xen: fix reading from an offset inside a page

src/driver/xen.rs

@@ -127,22 +127,26 @@ impl Introspectable for Xen {
  cur_paddr = paddr + buf_offset;
  // get the current gfn
  let gfn = cur_paddr >> PAGE_SHIFT;
- let offset = u64::from(PAGE_SIZE - 1) & cur_paddr;
+ let page_offset = u64::from(PAGE_SIZE - 1) & cur_paddr;
  // map gfn
  let page = self
  .xen_fgn
  .map(self.domid, PROT_READ, gfn)
  .map_err(XenDriverError::from)?;
  // determine how much we can read
- let read_len = if (offset + count_mut as u64) > u64::from(PAGE_SIZE) {
- u64::from(PAGE_SIZE) - offset
+ let read_len = if (page_offset + count_mut as u64) > u64::from(PAGE_SIZE) {
+ u64::from(PAGE_SIZE) - page_offset
  } else {
  count_mut
  };
 
+ // prepare offsets
+ let buf_start = buf_offset as usize;
+ let buf_end = (buf_offset + read_len) as usize;
+ let page_start = page_offset as usize;
+ let page_end = (page_offset + read_len) as usize;
  // do the read
- let buf_offset_end = (buf_offset + read_len) as usize;
- buf[buf_offset as usize..buf_offset_end].copy_from_slice(&page[..read_len as usize]);
+ buf[buf_start..buf_end].copy_from_slice(&page[page_start..page_end]);
  // update loop variables
  count_mut -= read_len;
  buf_offset += read_len;