RE: [PATCH bpf-next] selftests/bpf: Fix rare segfault in sock_fields prog test

[John Fastabend <john.fastabend@gmail.com>]

Jörn-Thorben Hinz wrote:
> test_sock_fields__detach() got called with a null pointer here when one
> of the CHECKs or ASSERTs up to the test_sock_fields__open_and_load()
> call resulted in a jump to the "done" label.
> 
> A skeletons *__detach() is not safe to call with a null pointer, though.
> This led to a segfault.
> 
> Go the easy route and only call test_sock_fields__destroy() which is
> null-pointer safe and includes detaching.
> 
> Came across this while looking[1] to introduce the usage of
> bpf_tcp_helpers.h (included in progs/test_sock_fields.c) together with
> vmlinux.h.
> 
> [1] https://lore.kernel.org/bpf/629bc069dd807d7ac646f836e9dca28bbc1108e2.camel@mailbox.tu-berlin.de/
> 
> Fixes: 8f50f16ff39d ("selftests/bpf: Extend verifier and bpf_sock tests for dst_port loads")
> Signed-off-by: Jörn-Thorben Hinz <jthinz@mailbox.tu-berlin.de>
> ---
>  tools/testing/selftests/bpf/prog_tests/sock_fields.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/sock_fields.c b/tools/testing/selftests/bpf/prog_tests/sock_fields.c
> index 9d211b5c22c4..7d23166c77af 100644
> --- a/tools/testing/selftests/bpf/prog_tests/sock_fields.c
> +++ b/tools/testing/selftests/bpf/prog_tests/sock_fields.c
> @@ -394,7 +394,6 @@ void serial_test_sock_fields(void)
>  	test();
>  
>  done:
> -	test_sock_fields__detach(skel);
>  	test_sock_fields__destroy(skel);
>  	if (child_cg_fd >= 0)
>  		close(child_cg_fd);
> -- 
> 2.30.2
> 

But we should still call __detach(skel) after the !skel check
is done I assume. So rather than remove it should add a new label
and jump to that,

  
 done:
   test_sock_fields__detach();
 done_no_skel:
   test_sock_fields__destroy()