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

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Date:   Wed, 25 Jul 2018 19:11:49 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Taehee Yoo <ap420073@...il.com>
Cc:     daniel@...earbox.net, ast@...nel.org, bjorn.topel@...el.com,
        brouer@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH V2 bpf] xdp: add NULL pointer check in __xdp_return()

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

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