mmap_xen: Don't drop the FileOffset while in use

The drop implementation of `struct MmapXenGrant` issues an Ioctl call,
which uses the file descriptor.

Currently the FileOffset is getting dropped before that, and results in
"Bad File Descriptor" error from the Ioctl. This commit fixes it by
keeping a copy of the same in `struct MmapXenGrant`.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

Author: vireshk

src/mmap_xen.rs

@@ -787,32 +787,32 @@ fn ioctl_gntdev_unmap_grant_ref() -> c_ulong {
 }
 
 // Xen grant memory specific implementation.
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, Debug)]
 struct MmapXenGrant {
     guest_base: GuestAddress,
     unix_mmap: Option<MmapUnix>,
+    file_offset: FileOffset,
     flags: i32,
-    fd: i32,
     size: usize,
     index: u64,
     domid: u32,
 }
 
 impl AsRawFd for MmapXenGrant {
     fn as_raw_fd(&self) -> i32 {
-        self.fd
+        self.file_offset.file().as_raw_fd()
     }
 }
 
 impl MmapXenGrant {
     fn new(range: &MmapRange, mmap_flags: MmapXenFlags) -> Result<Self> {
-        let (fd, _) = validate_file(&range.file_offset)?;
+        validate_file(&range.file_offset)?;
 
         let mut grant = Self {
             guest_base: range.addr,
             unix_mmap: None,
+            file_offset: range.file_offset.as_ref().unwrap().clone(),
             flags: range.flags.ok_or(Error::UnexpectedError)?,
-            fd,
             size: 0,
             index: 0,
             domid: range.mmap_data,
@@ -838,7 +838,7 @@ impl MmapXenGrant {
     fn mmap_range(&self, addr: GuestAddress, size: usize, prot: i32) -> Result<(MmapUnix, u64)> {
         let (count, size) = pages(size);
         let index = self.mmap_ioctl(addr, count)?;
-        let unix_mmap = MmapUnix::new(size, prot, self.flags, self.fd, index)?;
+        let unix_mmap = MmapUnix::new(size, prot, self.flags, self.as_raw_fd(), index)?;
 
         Ok((unix_mmap, index))
     }