devfs: BKL *not* taken while opening devices

[Russell King <rmk@arm.linux.org.uk>]

Hi,

Nice simple bug report.

Kernel 2.5.8... Devfs devfs_open() bypasses the normal chrdev_open and
blkdev_open functions, and misses out taking the BKL.  2.5.10 is the
same.

Certainly the tty layer (and probably many of the other devices as well)
require the BKL to be taken before calling the open method.

Also, the following looks wrong:

    if ( S_ISBLK (inode->i_mode) )
    {
        file->f_op = &def_blk_fops;
        if (df->ops) inode->i_bdev->bd_op = df->ops;
    }
    else file->f_op = fops_get ( (struct file_operations *) df->ops );
    if (file->f_op)
        err = file->f_op->open ? (*file->f_op->open) (inode, file) : 0;
    else
    {
        /*  Fallback to legacy scheme  */
        if ( S_ISCHR (inode->i_mode) ) err = chrdev_open (inode, file);
        else err = -ENODEV;
    }
    if (err < 0) return err;

We can return without releasing the file operations after fops_get(),
thereby effectively locking modules in memory.

-- 
Russell King (rmk@arm.linux.org.uk)                The developer of ARM Linux
             http://www.arm.linux.org.uk/personal/aboutme.html