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