netdev - Re: [PATCH] net/mlx4_en: Fix bpf_prog_add ref

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Date:   Wed, 09 Nov 2016 10:57:32 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Zhiyi Sun <zhiyisun@...il.com>
CC:     bblanco@...mgrid.com, Tariq Toukan <tariqt@...lanox.com>,
        Yishai Hadas <yishaih@...lanox.com>, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        alexei.starovoitov@...il.com
Subject: Re: [PATCH] net/mlx4_en: Fix bpf_prog_add ref_cnt in mlx4

On 11/09/2016 10:45 AM, Zhiyi Sun wrote:
> On Wed, Nov 09, 2016 at 10:05:31AM +0100, Daniel Borkmann wrote:
>> On 11/09/2016 08:35 AM, Zhiyi Sun wrote:
>>> There are rx_ring_num queues. Each queue will load xdp prog. So
>>> bpf_prog_add() should add rx_ring_num to ref_cnt.
>>>
>>> Signed-off-by: Zhiyi Sun <zhiyisun@...il.com>
>>
>> Your analysis looks incorrect to me. Please elaborate in more detail why
>> you think current code is buggy ...
>
> Yes, you are correct. My patch is incorrect. It is not a bug.
>
>> Call path is dev_change_xdp_fd(), which does bpf_prog_get_type() on the
>> fd. This already takes a ref and only drops it in case of error. Thus
>> in mlx4_xdp_set(), you only need priv->rx_ring_num - 1 refs for the rest
>> of the rings, so that dropping refs from old_prog makes sure we release
>> it again. Looks correct to me (maybe a comment would have helped there).
>
> I thought mlx4's code is incorrect because in mlx5's driver, function
> mlx5e_xdp_set() calls a pair of bpf_prog_add/put, the number of add and
> put to the refs are same. I didn't notice that one "add" has been called in its
> calller. So, it seems that mlx5's code is incorrect, right?

Yep, I think the two attached patches are needed.

The other thing I noticed in mlx5e_create_rq() is that it calls
bpf_prog_add(rq->xdp_prog, 1) without actually checking for errors.

View attachment "0001-bpf-mlx4-fix-prog-refcount-in-mlx4_en_try_alloc_reso.patch" of type "text/x-patch" (3022 bytes)

View attachment "0002-bpf-mlx5-fix-prog-refcount-in-mlx5e_xdp_set.patch" of type "text/x-patch" (1475 bytes)