From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Alexander Lobakin <alexandr.lobakin@intel.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>, bpf@vger.kernel.org,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH bpf] bpf, test_run: fix &xdp_frame misplacement for
 LIVE_FRAMES
In-Reply-To: <8d3a9feb-9ee5-4a49-330a-9a475e459228@intel.com>
References: <20230209172827.874728-1-alexandr.lobakin@intel.com>
 <87v8ka7gh5.fsf@toke.dk> <8d3a9feb-9ee5-4a49-330a-9a475e459228@intel.com>
Date:   Fri, 10 Feb 2023 18:38:45 +0100
Message-ID: <87lel5774q.fsf@toke.dk>
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Alexander Lobakin <alexandr.lobakin@intel.com> writes:

> From: Toke H=C3=B8iland-J=C3=B8rgensen <toke@redhat.com>
> Date: Thu, 09 Feb 2023 21:04:38 +0100
>
>> Alexander Lobakin <alexandr.lobakin@intel.com> writes:
>>=20
>>> &xdp_buff and &xdp_frame are bound in a way that
>>>
>>> xdp_buff->data_hard_start =3D=3D xdp_frame
>>>
>>> It's always the case and e.g. xdp_convert_buff_to_frame() relies on
>>> this.
>>> IOW, the following:
>>>
>>> 	for (u32 i =3D 0; i < 0xdead; i++) {
>>> 		xdpf =3D xdp_convert_buff_to_frame(&xdp);
>>> 		xdp_convert_frame_to_buff(xdpf, &xdp);
>>> 	}
>>>
>>> shouldn't ever modify @xdpf's contents or the pointer itself.
>>> However, "live packet" code wrongly treats &xdp_frame as part of its
>>> context placed *before* the data_hard_start. With such flow,
>>> data_hard_start is sizeof(*xdpf) off to the right and no longer points
>>> to the XDP frame.
>>=20
>> Oh, nice find!
>>=20
>>> Instead of replacing `sizeof(ctx)` with `offsetof(ctx, xdpf)` in several
>>> places and praying that there are no more miscalcs left somewhere in the
>>> code, unionize ::frm with ::data in a flex array, so that both starts
>>> pointing to the actual data_hard_start and the XDP frame actually starts
>>> being a part of it, i.e. a part of the headroom, not the context.
>>> A nice side effect is that the maximum frame size for this mode gets
>>> increased by 40 bytes, as xdp_buff::frame_sz includes everything from
>>> data_hard_start (-> includes xdpf already) to the end of XDP/skb shared
>>> info.
>>=20
>> I like the union approach, however...
>>=20
>>> (was found while testing XDP traffic generator on ice, which calls
>>>  xdp_convert_frame_to_buff() for each XDP frame)
>>>
>>> Fixes: b530e9e1063e ("bpf: Add "live packet" mode for XDP in BPF_PROG_R=
UN")
>>> Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
>>> ---
>>>  net/bpf/test_run.c | 13 ++++++++-----
>>>  1 file changed, 8 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
>>> index 2723623429ac..c3cce7a8d47d 100644
>>> --- a/net/bpf/test_run.c
>>> +++ b/net/bpf/test_run.c
>>> @@ -97,8 +97,11 @@ static bool bpf_test_timer_continue(struct bpf_test_=
timer *t, int iterations,
>>>  struct xdp_page_head {
>>>  	struct xdp_buff orig_ctx;
>>>  	struct xdp_buff ctx;
>>> -	struct xdp_frame frm;
>>> -	u8 data[];
>>> +	union {
>>> +		/* ::data_hard_start starts here */
>>> +		DECLARE_FLEX_ARRAY(struct xdp_frame, frm);
>>> +		DECLARE_FLEX_ARRAY(u8, data);
>>> +	};
>>=20
>> ...why does the xdp_frame need to be a flex array? Shouldn't this just b=
e:
>>=20
>>  +	union {
>>  +		/* ::data_hard_start starts here */
>>  +		struct xdp_frame frm;
>>  +		DECLARE_FLEX_ARRAY(u8, data);
>>  +	};
>>=20
>> which would also get rid of the other three hunks of the patch?
>
> That was my first thought. However, as I mentioned in between the lines
> in the commitmsg, this doesn't decrease the sizeof(ctx), so we'd have to
> replace those sizeofs with offsetof() in a couple places (-> the patch
> length would be the same). So I went this way to declare that frm
> doesn't belong to ctx but to the headroom.

Ah, right, I see! Okay, let's keep both as flex arrays, then. One other
nit, though: after your patch, we'll end up with this:

	frm =3D head->frm;
	data =3D &head->data;

both of those assignments refer to flex arrays, which seems a bit
inconsistent. The second one works because it's assigning to a void
pointer, so the compiler doesn't complain about the type mismatch; but
it should work with just 'data =3D head->data' as well, so can we update
that as well for consistency?

-Toke