Re: [PATCH] bpf: fix building without CONFIG_INET

[Joe Stringer]

On Fri, 5 Oct 2018 at 09:16, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> The newly added TCP and UDP handling fails to link when CONFIG_INET
> is disabled:
>
> net/core/filter.o: In function `sk_lookup':
> filter.c:(.text+0x7ff8): undefined reference to `tcp_hashinfo'
> filter.c:(.text+0x7ffc): undefined reference to `tcp_hashinfo'
> filter.c:(.text+0x8020): undefined reference to `__inet_lookup_established'
> filter.c:(.text+0x8058): undefined reference to `__inet_lookup_listener'
> filter.c:(.text+0x8068): undefined reference to `udp_table'
> filter.c:(.text+0x8070): undefined reference to `udp_table'
> filter.c:(.text+0x808c): undefined reference to `__udp4_lib_lookup'
> net/core/filter.o: In function `bpf_sk_release':
> filter.c:(.text+0x82e8): undefined reference to `sock_gen_put'
>
> The compiler can optimize it out and avoid those references for
> the most part, but we are missing a few steps here:
>
> - sk_lookup() should always have been marked 'static', this also
>   avoids a warning about a missing prototype when building with
>   'make W=1'.
> - The BPF_CALL_x() macro needs a little change to allow marking
>   the unneeded BPF call as 'static' and having the compiler
>   drop them.
> - The reference to the bpf_func_proto must be made conditional.
>
> Fixes: 6acc9b432e67 ("bpf: Add helper to retrieve socket in BPF")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---

Thanks for the fix.

>  include/linux/filter.h |  2 +-
>  net/core/filter.c      | 18 +++++++++++-------
>  2 files changed, 12 insertions(+), 8 deletions(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 6791a0ac0139..d9ec9d908bbe 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -428,9 +428,9 @@ struct sock_reuseport;
>                   u64, __ur_3, u64, __ur_4, u64, __ur_5)
>
>  #define BPF_CALL_x(x, name, ...)                                              \
> +       u64 name(__BPF_REG(x, __BPF_DECL_REGS, __BPF_N, __VA_ARGS__));         \
>         static __always_inline                                                 \
>         u64 ____##name(__BPF_MAP(x, __BPF_DECL_ARGS, __BPF_V, __VA_ARGS__));   \
> -       u64 name(__BPF_REG(x, __BPF_DECL_REGS, __BPF_N, __VA_ARGS__));         \
>         u64 name(__BPF_REG(x, __BPF_DECL_REGS, __BPF_N, __VA_ARGS__))          \
>         {                                                                      \
>                 return ____##name(__BPF_MAP(x,__BPF_CAST,__BPF_N,__VA_ARGS__));\

For what it's worth, other similar cases in net/core/filter.c avoid
this by just wrapping the relevant sections of the code in the #ifdef
as well.

Might be a bit simpler to follow that style (only checked with make
M=net/core W=1):

$ git di
diff --git a/net/core/filter.c b/net/core/filter.c
index 30c6b2d3ef16..4bbc6567fcb8 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4817,8 +4817,9 @@ static const struct bpf_func_proto
bpf_lwt_seg6_adjust_srh_proto = {
};
#endif /* CONFIG_IPV6_SEG6_BPF */

-struct sock *sk_lookup(struct net *net, struct bpf_sock_tuple *tuple,
-                      struct sk_buff *skb, u8 family, u8 proto)
+#ifdef CONFIG_INET
+static struct sock *sk_lookup(struct net *net, struct bpf_sock_tuple *tuple,
+                             struct sk_buff *skb, u8 family, u8 proto)
{
       int dif = skb->dev->ifindex;
       bool refcounted = false;
@@ -4951,6 +4952,7 @@ static const struct bpf_func_proto
bpf_sk_release_proto = {
       .ret_type       = RET_INTEGER,
       .arg1_type      = ARG_PTR_TO_SOCKET,
};
+#endif /* CONFIG_INET */

bool bpf_helper_changes_pkt_data(void *func)
{
@@ -5158,12 +5160,14 @@ tc_cls_act_func_proto(enum bpf_func_id
func_id, const struct bpf_prog *prog)
       case BPF_FUNC_skb_ancestor_cgroup_id:
               return &bpf_skb_ancestor_cgroup_id_proto;
#endif
+#ifdef CONFIG_INET
       case BPF_FUNC_sk_lookup_tcp:
               return &bpf_sk_lookup_tcp_proto;
       case BPF_FUNC_sk_lookup_udp:
               return &bpf_sk_lookup_udp_proto;
       case BPF_FUNC_sk_release:
               return &bpf_sk_release_proto;
+#endif
       default:
               return bpf_base_func_proto(func_id);
       }
@@ -5264,12 +5268,14 @@ sk_skb_func_proto(enum bpf_func_id func_id,
const struct bpf_prog *prog)
               return &bpf_sk_redirect_hash_proto;
       case BPF_FUNC_get_local_storage:
               return &bpf_get_local_storage_proto;
+#ifdef CONFIG_INET
       case BPF_FUNC_sk_lookup_tcp:
               return &bpf_sk_lookup_tcp_proto;
       case BPF_FUNC_sk_lookup_udp:
               return &bpf_sk_lookup_udp_proto;
       case BPF_FUNC_sk_release:
               return &bpf_sk_release_proto;
+#endif
       default:
               return bpf_base_func_proto(func_id);
       }