[PATCH bpf 1/2] bpf: Add dummy type reference to nf_conn___init to fix type deduplication

["Toke Høiland-Jørgensen" <toke@redhat.com>]

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/

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@redhat.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
-- 
2.38.1