Re: [PATCH] mm: shrinkers: fix race condition on debugfs cleanup

[Matthew Wilcox <willy@infradead.org>]

On Wed, Jul 26, 2023 at 12:08:26AM +0000, Joan Bruguera Micó wrote:
> Hmmm, indeed linux-next crashes with a null pointer dereference when
> calling `ida_free(..., -1)`.
> 
> It appears to me that 69cb69ea5542 ("ida: Remove assertions that an ID was
> allocated") didn't intend to make `ida_free(..., -1)` invalid; after all,
> it was authored & introduced immediately after fc82bbf4dede ("ida: don't
> use BUG_ON() for debugging") whose commit message calls for making it
> legal, with Matthew's support.
> And the referenced Bluetooth HCI code that also calls `ida_free(..., -1)`
> is still there, as far as I can tell ([1]).
> 
> Rather, probably the `((int)id < 0)` was accidentally dropped, or the idea
> was that it would be handled by the `not_found:` label in `ida_free`, but
> for that to work you'd need to change the `!test_bit(bit, bitmap->bitmap)`
> condition to `!bitmap || !test_bit(bit, bitmap->bitmap)` since otherwise
> `bitmap->bitmap` is a null pointer dereference.

I've been in two minds about that patch ever since I wrote it.  I've
dropped it from my tree for now.

But, er, you have pointed out a bug which is that we don't handle
!bitmap properly.  That should be fixed.