Fix user-space data write with correct offset

	The simplefs_write will copy data from user space. If the
input length is larger than BLOCK_SIZE, the data will not be copied
from user space buffer.

Author: RoyWFHuang

file.c

@@ -413,7 +413,7 @@ static ssize_t simplefs_write(struct file *file,
         size_t bytes_to_write =
             min_t(size_t, len, SIMPLEFS_BLOCK_SIZE - pos % SIMPLEFS_BLOCK_SIZE);
 
-        if (copy_from_user(bh_data->b_data + pos % SIMPLEFS_BLOCK_SIZE, buf,
+        if (copy_from_user(bh_data->b_data + pos % SIMPLEFS_BLOCK_SIZE, buf + bytes_write,
                            bytes_to_write)) {
             brelse(bh_data);
             bytes_write = -EFAULT;

script/test.sh

@@ -123,6 +123,18 @@ test_op 'dd if=/dev/zero of=file bs=1M count=12 status=none'
 filesize=$(sudo ls -lR  | grep -e "$F_MOD 2".*file | awk '{print $5}')
 test $filesize -le $MAXFILESIZE || echo "Failed, file size over the limit"
 
+# Write the file size larger than BLOCK_SIZE
+# test serial to write
+test_op 'bash -c "printf \"%.0s123456789\" {1..1600} > file.txt"'
+count=$(awk '{count += gsub(/123456789/, "")} END {print count}' "file.txt")
+echo "test $count"
+test "$count" -eq 1600 || echo "Failed, file size not matching"
+# test block to write
+test_op 'cat file.txt > checkfile.txt'
+count=$(awk '{count += gsub(/123456789/, "")} END {print count}' "checkfile.txt")
+echo "test $count"
+test "$count" -eq 1600 || echo "Failed, file size not matching"
+
 # test remove symbolic link
 test_op 'ln -s file symlink_fake'
 test_op 'rm -f symlink_fake'