Subject: Re: [PATCH bpf] selftests/bpf: fix v4_to_v6 in sk_lookup

From: Jakub Sitnicki <[email-masked]>
Date: Mon, 10 Aug 2020 22:30:39 +0200

On Mon, Aug 10, 2020 at 06:14 PM CEST, Stanislav Fomichev wrote:
> On Sat, Aug 8, 2020 at 11:46 AM Jakub Sitnicki <[email-masked]> wrote:
>>
>> On Sat, Aug 08, 2020 at 12:38 AM CEST, Stanislav Fomichev wrote:
>> > I'm getting some garbage in bytes 8 and 9 when doing conversion
>> > from sockaddr_in to sockaddr_in6 (leftover from AF_INET?).
>> > Let's explicitly clear the higher bytes.
>> >
>> > Signed-off-by: Stanislav Fomichev <[email-masked]>
>> > ---
>> >  tools/testing/selftests/bpf/prog_tests/sk_lookup.c | 1 +
>> >  1 file changed, 1 insertion(+)
>> >
>> > diff --git a/tools/testing/selftests/bpf/prog_tests/sk_lookup.c b/tools/testing/selftests/bpf/prog_tests/sk_lookup.c
>> > index c571584c00f5..9ff0412e1fd3 100644
>> > --- a/tools/testing/selftests/bpf/prog_tests/sk_lookup.c
>> > +++ b/tools/testing/selftests/bpf/prog_tests/sk_lookup.c
>> > @@ -309,6 +309,7 @@ static void v4_to_v6(struct sockaddr_storage *ss)
>> >       v6->sin6_addr.s6_addr[10] = 0xff;
>> >       v6->sin6_addr.s6_addr[11] = 0xff;
>> >       memcpy(&v6->sin6_addr.s6_addr[12], &v4.sin_addr.s_addr, 4);
>> > +     memset(&v6->sin6_addr.s6_addr[0], 0, 10);
>> >  }
>> >
>> >  static int udp_recv_send(int server_fd)
>>
>> That was badly written. Sorry about that. And thanks for the fix.
>>
>> I'd even zero out the whole thing:
>>
>>         memset(v6, 0, sizeof(*v6));
>>
>> ... because right now IPv4 address is left as sin6_flowinfo.  I can
>> follow up with that change, unless you'd like to roll a v2.
> Up to you, but I'm not sure zeroing out the whole v6 portion is the
> best way forward.
> IMO, it's a bit confusing when reading the code.
> It will work, but only because v4 and v6 address portions don't really
> overlap :-/

It's not that hacky :-) We copy sockaddr_in bits before overwriting ss:

	struct sockaddr_in v4 = *(struct sockaddr_in *)ss;

It could be easier to read, perhaps by copying just the fields we need:

	struct sockaddr_in *v4 = (struct sockaddr_in *)ss;
	uint32_t addr = v4->sin_addr.saddr;
	in_port_t port = v4->sin_port;

> I was thinking about adding new, on the stack sin6, fully initializing
> it and then doing memcpy into ss.
> But I decided that adding memset is probably good enough :-)

Makes sense. Either way sounds good to me.