[bug report] ext4: convert symlink external data block mapping to bdev

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

Hello Zhang Yi,

The patch 6493792d3299: "ext4: convert symlink external data block
mapping to bdev" from Apr 24, 2022 (linux-next), leads to the
following Smatch static checker warning:

	fs/ext4/namei.c:3353 ext4_init_symlink_block()
	error: potential NULL/IS_ERR bug 'bh'

fs/ext4/namei.c
    3337 static int ext4_init_symlink_block(handle_t *handle, struct inode *inode,
    3338                                    struct fscrypt_str *disk_link)
    3339 {
    3340         struct buffer_head *bh;
    3341         char *kaddr;
    3342         int err = 0;
    3343 
    3344         bh = ext4_bread(handle, inode, 0, EXT4_GET_BLOCKS_CREATE);
    3345         if (IS_ERR(bh))
    3346                 return PTR_ERR(bh);

>From reading the code, it looks like ext4_bread() can return both error
pointers and NULL.  (Second return statement).

    3347 
    3348         BUFFER_TRACE(bh, "get_write_access");
    3349         err = ext4_journal_get_write_access(handle, inode->i_sb, bh, EXT4_JTR_NONE);
    3350         if (err)
    3351                 goto out;
    3352 
--> 3353         kaddr = (char *)bh->b_data;
                                 ^^^^
Unchecked dereference

    3354         memcpy(kaddr, disk_link->name, disk_link->len);
    3355         inode->i_size = disk_link->len - 1;
    3356         EXT4_I(inode)->i_disksize = inode->i_size;
    3357         err = ext4_handle_dirty_metadata(handle, inode, bh);
    3358 out:
    3359         brelse(bh);
    3360         return err;
    3361 }

regards,
dan carpenter