mmap: get file size with seek instead of metadata

Currently mmap::check_file_offset attempts to validate the backing
object by checking, among other things, its size to confirm the
mapping will fit on it. This check is implemented by using
std::fs::metadata to obtain the file size, but this approach doesn't
work when the backing object is a block or char device, as the
returned value is always zero.

In this commit, we replace the use of std::fs:metadata with
std::io::Seek::seek(SeekEnd(0)), which works fine even with special
files such as block and char devices. As a reference, this is the
method that QEMU uses for the same goal.

Fixes: #195
Signed-off-by: Sergio Lopez <[email protected]>

Author: slp

src/mmap.rs

@@ -16,6 +16,8 @@ use std::borrow::Borrow;
 use std::error;
 use std::fmt;
 use std::io::{Read, Write};
+#[cfg(unix)]
+use std::io::{Seek, SeekFrom};
 use std::ops::Deref;
 use std::result;
 use std::sync::atomic::Ordering;
@@ -116,14 +118,17 @@ pub fn check_file_offset(
     file_offset: &FileOffset,
     size: usize,
 ) -> result::Result<(), MmapRegionError> {
-    let file = file_offset.file();
+    let mut file = file_offset.file();
     let start = file_offset.start();
 
     if let Some(end) = start.checked_add(size as u64) {
-        if let Ok(metadata) = file.metadata() {
-            if metadata.len() < end {
-                return Err(MmapRegionError::MappingPastEof);
-            }
+        let filesize = file
+            .seek(SeekFrom::End(0))
+            .map_err(MmapRegionError::SeekEnd)?;
+        file.seek(SeekFrom::Start(0))
+            .map_err(MmapRegionError::SeekStart)?;
+        if filesize < end {
+            return Err(MmapRegionError::MappingPastEof);
         }
     } else {
         return Err(MmapRegionError::InvalidOffsetLength);

src/mmap_unix.rs

@@ -37,6 +37,10 @@ pub enum Error {
     MappingPastEof,
     /// The `mmap` call returned an error.
     Mmap(io::Error),
+    /// Seeking the end of the file returned an error.
+    SeekEnd(io::Error),
+    /// Seeking the start of the file returned an error.
+    SeekStart(io::Error),
 }
 
 impl fmt::Display for Error {
@@ -60,6 +64,8 @@ impl fmt::Display for Error {
                 "The specified file offset and length is greater then file length"
             ),
             Error::Mmap(error) => write!(f, "{}", error),
+            Error::SeekEnd(error) => write!(f, "Error seeking the end of the file: {}", error),
+            Error::SeekStart(error) => write!(f, "Error seeking the start of the file: {}", error),
         }
     }
 }