netdev - Re: [PATCH bpf 1/2] bpf: Add dummy type reference to nf_conn__

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Date:   Wed, 30 Nov 2022 17:16:06 -0800
From:   Yonghong Song <yhs@...a.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Jiri Benc <jbenc@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH bpf 1/2] bpf: Add dummy type reference to nf_conn___init
 to fix type deduplication



On 11/30/22 6:42 AM, Toke Høiland-Jørgensen wrote:
> The bpf_ct_set_nat_info() kfunc is defined in the nf_nat.ko module, and
> takes as a parameter the nf_conn___init struct, which is allocated through
> the bpf_xdp_ct_alloc() helper defined in the nf_conntrack.ko module.
> However, because kernel modules can't deduplicate BTF types between each
> other, and the nf_conn___init struct is not referenced anywhere in vmlinux
> BTF, this leads to two distinct BTF IDs for the same type (one in each
> module). This confuses the verifier, as described here:
> 
> https://lore.kernel.org/all/87leoh372s.fsf@toke.dk/

We might have similar issues later for other types.
Not sure whether the root cause is in libbpf or verifier. But we know
the kfunc from (module, btf_id), so for arguments, we could first
search the corresponding module and then vmlinux for btf_id matching?
This way we might fix potential other cases?

> 
> As a workaround, add a dummy pointer to the type in net/filter.c, so the
> type definition gets included in vmlinux BTF. This way, both modules can
> refer to the same type ID (as they both build on top of vmlinux BTF), and
> the verifier is no longer confused.
> 
> Fixes: 820dc0523e05 ("net: netfilter: move bpf_ct_set_nat_info kfunc in nf_nat_bpf.c")
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>   net/core/filter.c | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index bb0136e7a8e4..1bdf9efe8593 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -80,6 +80,7 @@
>   #include <net/tls.h>
>   #include <net/xdp.h>
>   #include <net/mptcp.h>
> +#include <net/netfilter/nf_conntrack_bpf.h>
>   
>   static const struct bpf_func_proto *
>   bpf_sk_base_func_proto(enum bpf_func_id func_id);
> @@ -11531,3 +11532,17 @@ bpf_sk_base_func_proto(enum bpf_func_id func_id)
>   
>   	return func;
>   }
> +
> +#if IS_MODULE(CONFIG_NF_CONNTRACK) && IS_ENABLED(CONFIG_DEBUG_INFO_BTF_MODULES)
> +/* The nf_conn___init type is used in the NF_CONNTRACK kfuncs. The kfuncs are
> + * defined in two different modules, and we want to be able to use them
> + * interchangably with the same BTF type ID. Because modules can't de-duplicate
> + * BTF IDs between each other, we need the type to be referenced in the vmlinux
> + * BTF or the verifier will get confused about the different types. So we add
> + * this dummy pointer to serve as a type reference which will be included in
> + * vmlinux BTF, allowing both modules to refer to the same type ID.
> + *
> + * We use a pointer as that is smaller than an instance of the struct.
> + */
> +const struct nf_conn___init *ctinit;
> +#endif