Re: [PATCH] can: j1939: Remove unnecessary WARN_ON_ONCE in j1939_sk_queue_activate_next_locked()

[Fedor Pchelkin <pchelkin@ispras.ru>]

Hello Oleksij,

I'm sorry for late answering.

On 20.07.2022 22:13, Oleksij Rempel wrote:
>> Are you working on some system where this use case is valid?

No, we are fuzzing the kernel and analyzing different warnings and
crashes.

On 20.07.2022 22:13, Oleksij Rempel wrote:
 > yes

Well, there is a long story about where and for which purposes the
kernel warning macros should be correctly used and, overall,
WARN_ON_ONCE is not intended for user-space notification.

Linus Torvalds wrote:
 > WARN_ON() should only be used for "This cannot happen, but if it does,
 > I want to know how we got here".
 >
 > So if that j1939 thing is something that can be triggered by a user,
 > then the backtrace should be reported to the driver maintainer, and
 > then either
 >
 > (a) the WARN_ON_ONCE() should just be removed ("ok, this can happen,
 > we understand why it can happen, and it's fine")
 >
 > (b) the problem the WARN_ON_ONCE() reports about should be made
 > impossible some way
 >
 > (c) it might be downgraded to a pr_warn() if people really want to
 > tell user space that "guys, you're doing something wrong" and it's
 > considered a useful warning.

So WARN_ON_ONCE should be replaced with a more gentle variant - I think
pr_warn_once would suit this case. I've prepared a new patch for that,
it will follow this email.

Could you also look at the patch - [PATCH] can: j1939: fix memory leak 
of skbs - which I sent you on 08.07.2022, please?