Re: [PATCH bpf-next] net: core: Fix build with CONFIG_IPV6=m

On 10/04/2018 08:33 PM, Joe Stringer wrote:

On Thu, 4 Oct 2018 at 01:48, Daniel Borkmann [off-list ref] wrote:

quoted

On 10/03/2018 07:32 AM, Joe Stringer wrote:

quoted

Stephen Rothwell reports the following link failure with IPv6 as module:

  x86_64-linux-gnu-ld: net/core/filter.o: in function `sk_lookup':
  (.text+0x19219): undefined reference to `__udp6_lib_lookup'

Fix the build by only enabling the IPv6 socket lookup if IPv6 support is
compiled into the kernel.

Signed-off-by: Joe Stringer <redacted>
---
 net/core/filter.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index 591c698bc517..30c6b2d3ef16 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -4838,7 +4838,7 @@ struct sock *sk_lookup(struct net *net, struct bpf_sock_tuple *tuple,
                      sk = __udp4_lib_lookup(net, src4, tuple->ipv4.sport,
                                             dst4, tuple->ipv4.dport,
                                             dif, sdif, &udp_table, skb);
-#if IS_ENABLED(CONFIG_IPV6)
+#if IS_REACHABLE(CONFIG_IPV6)
      } else {
              struct in6_addr *src6 = (struct in6_addr *)&tuple->ipv6.saddr;
              struct in6_addr *dst6 = (struct in6_addr *)&tuple->ipv6.daddr;

Applied as a quick fix, thanks Joe, but ideally this should also work when ipv6
is compiled as a module. There's the ipv6_bpf_stub, which does that job for other
helpers that would call into v6 code out of the builtin filter.c, so I think we
should follow the same approach here as well. See commit d74bad4e74ee ("bpf:
Hooks for sys_connect").

Thanks for the pointers, I'll look into that.

To confirm my understanding, is it possible to unload the IPv6 module?
I don't see any code that uninitializes "ipv6_bpf_stub". Seems like a
simple conditional check on that variable should be enough to gate its
usage from packet paths where sk_lookup could be invoked (Given that
the system could receive any packets, including IPv6 when the module
is not loaded).

No unload, this has been removed via 8ce440610357 ("ipv6: do not allow
ipv6 module to be removed").

Thanks,
Daniel