selftests/bpf: Fix sendto() message size in lwt_seg6local

This test_progs test fails on 32-bit armhf:

  root@qemu-armhf:/usr/libexec/kselftests-bpf#  test_progs -a lwt_seg6local
  [...]
  test_lwt_seg6local:PASS:setup 0 nsec
  test_lwt_seg6local:PASS:open ns6 0 nsec
  test_lwt_seg6local:PASS:start server 0 nsec
  test_lwt_seg6local:PASS:open ns1 0 nsec
  test_lwt_seg6local:PASS:start client 0 nsec
  test_lwt_seg6local:PASS:build target addr 0 nsec
  test_lwt_seg6local:PASS:send packet 0 nsec
  test_lwt_seg6local:FAIL:receive packet unexpected receive packet: actual 4 != expected 7
  torvalds#183     lwt_seg6local:FAIL

This happens because a sendto() call mistakenly uses 'sizeof(char *)' as
message length rather than the actual string ("foobar\0") size. e.g.

  bytes = sendto(cfd, foobar, sizeof(foobar), 0, ...

This likely passed by accident till now because BPF CI only tests 64-bit
targets. Fix by using strlen() to determine the message length.

Fixes: 1041b8b ("selftests/bpf: lwt_seg6local: Move test to test_progs")
Signed-off-by: Tony Ambardar <[email protected]>

Author: guidosarducci

tools/testing/selftests/bpf/prog_tests/lwt_seg6local.c

@@ -153,9 +153,9 @@ void test_lwt_seg6local(void)
 		       "build target addr"))
 		goto close_client;
 
-	bytes = sendto(cfd, foobar, sizeof(foobar), 0,
+	bytes = sendto(cfd, foobar, strlen(foobar) + 1, 0,
 		       (struct sockaddr *)&server_addr, sizeof(server_addr));
-	if (!ASSERT_EQ(bytes, sizeof(foobar), "send packet"))
+	if (!ASSERT_EQ(bytes, strlen(foobar) + 1, "send packet"))
 		goto close_client;
 
 	/* Verify we received all expected bytes */