Fix problem with sendfile(2) writing less data

When the amount of data read is more than the amount written, sendfile would not
adjust 'in file' position and would resume from the wrong location.

Closes #33

PiperOrigin-RevId: 196731287
Change-Id: Ia219895dd765016ed9e571fd5b366963c99afb27

pkg/sentry/syscalls/linux/sys_file.go

@@ -1929,8 +1929,16 @@ func Sendfile(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sysc
 		// If we don't have a provided offset.
 	} else {
 		// Send data using readv.
+		inOff := inFile.Offset()
 		r := &io.LimitedReader{R: &fs.FileReader{t, inFile}, N: count}
 		n, err = io.Copy(w, r)
+		inOff += n
+		if inFile.Offset() != inOff {
+			// Adjust file position in case more bytes were read than written.
+			if _, err := inFile.Seek(t, fs.SeekSet, inOff); err != nil {
+				return 0, nil, syserror.EIO
+			}
+		}
 	}
 
 	// We can only pass a single file to handleIOError, so pick inFile