Re: [PATCH net-next v1 2/2] netpoll: avoid reference leaks

[David Decotigny]

Hi,

Thanks for the feedback. This patch results from manual inspection of
the code. I agree my commit description is abusive: in the case of
bonding, I think everything is fine, there should be no ref leak,
cleanup paths seem clean.

My point was to make things more predictable: ndo_netpoll_cleanup
called anyways to acknowledge actual loss of a ref to npinfo,
irrespective of whether it's the last ref or not. Without this patch,
calling ndo_netpoll_cleanup would depend on some timing behavior, hard
to predict, and users of the API have better be careful to reclaim the
refs manually anyways: as a consequence, not sure this callback is
actually required in its current inception.

On Mon, Jul 7, 2014 at 7:35 PM, David Miller <davem@xxxxxxxxxxxxx> wrote:
> From: David Decotigny <decot@xxxxxxxxxxxx>
> Date: Mon, 30 Jun 2014 16:50:10 -0700
>
>> This ensures that the ndo_netpoll_cleanup callback is called for every
>> device that provides one. Otherwise there is a risk of reference leak
>> with bonding for example, which depends on this callback to cleanup
>> the slaves' references to netpoll info.
>>
>> Tested:
>>   see patch "netpoll: fix use after free"
>>
>> Signed-off-by: David Decotigny <decot@xxxxxxxxxxxx>
>
> I definitely don't understand this.
>
> Why would we call the cleanup function of an object before it's
> reference count hits zero?  It is exactly the act of reaching a
> zero refcount which should trigger invoking the cleanup callback.
>
> If a refcount is being released in another location without checking
> if it hits zero and invoking the cleanup if so, _THAT_ is the bug.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/