netdev - Re: [PATCH V2 bpf] xdp: add NULL pointer check in __xdp

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Date:   Thu, 26 Jul 2018 23:05:07 +0900
From:   Taehee Yoo <ap420073@...il.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Daniel Borkmann <daniel@...earbox.net>, ast@...nel.org,
        Björn Töpel <bjorn.topel@...el.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH V2 bpf] xdp: add NULL pointer check in __xdp_return()

2018-07-26 21:07 GMT+09:00 Björn Töpel <bjorn.topel@...il.com>:
> Den tors 26 juli 2018 kl 04:14 skrev Jakub Kicinski
> <jakub.kicinski@...ronome.com>:
>>
>> On Thu, 26 Jul 2018 00:09:50 +0900, Taehee Yoo wrote:
>> > rhashtable_lookup() can return NULL. so that NULL pointer
>> > check routine should be added.
>> >
>> > Fixes: 02b55e5657c3 ("xdp: add MEM_TYPE_ZERO_COPY")
>> > Signed-off-by: Taehee Yoo <ap420073@...il.com>
>> > ---
>> > V2 : add WARN_ON_ONCE when xa is NULL.
>> >
>> >  net/core/xdp.c | 5 ++++-
>> >  1 file changed, 4 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/net/core/xdp.c b/net/core/xdp.c
>> > index 9d1f220..786fdbe 100644
>> > --- a/net/core/xdp.c
>> > +++ b/net/core/xdp.c
>> > @@ -345,7 +345,10 @@ static void __xdp_return(void *data, struct xdp_mem_info *mem, bool napi_direct,
>> >               rcu_read_lock();
>> >               /* mem->id is valid, checked in xdp_rxq_info_reg_mem_model() */
>> >               xa = rhashtable_lookup(mem_id_ht, &mem->id, mem_id_rht_params);
>> > -             xa->zc_alloc->free(xa->zc_alloc, handle);
>> > +             if (!xa)
>> > +                     WARN_ON_ONCE(1);
>>
>> nit: is compiler smart enough to figure out the fast path here?
>> WARN_ON_ONCE() has the nice side effect of wrapping the condition in
>> unlikely().  It could save us both LoC and potentially cycles to do:
>>
>> if (!WARN_ON_ONCE(!xa))
>>         xa->zc_alloc->free(xa->zc_alloc, handle);
>>
>> Although it admittedly looks a bit awkward.  I'm not sure if we have
>> some form of assert (i.e. positive check) in tree :S
>>
>
> I'm kind of in favor of this ^^^. Hopefully, Taehee is ok with another spin.
>

I like this code style and I think it has performance benefit.
So I will send v3 patch!

Thanks!

> Björn
>
>> > +             else
>> > +                     xa->zc_alloc->free(xa->zc_alloc, handle);
>> >               rcu_read_unlock();
>> >       default:
>> >               /* Not possible, checked in xdp_rxq_info_reg_mem_model() */