Ensure BPF read buffer is 4-byte aligned

pnet_datalink/src/bpf.rs

@@ -22,6 +22,7 @@ use std::sync::Arc;
 use std::time::Duration;
 
 static ETHERNET_HEADER_SIZE: usize = 14;
+static NULL_HEADER_SIZE: usize = 4;
 
 /// The BPF-specific configuration.
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
@@ -184,14 +185,21 @@ pub fn channel(network_interface: &NetworkInterface, config: Config) -> io::Resu
     }
 
     let mut loopback = false;
+    let mut buffer_offset = 0;
     let mut allocated_read_buffer_size = config.read_buffer_size;
     // The loopback device does weird things
     // FIXME This should really just be another L2 packet header type
     if dlt == bpf::DLT_NULL {
         loopback = true;
-        // So we can guaranatee that we can have a header before the packet.
-        // Loopback packets arrive without the header.
-        allocated_read_buffer_size += ETHERNET_HEADER_SIZE;
+        // The loopback device provides a smaller (4-byte) header than ethernet (14-byte).
+        // We deal with this by offseting the write buffer, then overwriting the null header
+        // with a zeroed ethernet header. This is complicated by the fact that the buffer
+        // offset must be a multiple of four for pointer alignment, and that the write itself
+        // must be 4096 bytes.
+        //
+        // & !x is a bit-hack to round down to nearest multiple of x + 1.
+        buffer_offset = (ETHERNET_HEADER_SIZE - NULL_HEADER_SIZE + 4 - 1) & !(4 - 1);
+        allocated_read_buffer_size += buffer_offset;
 
         // Allow packets to be read back after they are written
         if let Err(e) = set_feedback(fd) {
@@ -235,6 +243,7 @@ pub fn channel(network_interface: &NetworkInterface, config: Config) -> io::Resu
         fd: fd.clone(),
         fd_set: unsafe { mem::zeroed() },
         read_buffer: vec![0; allocated_read_buffer_size],
+        buffer_offset,
         loopback: loopback,
         timeout: config
             .read_timeout
@@ -362,22 +371,18 @@ struct DataLinkReceiverImpl {
     fd: Arc<pnet_sys::FileDesc>,
     fd_set: libc::fd_set,
     read_buffer: Vec<u8>,
+    buffer_offset: usize,
     loopback: bool,
     timeout: Option<libc::timespec>,
     packets: VecDeque<(usize, usize)>,
 }
 
 impl DataLinkReceiver for DataLinkReceiverImpl {
     fn next(&mut self) -> io::Result<&[u8]> {
-        // Loopback packets arrive with a 4 byte header instead of normal ethernet header.
-        // Discard that header and replace with zeroed out ethernet header.
-        let (header_size, buffer_offset) = if self.loopback {
-            (4, ETHERNET_HEADER_SIZE)
-        } else {
-            (0, 0)
-        };
+        let header_size = if self.loopback { NULL_HEADER_SIZE } else { 0 };
+
         if self.packets.is_empty() {
-            let buffer = &mut self.read_buffer[buffer_offset..];
+            let buffer = &mut self.read_buffer[self.buffer_offset..];
             let ret = unsafe {
                 libc::FD_SET(self.fd.fd, &mut self.fd_set as *mut libc::fd_set);
                 libc::pselect(
@@ -425,9 +430,9 @@ impl DataLinkReceiver for DataLinkReceiverImpl {
             }
         }
         let (start, mut len) = self.packets.pop_front().unwrap();
-        len += buffer_offset;
+        len += self.buffer_offset;
         // Zero out part that will become fake ethernet header if on loopback.
-        for i in (&mut self.read_buffer[start..start + buffer_offset]).iter_mut() {
+        for i in (&mut self.read_buffer[start..start + self.buffer_offset]).iter_mut() {
             *i = 0;
         }
         Ok(&self.read_buffer[start..start + len])