fix(vmnet): skip zero-length buffers

The second buffer returned from split_at_checked may have length 0.

Signed-off-by: Changyuan Lyu <changyuanl@google.com>

Author: Lencerf

alioth/src/virtio/dev/net/vmnet.rs

@@ -404,28 +404,7 @@ impl VirtioMio for Net {
 
 fn read_from_vmnet(interface: *mut VmnetInterface) -> impl FnMut(&mut DescChain) -> Result<Status> {
     move |chain: &mut DescChain| {
-        let mut trim_len = size_of::<VirtioNetHdr>();
-        let mut iov = Vec::with_capacity(chain.writable.len());
-
-        for buf in chain.writable.iter_mut() {
-            if trim_len > 0 {
-                if let Some((_, tail)) = buf.split_at_mut_checked(trim_len) {
-                    iov.push(libc::iovec {
-                        iov_base: tail.as_ptr() as *mut c_void,
-                        iov_len: tail.len(),
-                    });
-                    trim_len = 0;
-                } else {
-                    trim_len -= buf.len();
-                }
-            } else {
-                iov.push(libc::iovec {
-                    iov_base: buf.as_ptr() as *mut c_void,
-                    iov_len: buf.len(),
-                });
-            }
-        }
-
+        let mut iov = trim_desc_chain(chain.writable.iter().map(|b| &**b));
         let size = iov.iter().map(|s| s.iov_len).sum();
         let mut packets = VmPktDesc {
             vm_pkt_size: size,
@@ -455,32 +434,11 @@ fn read_from_vmnet(interface: *mut VmnetInterface) -> impl FnMut(&mut DescChain)
 
 fn write_to_vmnet(interface: *mut VmnetInterface) -> impl FnMut(&mut DescChain) -> Result<Status> {
     move |chain: &mut DescChain| {
-        let mut trim_len = size_of::<VirtioNetHdr>();
-        let mut iov = Vec::with_capacity(chain.readable.len());
-
-        for buf in chain.readable.iter() {
-            if trim_len > 0 {
-                if let Some((_, tail)) = buf.split_at_checked(trim_len) {
-                    iov.push(libc::iovec {
-                        iov_base: tail.as_ptr() as *mut c_void,
-                        iov_len: tail.len(),
-                    });
-                    trim_len = 0;
-                } else {
-                    trim_len -= buf.len();
-                }
-            } else {
-                iov.push(libc::iovec {
-                    iov_base: buf.as_ptr() as *mut c_void,
-                    iov_len: buf.len(),
-                });
-            }
-        }
-
+        let mut iov = trim_desc_chain(chain.readable.iter().map(|b| &**b));
         let size = iov.iter().map(|s| s.iov_len).sum();
         let mut packets = VmPktDesc {
             vm_pkt_size: size,
-            vm_pkt_iov: iov.as_ptr() as *mut libc::iovec,
+            vm_pkt_iov: iov.as_mut_ptr(),
             vm_pkt_iovcnt: iov.len() as u32,
             vm_flags: 0,
         };
@@ -508,3 +466,29 @@ impl DevParam for NetVmnetParam {
         Net::new(self, name)
     }
 }
+
+fn trim_desc_chain<'m>(bufs: impl Iterator<Item = &'m [u8]>) -> Vec<libc::iovec> {
+    let mut iov = Vec::new();
+    let mut trim_len = size_of::<VirtioNetHdr>();
+
+    for buf in bufs {
+        let b = if trim_len > 0 {
+            if let Some((_, tail)) = buf.split_at_checked(trim_len)
+                && !tail.is_empty()
+            {
+                trim_len = 0;
+                tail
+            } else {
+                trim_len -= buf.len();
+                continue;
+            }
+        } else {
+            buf
+        };
+        iov.push(libc::iovec {
+            iov_base: b.as_ptr() as *mut c_void,
+            iov_len: b.len(),
+        });
+    }
+    iov
+}