Re: [PATCH] Input: serio - fix O(n^2) complexity in serio_unregister_driver()

[Mohamad Raizudeen]

On Wed, Apr 08, 2026 at 10:56:26AM -0700, Dmitry Torokhov wrote:
> On Wed, Apr 08, 2026 at 11:21:30PM +0530, Mohamad Raizudeen wrote:
> > Hi Dmitry,
> 
> Please do not top-post.
> 
> > 
> > *We do not have such setups at the moment, but what about parent's parent's
> > parent?*
> > You are right. Even though we don't have such setups today, let me explain
> > why the patch works for arbitrary depth.
> > 
> > If we have three ports linked like A->B->C (A is top, B is child of A, C is
> > child of B) and all use the same driver.
> 
> What happens if B uses different driver from A?
> 
> > 
> > C sees its parent B is using the same driver, skip C
> > B sees its parent A is using the same driver, skip B
> > A has no parent using the same driver, collect A
> > 
> > When we disconnect A, it automatically destroys B and C. So all ports are
> > cleaned up. The logic works for any number of levels.
> > 
> > * Could you explain more about the use-after-free scenario?*
> > If we collected both A and B, disconnecting A would free B. Then when we
> > try to process B from the list, we would use memory that is already freed
> > that leads to crash. My patch avoids this by never collecting a port whose
> > parent is also using the same driver.
> 
> But currently we restart scanning the list, so there won't be any stale
> entries. How would we end up with touching freed memory?
> 
> Thanks.
> 
> -- 
> Dmitry
Thank you for your careful review and for pointing out the mixed driver nestingscenario (A bound to driver X, B bound to driver Y, C bound to driver X). I completely missed that case.
You are right my 
My patch would collect both A and C, then disconnecting A would detroy B and C, leading to a use-after-free when C is later processed from the temporary list. The original goto approach handles this correctly by restarting the scan. 

I am sorry for sending a flawed patch. I will withdraw it.

I will try to deisign a better solution that works for all cases, includes mixed driver nesting, before submitting again.

Thank you again for your guidance.

Regards,
Mohamad Raizudeen.