Re: [PATCH] usb: gadget: u_serial: Add null pointer check in gserial_resume

[Prashanth K <quic_prashk@quicinc.com>]

On 09-02-23 09:33 pm, Alan Stern wrote:
> On Thu, Feb 09, 2023 at 09:13:37PM +0530, Prashanth K wrote:
>>
>>
>> On 09-02-23 08:39 pm, Alan Stern wrote:
>>> You should consider having _two_ spinlocks: One in the gs_port structure
>>> (the way it is now) and a separate global lock.  The first would be used
>>> in situations where you know you have a valid pointer.  The second would
>>> be used in situations where you don't know if the pointer is non-NULL
>>> or where you are changing the pointer's value.
>> Lets say we replaced the existing spinlock in gserial_resume and
>> gserial_disconnect with a new static spinlock, and kept the spinlocks in
>> other functions unchanged. In that case, wouldn't it cause additional race
>> conditions as we are using 2 different locks.
> 
> Not race conditions, but possibilities for deadlock.
> 
> Indeed, you would have to be very careful about avoiding deadlock
> scenarios.  In particular, you would have to ensure that the code never
> tries to acquire the global spinlock while already holding one of the
> per-port spinlocks.
> 
> Alan Stern
Hi Alan, instead of doing these and causing potential regressions, can 
we just have the null pointer check which i suggested in the beginning? 
The major concern was that port might become null after the null pointer 
check. We mark gser->ioport as null pointer in gserial_disconnect, and 
in gserial_resume we copy the gser->ioport to *port in the beginning.

struct gs_port *port = gser->ioport;

And hence it wont cause null pointer deref after the check as we don't 
de-reference anything from gser->ioport afterwards. We only use the 
local pointer *port afterwards.

Thanks,
Prashanth K