SO_SNDTIMEO, SO_SNDTIMEO_OLD, SO_TYPE, TCP_CONGESTION,
         TCP_CORK, TCP_KEEPCNT, TCP_KEEPIDLE, TCP_KEEPINTVL, TCP_NODELAY, TCP_QUICKACK,

src/backend/linux_raw/net/send_recv.rs

@@ -58,3 +58,30 @@ bitflags! {
         const _ = !0;
     }
 }
+
+bitflags! {
+    /// `MSG_*` flags returned from [`recvmsg`], in the `flags` field of
+    /// [`RecvMsgReturn`]
+    ///
+    /// [`recvmsg`]: crate::net::recvmsg
+    /// [`RecvMsgReturn`]: crate::net::RecvMsgReturn
+    #[repr(transparent)]
+    #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
+    pub struct ReturnFlags: u32 {
+        /// `MSG_OOB`
+        const OOB = c::MSG_OOB;
+        /// `MSG_EOR`
+        const EOR = c::MSG_EOR;
+        /// `MSG_TRUNC`
+        const TRUNC = c::MSG_TRUNC;
+        /// `MSG_CTRUNC`
+        const CTRUNC = c::MSG_CTRUNC;
+        /// `MSG_ERRQUEUE`
+        const ERRQUEUE = c::MSG_ERRQUEUE;
+        /// `MSG_CMSG_CLOEXEC`
+        const CMSG_CLOEXEC = c::MSG_CMSG_CLOEXEC;
+
+        /// <https://docs.rs/bitflags/*/bitflags/#externally-defined-flags>
+        const _ = !0;
+    }
+}

src/backend/linux_raw/net/syscalls.rs

@@ -11,7 +11,7 @@ use super::msghdr::{
     with_noaddr_msghdr, with_recv_msghdr, with_unix_msghdr, with_v4_msghdr, with_v6_msghdr,
 };
 use super::read_sockaddr::{initialize_family_to_unspec, maybe_read_sockaddr_os, read_sockaddr_os};
-use super::send_recv::{RecvFlags, SendFlags};
+use super::send_recv::{RecvFlags, ReturnFlags, SendFlags};
 #[cfg(target_os = "linux")]
 use super::write_sockaddr::encode_sockaddr_xdp;
 use super::write_sockaddr::{encode_sockaddr_v4, encode_sockaddr_v6};
@@ -293,7 +293,7 @@ pub(crate) fn recvmsg(
             RecvMsgReturn {
                 bytes,
                 address: addr,
-                flags: RecvFlags::from_bits_retain(msghdr.msg_flags),
+                flags: ReturnFlags::from_bits_retain(msghdr.msg_flags),
             }
         })
     })

src/net/send_recv/mod.rs

@@ -13,7 +13,7 @@ use backend::fd::{AsFd, BorrowedFd};
 use core::cmp::min;
 use core::mem::MaybeUninit;
 
-pub use backend::net::send_recv::{RecvFlags, SendFlags};
+pub use backend::net::send_recv::{RecvFlags, ReturnFlags, SendFlags};
 
 #[cfg(not(any(
     windows,

src/net/send_recv/msg.rs

@@ -15,7 +15,7 @@ use core::mem::{align_of, size_of, size_of_val, take};
 use core::ptr::addr_of;
 use core::{ptr, slice};
 
-use super::{RecvFlags, SendFlags, SocketAddrAny, SocketAddrV4, SocketAddrV6};
+use super::{RecvFlags, ReturnFlags, SendFlags, SocketAddrAny, SocketAddrV4, SocketAddrV6};
 
 /// Macro for defining the amount of space to allocate in a buffer for use with
 /// [`RecvAncillaryBuffer::new`] and [`SendAncillaryBuffer::new`].
@@ -817,12 +817,17 @@ pub fn recvmsg(
 }
 
 /// The result of a successful [`recvmsg`] call.
+#[derive(Debug)]
 pub struct RecvMsgReturn {
     /// The number of bytes received.
+    ///
+    /// When `RecvFlags::TRUNC` is in use, this may be greater than the
+    /// length of the buffer, as it reflects the number of bytes received
+    /// before truncation into the buffer.
     pub bytes: usize,
 
     /// The flags received.
-    pub flags: RecvFlags,
+    pub flags: ReturnFlags,
 
     /// The address of the socket we received from, if any.
     pub address: Option<SocketAddrAny>,

tests/net/recv_trunc.rs

@@ -1,4 +1,5 @@
-use rustix::net::{AddressFamily, RecvFlags, SendFlags, SocketAddrUnix, SocketType};
+use rustix::io::IoSliceMut;
+use rustix::net::{AddressFamily, RecvFlags, ReturnFlags, SendFlags, SocketAddrUnix, SocketType};
 use std::mem::MaybeUninit;
 
