Alexander Lobakin <alexandr.lobakin@xxxxxxxxx> writes:
> From: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
> Date: Thu, 09 Feb 2023 21:04:38 +0100
>
>> Alexander Lobakin <alexandr.lobakin@xxxxxxxxx> writes:
>>
>>> &xdp_buff and &xdp_frame are bound in a way that
>>>
>>> xdp_buff->data_hard_start == xdp_frame
>>>
>>> It's always the case and e.g. xdp_convert_buff_to_frame() relies on
>>> this.
>>> IOW, the following:
>>>
>>> for (u32 i = 0; i < 0xdead; i++) {
>>> xdpf = 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.
>>
>> Oh, nice find!
>>
>>> 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.
>>
>> I like the union approach, however...
>>
>>> (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_RUN")
>>> Signed-off-by: Alexander Lobakin <alexandr.lobakin@xxxxxxxxx>
>>> ---
>>> 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);
>>> + };
>>
>> ...why does the xdp_frame need to be a flex array? Shouldn't this just be:
>>
>> + union {
>> + /* ::data_hard_start starts here */
>> + struct xdp_frame frm;
>> + DECLARE_FLEX_ARRAY(u8, data);
>> + };
>>
>> 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 = head->frm;
data = &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 = head->data' as well, so can we update
that as well for consistency?
-Toke
