xdp: add a third way of loading the BPF program

We need that in kresd, as overwriting the program would detach
all the AF_XDP sockets and only the last kresd could receive.
It's not perfect, but I suppose we can bear that extraneous message.

src/libknot/xdp/af_xdp.c

@@ -236,7 +236,6 @@ static struct knot_xsk_socket *xsk_configure_socket(struct xsk_umem_info *umem,
 		.tx_size = XSK_RING_PROD__DEFAULT_NUM_DESCS,
 		.rx_size = XSK_RING_CONS__DEFAULT_NUM_DESCS,
 		.libbpf_flags = XSK_LIBBPF_FLAGS__INHIBIT_PROG_LOAD,
-		.xdp_flags = XDP_FLAGS_UPDATE_IF_NOEXIST,
 	};
 
 	errno = xsk_socket__create(&xsk_info->xsk, iface->ifname,
@@ -600,7 +599,7 @@ void knot_xsk_free_recvd(struct knot_xsk_socket *socket, const knot_xsk_msg_t *m
 
 _public_
 int knot_xsk_init(struct knot_xsk_socket **socket, const char *ifname, int if_queue,
-                  int listen_port, bool load_bpf)
+                  int listen_port, knot_xsk_load_bpf_t load_bpf)
 {
 	if (socket == NULL || *socket != NULL) {
 		return KNOT_EINVAL;

src/libknot/xdp/af_xdp.h

@@ -30,6 +30,19 @@ typedef struct {
 	struct iovec payload;
 } knot_xsk_msg_t;
 
+/*! \brief Styles of loading BPF program.
+ *
+ * \note In *all* the cases loading can only succeed if at the end
+ *   a compatible BPF program is loaded on the interface.
+ */
+typedef enum {
+	KNOT_XSK_LOAD_BPF_NEVER,  /*!< Do not load; error out if not loaded already. */
+	KNOT_XSK_LOAD_BPF_ALWAYS, /*!< Always load a program (overwrite it). */
+	KNOT_XSK_LOAD_BPF_MAYBE,  /*!< Try with present program or load if none. */
+	/* Implementation caveat: when re-using program in _MAYBE case, we get a message:
+	 * libbpf: Kernel error message: XDP program already attached */
+} knot_xsk_load_bpf_t;
+
 /*! \brief Context structure for une XDP socket. */
 struct knot_xsk_socket;
 
@@ -45,7 +58,7 @@ struct knot_xsk_socket;
  * \return KNOT_E*
  */
 int knot_xsk_init(struct knot_xsk_socket **socket, const char *ifname, int if_queue,
-                  int listen_port, bool load_bpf);
+                  int listen_port, knot_xsk_load_bpf_t load_bpf);
 
 /*! \brief De-init XDP socket. */
 void knot_xsk_deinit(struct knot_xsk_socket *socket);

src/libknot/xdp/bpf-user.c

@@ -26,9 +26,10 @@
 #include <unistd.h>
 
 #include <bpf/bpf.h>
+#include <linux/if_link.h>
 #include <net/if.h>
 
-static int ensure_udp_prog(struct kxsk_iface *iface, const char *prog_fname)
+static int ensure_udp_prog(struct kxsk_iface *iface, const char *prog_fname, bool overwrite)
 {
 	/* Use libbpf for extracting BPF byte-code from BPF-ELF object, and
 	 * loading this into the kernel via bpf-syscall */
@@ -38,12 +39,23 @@ static int ensure_udp_prog(struct kxsk_iface *iface, const char *prog_fname)
 		return KNOT_EPROGRAM;
 	}
 
-	ret = bpf_set_link_xdp_fd(iface->ifindex, prog_fd, 0);
+	ret = bpf_set_link_xdp_fd(iface->ifindex, prog_fd,
+			overwrite ? 0 : XDP_FLAGS_UPDATE_IF_NOEXIST);
 	if (ret) {
+		close(prog_fd);
+	}
+	if (ret == -EBUSY && !overwrite) { /* We try accepting the present program. */
+		uint32_t prog_id = 0;
+		ret = bpf_get_link_xdp_id(iface->ifindex, &prog_id, 0);
+		if (!ret && prog_id) {
+			ret = prog_fd = bpf_prog_get_fd_by_id(prog_id);
+		}
+	}
+	if (ret < 0) {
 		return KNOT_EFD;
+	} else {
+		return prog_fd;
 	}
-
-	return prog_fd;
 }
 
 static int array2file(char *filename, const uint8_t *array, unsigned len)
@@ -66,7 +78,7 @@ static int array2file(char *filename, const uint8_t *array, unsigned len)
 	return KNOT_EOK;
 }
 
-static int ensure_udp_prog_builtin(struct kxsk_iface *iface)
+static int ensure_udp_prog_builtin(struct kxsk_iface *iface, bool overwrite)
 {
 	if (bpf_kernel_o_len < 2) {
 		return KNOT_ENOTSUP;
@@ -78,7 +90,7 @@ static int ensure_udp_prog_builtin(struct kxsk_iface *iface)
 		return ret;
 	}
 
-	ret = ensure_udp_prog(iface, filename);
+	ret = ensure_udp_prog(iface, filename, overwrite);
 	unlink(filename);
 	return ret;
 }
@@ -194,7 +206,8 @@ int kxsk_socket_stop(const struct kxsk_iface *iface, int queue_id)
 	return err;
 }
 
-int kxsk_iface_new(const char *ifname, bool load_bpf, struct kxsk_iface **out_iface)
+int kxsk_iface_new(const char *ifname, knot_xsk_load_bpf_t load_bpf,
+		   struct kxsk_iface **out_iface)
 {
 	struct kxsk_iface *iface = calloc(1, sizeof(*iface));
 	if (iface == NULL) {
@@ -209,14 +222,23 @@ int kxsk_iface_new(const char *ifname, bool load_bpf, struct kxsk_iface **out_if
 	iface->qidconf_map_fd = iface->xsks_map_fd = -1;
 
 	int ret;
-	if (load_bpf) {
-		ret = ensure_udp_prog_builtin(iface);
-	} else {
+	switch (load_bpf) {
+	case KNOT_XSK_LOAD_BPF_NEVER:
+		(void)0;
 		uint32_t prog_id = 0;
 		ret = bpf_get_link_xdp_id(iface->ifindex, &prog_id, 0);
 		if (!ret && prog_id) {
 			ret = bpf_prog_get_fd_by_id(prog_id);
 		}
+		break;
+	case KNOT_XSK_LOAD_BPF_ALWAYS:
+		ret = ensure_udp_prog_builtin(iface, true);
+		break;
+	case KNOT_XSK_LOAD_BPF_MAYBE:
+		ret = ensure_udp_prog_builtin(iface, false);
+		break;
+	default:
+		return KNOT_EINVAL;
 	}
 
 	if (ret >= 0) {

src/libknot/xdp/bpf-user.h

@@ -16,7 +16,9 @@
 
 #pragma once
 
-#include  <stdint.h>
+#include <stdint.h>
+
+#include "libknot/xdp/af_xdp.h"
 
 #include <bpf/xsk.h>
 
@@ -98,7 +100,8 @@ typedef struct knot_xsk_socket {
  *
  * \return KNOT_E*
  */
-int kxsk_iface_new(const char *ifname, bool load_bpf, struct kxsk_iface **out_iface);
+int kxsk_iface_new(const char *ifname, knot_xsk_load_bpf_t load_bpf,
+		   struct kxsk_iface **out_iface);
 
 /*!
  * \brief Unload BPF maps for a socket.