 /// Test `recv_uninit` with the `RecvFlags::Trunc` flag.
@@ -17,7 +18,6 @@ fn net_recv_uninit_trunc() {
     let request = b"Hello, World!!!";
     let n = rustix::net::sendto_unix(&sender, request, SendFlags::empty(), &name).expect("send");
     assert_eq!(n, request.len());
-    drop(sender);
 
     let mut response = [MaybeUninit::<u8>::zeroed(); 5];
     let (init, uninit) =
@@ -26,4 +26,109 @@ fn net_recv_uninit_trunc() {
     // We used the `TRUNC` flag, so we should have only gotten 5 bytes.
     assert_eq!(init, b"Hello");
     assert!(uninit.is_empty());
+
+    // Send the message again.
+    let n = rustix::net::sendto_unix(&sender, request, SendFlags::empty(), &name).expect("send");
+    assert_eq!(n, request.len());
+
+    // This time receive it without `TRUNC`. This should fail.
+    let mut response = [MaybeUninit::<u8>::zeroed(); 5];
+    let (init, uninit) = rustix::net::recv_uninit(&receiver, &mut response, RecvFlags::empty())
+        .expect("recv_uninit");
+
+    // We didn't use the `TRUNC` flag, so we should have received 15 bytes,
+    // truncated to 5 bytes.
+    assert_eq!(init, b"Hello");
+    assert!(uninit.is_empty());
+}
+
+/// Test `recvmsg` with the `RecvFlags::Trunc` flag.
+#[test]
+fn net_recvmsg_trunc() {
+    crate::init();
+
+    let tmpdir = tempfile::tempdir().unwrap();
+    let path = tmpdir.path().join("recv_uninit_trunc");
+    let name = SocketAddrUnix::new(&path).unwrap();
+
+    let receiver = rustix::net::socket(AddressFamily::UNIX, SocketType::DGRAM, None).unwrap();
+    rustix::net::bind_unix(&receiver, &name).expect("bind");
+
+    let sender = rustix::net::socket(AddressFamily::UNIX, SocketType::DGRAM, None).unwrap();
+    let request = b"Hello, World!!!";
+    let n = rustix::net::sendto_unix(&sender, request, SendFlags::empty(), &name).expect("send");
+    assert_eq!(n, request.len());
+
+    let mut response = [0_u8; 5];
+    let result = rustix::net::recvmsg(
+        &receiver,
+        &mut [IoSliceMut::new(&mut response)],
+        &mut Default::default(),
+        RecvFlags::TRUNC,
+    )
+    .expect("recvmsg");
+
+    // We used the `TRUNC` flag, so we should have received 15 bytes,
+    // truncated to 5 bytes, and the `TRUNC` flag should have been returned.
+    assert_eq!(&response, b"Hello");
+    assert_eq!(result.bytes, 15);
+    assert_eq!(result.flags, ReturnFlags::TRUNC);
+
+    // Send the message again.
+    let n = rustix::net::sendto_unix(&sender, request, SendFlags::empty(), &name).expect("send");
+    assert_eq!(n, request.len());
+
+    // This time receive it with `TRUNC` and a big enough buffer.
+    let mut response = [0_u8; 30];
+    let result = rustix::net::recvmsg(
+        &receiver,
+        &mut [IoSliceMut::new(&mut response)],
+        &mut Default::default(),
+        RecvFlags::TRUNC,
+    )
+    .expect("recvmsg");
+
+    // We used the `TRUNC` flag, so we should have received 15 bytes
+    // and the buffer was big enough so the `TRUNC` flag should not have
+    // been returned.
+    assert_eq!(&response[..result.bytes], request);
+    assert_eq!(result.flags, ReturnFlags::empty());
+
+    // Send the message again.
+    let n = rustix::net::sendto_unix(&sender, request, SendFlags::empty(), &name).expect("send");
+    assert_eq!(n, request.len());
+
+    // This time receive it without `TRUNC` but a big enough buffer.
+    let mut response = [0_u8; 30];
+    let result = rustix::net::recvmsg(
+        &receiver,
+        &mut [IoSliceMut::new(&mut response)],
+        &mut Default::default(),
+        RecvFlags::empty(),
+    )
+    .expect("recvmsg");
+
+    // We used the `TRUNC` flag, so we should have received 15 bytes,
+    // truncated to 5 bytes, and the `TRUNC` flag should have been returned.
+    assert_eq!(&response[..result.bytes], request);
+    assert_eq!(result.flags, ReturnFlags::empty());
+
+    // Send the message again.
+    let n = rustix::net::sendto_unix(&sender, request, SendFlags::empty(), &name).expect("send");
+    assert_eq!(n, request.len());
+
+    // This time receive it without `TRUNC` and a small buffer.
+    let mut response = [0_u8; 5];
+    let result = rustix::net::recvmsg(
+        &receiver,
+        &mut [IoSliceMut::new(&mut response)],
+        &mut Default::default(),
+        RecvFlags::empty(),
+    )
+    .expect("recvmsg");
+
+    // We didn't use the `TRUNC` flag, so we should have received 15 bytes,
+    // truncated to 5 bytes, and the `TRUNC` flag should have been returned.
+    assert_eq!(&response[..result.bytes], b"Hello");
+    assert_eq!(result.flags, ReturnFlags::TRUNC);
 }