bpf, samples: Fix xdpsock with '-M' parameter missing unload process

Execute the following command and exit, then execute it again, the following
error will be reported:

$ sudo ./samples/bpf/xdpsock -i ens4f2 -M
^C
$ sudo ./samples/bpf/xdpsock -i ens4f2 -M
libbpf: elf: skipping unrecognized data section(16) .eh_frame
libbpf: elf: skipping relo section(17) .rel.eh_frame for section(16) .eh_frame
libbpf: Kernel error message: XDP program already attached
ERROR: link set xdp fd failed

Commit c9d27c9e8dc7 ("samples: bpf: Do not unload prog within xdpsock") removed
the unloading prog code because of the presence of bpf_link. This is fine if
XDP_SHARED_UMEM is disabled, but if it is enabled, unloading the prog is still
needed.

Fixes: c9d27c9e8dc7 ("samples: bpf: Do not unload prog within xdpsock")
Signed-off-by: Wang Hai <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Magnus Karlsson <[e-mail]>
Cc: Maciej Fijalkowski <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

samples/bpf/xdpsock_user.c

···
 static int opt_timeout = 1000;
 static bool opt_need_wakeup = true;
 static u32 opt_num_xsks = 1;
+static u32 prog_id;
 static bool opt_busy_poll;
 static bool opt_reduced_cap;
 
···
 	return NULL;
 }
 
+static void remove_xdp_program(void)
+{
+	u32 curr_prog_id = 0;
+
+	if (bpf_get_link_xdp_id(opt_ifindex, &curr_prog_id, opt_xdp_flags)) {
+		printf("bpf_get_link_xdp_id failed\n");
+		exit(EXIT_FAILURE);
+	}
+
+	if (prog_id == curr_prog_id)
+		bpf_set_link_xdp_fd(opt_ifindex, -1, opt_xdp_flags);
+	else if (!curr_prog_id)
+		printf("couldn't find a prog id on a given interface\n");
+	else
+		printf("program on interface changed, not removing\n");
+}
+
 static void int_exit(int sig)
 {
 	benchmark_done = true;
···
 {
 	fprintf(stderr, "%s:%s:%i: errno: %d/\"%s\"\n", file, func,
 		line, error, strerror(error));
+
+	if (opt_num_xsks > 1)
+		remove_xdp_program();
 	exit(EXIT_FAILURE);
 }
 
···
 		if (write(sock, &cmd, sizeof(int)) < 0)
 			exit_with_error(errno);
 	}
+
+	if (opt_num_xsks > 1)
+		remove_xdp_program();
 }
 
 static void swap_mac_addresses(void *data)
···
 	txr = tx ? &xsk->tx : NULL;
 	ret = xsk_socket__create(&xsk->xsk, opt_if, opt_queue, umem->umem,
 				 rxr, txr, &cfg);
+	if (ret)
+		exit_with_error(-ret);
+
+	ret = bpf_get_link_xdp_id(opt_ifindex, &prog_id, opt_xdp_flags);
 	if (ret)
 		exit_with_error(-ret);