datalink(linux): add feature to pass the fd (socket) to ::channel()

This can be useful if the caller want to manipulate the
socket differently at creating. For example setting a
BPF filter on the socket before passing it in the channel:

```
datalink::linux::channel(interface, config, socket_fd)
```

Signed-off-by: Martin Andre <martin.andre@tessares.net>

Cargo.toml

@@ -21,6 +21,7 @@ travis = []
 serde = ["pnet_base/serde", "pnet_datalink?/serde"]
 std = ["pnet_base/std", "pnet_sys", "pnet_datalink", "pnet_transport", "ipnetwork"]
 default = ["pnet_base/default", "pnet_datalink/default", "pnet_transport/default", "pnet_packet/default"]
+psocket = ["pnet_datalink/psocket"]
 
 [dependencies]
 ipnetwork = { version = "0.19.0", optional = true }

pnet_datalink/Cargo.toml

@@ -15,6 +15,7 @@ edition = "2021"
 netmap = []
 std = ["pnet_base/std"]
 default = ["std"]
+psocket = []
 
 [dependencies]
 libc = "0.2.126"

pnet_datalink/src/lib.rs

@@ -181,8 +181,18 @@ impl Default for Config {
 /// When matching on the returned channel, make sure to include a catch-all so that code doesn't
 /// break when new channel types are added.
 #[inline]
-pub fn channel(network_interface: &NetworkInterface, configuration: Config) -> io::Result<Channel> {
-    backend::channel(network_interface, (&configuration).into())
+pub fn channel(
+    network_interface: &NetworkInterface,
+    configuration: Config,
+    #[cfg(feature = "psocket")]
+    socket: i32
+) -> io::Result<Channel> {
+    backend::channel(
+        network_interface,
+        (&configuration).into(),
+        #[cfg(feature = "psocket")]
+        socket
+    )
 }
 
 /// Trait to enable sending `$packet` packets.

pnet_datalink/src/linux.rs

@@ -97,19 +97,25 @@ impl Default for Config {
 
 /// Create a data link channel using the Linux's `AF_PACKET` socket type.
 #[inline]
-pub fn channel(network_interface: &NetworkInterface, config: Config) -> io::Result<super::Channel> {
-    let eth_p_all = 0x0003;
-    let (typ, proto) = match config.channel_type {
-        super::ChannelType::Layer2 => (libc::SOCK_RAW, eth_p_all),
-        super::ChannelType::Layer3(proto) => (libc::SOCK_DGRAM, proto),
+pub fn channel(
+    network_interface: &NetworkInterface,
+    config: Config,
+    #[cfg(feature = "psocket")]
+    socket: i32
+) -> io::Result<super::Channel> {
+    let (_typ, proto) = match config.channel_type {
+        super::ChannelType::Layer2 => (libc::SOCK_RAW, libc::ETH_P_ALL),
+        super::ChannelType::Layer3(proto) => (libc::SOCK_DGRAM, proto as i32),
     };
-    let socket = unsafe { libc::socket(libc::AF_PACKET, typ, proto.to_be() as i32) };
-    if socket == -1 {
-        return Err(io::Error::last_os_error());
-    }
-    let mut addr: libc::sockaddr_storage = unsafe { mem::zeroed() };
-    let len = network_addr_to_sockaddr(network_interface, &mut addr, proto as i32);
 
+    #[cfg(not(feature = "psocket"))]
+    let socket = match unsafe { libc::socket(libc::AF_PACKET, _typ, proto.to_be()) } {
+        -1 => return Err(io::Error::last_os_error()),
+        fd => fd
+    };
+
+    let mut addr: libc::sockaddr_storage = unsafe { mem::zeroed() };
+    let len = network_addr_to_sockaddr(network_interface, &mut addr, proto);
     let send_addr = (&addr as *const libc::sockaddr_storage) as *const libc::sockaddr;
 
     // Bind to interface