questionable code: always copy data from the beginning of buf when file is writing

[zhuqianbin]

In the following function, buf pointer will not moved forward according to bytes_write and it always copies data from the beginning of buf

if (copy_from_user(bh_data->b_data + pos % SIMPLEFS_BLOCK_SIZE, buf,
bytes_to_write)):

static ssize_t simplefs_write(struct file *file,
                              const char __user *buf,
                              size_t len,
                              loff_t *ppos)
{
    struct inode *inode = file_inode(file);
    struct super_block *sb = inode->i_sb;
    ssize_t bytes_write = 0;
    loff_t pos = *ppos;

    if (pos > inode->i_size)
        return 0;
    len = min_t(size_t, len, SIMPLEFS_MAX_FILESIZE - pos);

    /* find extent block */
    struct buffer_head *bh = sb_bread(sb, SIMPLEFS_INODE(inode)->ei_block);
    if (!bh)
        return -EIO;
    struct simplefs_file_ei_block *ei_block =
        (struct simplefs_file_ei_block *) bh->b_data;

    /* count block position */
    sector_t block_index = pos / SIMPLEFS_BLOCK_SIZE;
    sector_t ei_index = block_index / SIMPLEFS_MAX_BLOCKS_PER_EXTENT;

    /* write data */
    while (len > 0) {
        /* check if block is allocated */
        if (ei_block->extents[ei_index].ee_start == 0) {
            int bno = get_free_blocks(sb, 8);
            if (!bno) {
                bytes_write = -ENOSPC;
                break;
            }
            ei_block->extents[ei_index].ee_start = bno;
            ei_block->extents[ei_index].ee_len = 8;
            ei_block->extents[ei_index].ee_block =
                ei_index ? ei_block->extents[ei_index - 1].ee_block +
                               ei_block->extents[ei_index - 1].ee_len
                         : 0;
        }

        struct buffer_head *bh_data =
            sb_bread(sb, ei_block->extents[ei_index].ee_start +
                             block_index % SIMPLEFS_MAX_BLOCKS_PER_EXTENT);
        if (!bh_data) {
            pr_err("Failed to read data block %llu\n",
                   ei_block->extents[ei_index].ee_start +
                       block_index % SIMPLEFS_MAX_BLOCKS_PER_EXTENT);
            bytes_write = -EIO;
            break;
        }
        /* copy data from buffer */
        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,
                           bytes_to_write)) {
            brelse(bh_data);
            bytes_write = -EFAULT;
            break;
        }

        mark_buffer_dirty(bh_data);
        sync_dirty_buffer(bh_data);
        brelse(bh_data);

        /* successfully write data */
        len = len - bytes_to_write;
        bytes_write += bytes_to_write;
        pos += bytes_to_write;

        /* count extent block */
        block_index = pos / SIMPLEFS_BLOCK_SIZE;
        ei_index = block_index / SIMPLEFS_MAX_BLOCKS_PER_EXTENT;
    }
    mark_buffer_dirty(bh);
    sync_dirty_buffer(bh);
    brelse(bh);

    inode->i_size = max(pos, inode->i_size);
    inode->i_blocks = DIV_ROUND_UP(inode->i_size, SIMPLEFS_BLOCK_SIZE) + 1;
#if SIMPLEFS_AT_LEAST(6, 7, 0)
    struct timespec64 cur_time = current_time(inode);
    inode_set_mtime_to_ts(inode, cur_time);
    inode_set_ctime_to_ts(inode, cur_time);
#elif SIMPLEFS_AT_LEAST(6, 6, 0)
    struct timespec64 cur_time = current_time(inode);
    inode->i_mtime = cur_time;
    inode_set_ctime_to_ts(inode, cur_time);
#else
    inode->i_mtime = inode->i_ctime = current_time(inode);
#endif
    mark_inode_dirty(inode);
    *ppos = pos;

    return bytes_write;
}