Subject: Re: [PATCH bpf-next 2/2] selftests/bpf: fix "metadata marker" getting overwritten by the netstack

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Thu, 16 Mar 2023 21:10:02 +0100

Alexander Lobakin <[email-masked]> writes:

> Alexei noticed xdp_do_redirect test on BPF CI started failing on
> BE systems after skb PP recycling was enabled:
>
> test_xdp_do_redirect:PASS:prog_run 0 nsec
> test_xdp_do_redirect:PASS:pkt_count_xdp 0 nsec
> test_xdp_do_redirect:PASS:pkt_count_zero 0 nsec
> test_xdp_do_redirect:FAIL:pkt_count_tc unexpected pkt_count_tc: actual
> 220 != expected 9998
> test_max_pkt_size:PASS:prog_run_max_size 0 nsec
> test_max_pkt_size:PASS:prog_run_too_big 0 nsec
> close_netns:PASS:setns 0 nsec
>  #289 xdp_do_redirect:FAIL
> Summary: 270/1674 PASSED, 30 SKIPPED, 1 FAILED
>
> and it doesn't happen on LE systems.
> Ilya then hunted it down to:
>
>  #0  0x0000000000aaeee6 in neigh_hh_output (hh=0x83258df0,
> skb=0x88142200) at linux/include/net/neighbour.h:503
>  #1  0x0000000000ab2cda in neigh_output (skip_cache=false,
> skb=0x88142200, n=<optimized out>) at linux/include/net/neighbour.h:544
>  #2  ip6_finish_output2 (net=net@entry=0x88edba00, sk=sk@entry=0x0,
> skb=skb@entry=0x88142200) at linux/net/ipv6/ip6_output.c:134
>  #3  0x0000000000ab4cbc in __ip6_finish_output (skb=0x88142200, sk=0x0,
> net=0x88edba00) at linux/net/ipv6/ip6_output.c:195
>  #4  ip6_finish_output (net=0x88edba00, sk=0x0, skb=0x88142200) at
> linux/net/ipv6/ip6_output.c:206
>
> xdp_do_redirect test places a u32 marker (0x42) right before the Ethernet
> header to check it then in the XDP program and return %XDP_ABORTED if it's
> not there. Neigh xmit code likes to round up hard header length to speed
> up copying the header, so it overwrites two bytes in front of the Eth
> header. On LE systems, 0x42 is one byte at `data - 4`, while on BE it's
> `data - 1`, what explains why it happens only there.
> It didn't happen previously due to that %XDP_PASS meant the page will be
> discarded and replaced by a new one, but now it can be recycled as well,
> while bpf_test_run code doesn't reinitialize the content of recycled
> pages. This mark is limited to this particular test and its setup though,
> so there's no need to predict 1000 different possible cases. Just move
> it 4 bytes to the left, still keeping it 32 bit to match on more
> bytes.

Wow, this must have been annoying to track down - nice work :)

> Fixes: 9c94bbf9a87b ("xdp: recycle Page Pool backed skbs built from XDP frames")
> Reported-by: Alexei Starovoitov <[email-masked]>
> Link: https://lore.kernel.org/bpf/CAADnVQ+B_JOU+EpP=[email-masked]
> Reported-by: Ilya Leoshkevich <[email-masked]> # + debugging
> Link: https://lore.kernel.org/bpf/[email-masked]
> Signed-off-by: Alexander Lobakin <[email-masked]>

Acked-by: Toke Høiland-Jørgensen <[email-masked]>