mmap: support for devdax and block devices

metadata.len() returns 0 when trying to mmap files such as block
devices and devdax (character devices) that are not regular files,
hence returning MappingPastEof even if the mapping would fit at the
provided file_offset.

This patch adds support for checking the size of devdax and block
devices, and returns a new error, InvalidFileType, if the mmap being
created is not for a regular file, block or devdax device, or if the
size the devices couldn't be found in sysfs.

Usecase:
Devdax and block devices can be used in cloud-hypervisor as
memory-zone. MmapRegion::build from vm-memory is called while
creating a GuestRegionMmap for the VM memory-zone.

Reviewed-by: Fam Zheng <[email protected]>
Signed-off-by: Usama Arif <[email protected]>

src/mmap.rs

@@ -15,8 +15,14 @@
 use std::borrow::Borrow;
 use std::error;
 use std::fmt;
+#[cfg(unix)]
+use std::fs::{read_link, read_to_string, File};
 use std::io::{Read, Write};
 use std::ops::Deref;
+#[cfg(unix)]
+use std::os::unix::fs::FileTypeExt;
+#[cfg(unix)]
+use std::os::unix::io::AsRawFd;
 use std::result;
 use std::sync::atomic::Ordering;
 use std::sync::Arc;
@@ -106,6 +112,56 @@ impl fmt::Display for Error {
 
 impl error::Error for Error {}
 
+#[cfg(unix)]
+/// Return the size of `file`.
+///
+/// For regular files this returns the files' metadata.length().
+/// For devdax and block devices, the size is checked in sysfs.
+fn calculate_file_size(file: &File) -> Option<u64> {
+    let metadata = file.metadata().ok()?;
+    let file_type = metadata.file_type();
+
+    if file_type.is_file() {
+        return Some(metadata.len());
+    }
+
+    let mut device_size_multiplier = 0;
+    let mut sys_fs_location = "";
+    let proc_file_name = format!("/proc/self/fd/{}", file.as_raw_fd());
+    let file_name = read_link(proc_file_name).ok()?;
+
+    if file_type.is_block_device() {
+        // block device size are reported as 512 byte blocks.
+        device_size_multiplier = 512;
+        sys_fs_location = "/sys/block";
+    } else if file_type.is_char_device() {
+        if let Some(file_name) = file_name.to_str() {
+            if file_name.starts_with("/dev/dax") {
+                device_size_multiplier = 1;
+                sys_fs_location = "/sys/bus/dax/devices";
+            } else {
+                return None;
+            }
+        }
+    } else {
+        return None;
+    }
+
+    if let Some(file_name) = file_name.to_str() {
+        if let Some(device_name) = file_name.split('/').last() {
+            if let Ok(device_size_str) =
+                read_to_string(format!("{}/{}/size", sys_fs_location, device_name))
+            {
+                if let Ok(device_size_int) = device_size_str.trim().parse::<u64>() {
+                    return Some(device_size_int * device_size_multiplier);
+                }
+            }
+        }
+    }
+
+    None
+}
+
 // TODO: use this for Windows as well after we redefine the Error type there.
 #[cfg(unix)]
 /// Checks if a mapping of `size` bytes fits at the provided `file_offset`.
@@ -119,14 +175,16 @@ pub fn check_file_offset(
     let 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 {
+    if let Some(file_size) = calculate_file_size(file) {
+        if let Some(end) = start.checked_add(size as u64) {
+            if file_size < end {
                 return Err(MmapRegionError::MappingPastEof);
             }
+        } else {
+            return Err(MmapRegionError::InvalidOffsetLength);
         }
     } else {
-        return Err(MmapRegionError::InvalidOffsetLength);
+        return Err(MmapRegionError::InvalidFileType);
     }
 
     Ok(())

src/mmap_unix.rs

@@ -29,6 +29,8 @@ pub enum Error {
     InvalidOffsetLength,
     /// The specified pointer to the mapping is not page-aligned.
     InvalidPointer,
+    /// The specified file type is invalid.
+    InvalidFileType,
     /// The forbidden `MAP_FIXED` flag was specified.
     MapFixed,
     /// Mappings using the same fd overlap in terms of file offset and length.
@@ -50,6 +52,7 @@ impl fmt::Display for Error {
                 f,
                 "The specified pointer to the mapping is not page-aligned",
             ),
+            Error::InvalidFileType => write!(f, "The specified file type is invalid"),
             Error::MapFixed => write!(f, "The forbidden `MAP_FIXED` flag was specified"),
             Error::MappingOverlap => write!(
                 f,