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SubjectRe: [PATCH] loop: fput() called in loop_clr_fd() may cause bd_mutex recursive locking
On (12/17/11 22:30), Al Viro wrote:
> > Sorry, why is that a false positive?
> >
> > blkdev_put() calls lo_release() while holding bd_mutex,
> > lo_release() calls loop_clr_fd() -> fput(). fput() once again
> > attempts to grub already held bd_mutex calling blkdev_put().
> > Looks like a recursion to me.
>
> Because of this:
> /* Avoid recursion */
> f = file;
> while (is_loop_device(f)) {
> struct loop_device *l;
>
> if (f->f_mapping->host->i_bdev == bdev)
> goto out_putf;
>
> l = f->f_mapping->host->i_bdev->bd_disk->private_data;
> if (l->lo_state == Lo_unbound) {
> error = -EINVAL;
> goto out_putf;
> }
> f = l->lo_backing_file;
> }
> in loop_set_fd().

Oh, thanks. I didn't notice that one.

> Think of it for a minute - if we could run into the
> same bdev in that recursion, what would have happened on read() from
> that sucker? So yes, it is a false positive.

I've tried read()/write() some time ago and it worked. Perhaps, I just
wasn't "lucky" enough to hit any problems.

> And your patch would simply leave the underlying device opened,
> with all the consequences...
>
well, that sucks.


Sergey


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