readat/writeat (#249)

* change read and write to use readat and writeat, removing excess seek call

* add comments

* add comments

* add comments

Author: rennergade

src/interface/file.rs

@@ -19,6 +19,7 @@ use libc::{mmap, mremap, munmap, off64_t, MAP_SHARED, MREMAP_MAYMOVE, PROT_READ,
 use std::convert::TryInto;
 use std::ffi::c_void;
 use std::os::unix::io::{AsRawFd, RawFd};
+use std::os::unix::fs::{FileExt};
 
 pub fn removefile(filename: String) -> std::io::Result<()> {
     let path: RustPathBuf = [".".to_string(), filename].iter().collect();
@@ -141,7 +142,9 @@ impl EmulatedFile {
         unsafe { libc::sync_file_range(*fd, offset as off64_t, nbytes as off64_t, flags) }
     }
 
-    // Read from file into provided C-buffer
+    // Wrapper around Rust's file object read_at function
+    // Reads from file at specified offset into provided C-buffer
+    // We need to specify the offset for read/write operations because multiple cages may refer to same system file handle
     pub fn readat(&self, ptr: *mut u8, length: usize, offset: usize) -> std::io::Result<usize> {
         let buf = unsafe {
             assert!(!ptr.is_null());
@@ -151,18 +154,19 @@ impl EmulatedFile {
         match &self.fobj {
             None => panic!("{} is already closed.", self.filename),
             Some(f) => {
-                let mut fobj = f.lock();
+                let fobj = f.lock();
                 if offset > self.filesize {
                     panic!("Seek offset extends past the EOF!");
                 }
-                fobj.seek(SeekFrom::Start(offset as u64))?;
-                let bytes_read = fobj.read(buf)?;
+                let bytes_read = fobj.read_at(buf, offset as u64)?;
                 Ok(bytes_read)
             }
         }
     }
 
-    // Write to file from provided C-buffer
+    // Wrapper around Rust's file object write_at function
+    // Writes from provided C-buffer into file at specified offset
+    // We need to specify the offset for read/write operations because multiple cages may refer to same system file handle
     pub fn writeat(
         &mut self,
         ptr: *const u8,
@@ -179,15 +183,15 @@ impl EmulatedFile {
         match &self.fobj {
             None => panic!("{} is already closed.", self.filename),
             Some(f) => {
-                let mut fobj = f.lock();
+                let fobj = f.lock();
                 if offset > self.filesize {
                     panic!("Seek offset extends past the EOF!");
                 }
-                fobj.seek(SeekFrom::Start(offset as u64))?;
-                bytes_written = fobj.write(buf)?;
+                bytes_written = fobj.write_at(buf, offset as u64)?;
             }
         }
 
+        // update our recorded filesize if we've written past the old filesize
         if offset + length > self.filesize {
             self.filesize = offset + length;
         }