bpf: egressgw: clarify egress ifindex for BPF redirect

When the FIB lookup in to-netdev returns the same ifindex as the current
interface, we want to skip the redirect. But that doesn't apply for the
lookup in from-overlay - here we *always* need to redirect to the selected
interface.

As we previously handled the !is_defined(HAVE_FIB_IFINDEX) case, we can
pass oif = 0 and trust that fib_do_redirect() will always use the ifindex
from the fib_params.

Signed-off-by: Julian Wiedmann <jwi@isovalent.com>

bpf/lib/egress_gateway.h

@@ -29,7 +29,7 @@ int egress_gw_fib_lookup_and_redirect(struct __ctx_buff *ctx, __be32 egress_ip,
 				      __s8 *ext_err)
 {
 	struct bpf_fib_lookup_padded fib_params = {};
-	__u32 old_oif = ctx_get_ifindex(ctx);
+	int oif = 0;
 
 	*ext_err = (__s8)fib_lookup_v4(ctx, &fib_params, egress_ip, daddr, 0);
 
@@ -49,10 +49,11 @@ int egress_gw_fib_lookup_and_redirect(struct __ctx_buff *ctx, __be32 egress_ip,
 		return DROP_NO_FIB;
 	}
 
-	if (old_oif == fib_params.l.ifindex)
+	/* Skip redirect in to-netdev if we stay on the same iface: */
+	if (is_defined(IS_BPF_HOST) && fib_params.l.ifindex == ctx_get_ifindex(ctx))
 		return CTX_ACT_OK;
 
-	return fib_do_redirect(ctx, true, &fib_params, false, ext_err, (int *)&old_oif);
+	return fib_do_redirect(ctx, true, &fib_params, false, ext_err, &oif);
 }
 
 #ifdef ENABLE_EGRESS_GATEWAY