Re: [PATCH] vsprintf: do not append unset Scope ID to IPv6

[Daniel Borkmann <daniel@iogearbox.net>]

On 02/03/2016 11:41 AM, Jason A. Donenfeld wrote:
> The sockaddr_in6 has the sin6_scope_id parameter. This contains the
> netdev index of the output device. When set to 0, sin6_scope_id is
> considered to be "unset" -- it has no Scope ID (see RFC4007). When it is
> set to >0, it has a Scope ID.
[...]
> So, here, either it has Scope ID, in which case it's returned, or it
> doesn't need a Scope ID, in which case 0 - "no scope id" - is returned.
>
> Therefore, vsprintf should treat the 0 Scope ID the same way the rest of
> the v6 stack and the RFC treats it: 0 Scope ID means no Scope ID. And if
> there is no Scope ID, there is nothing to be printed.

I don't see an "issue" here. I.e. if the developer requested to append 's'
to the specifier, then it's expected to dump what is in the scope id, even
if that is 0 meaning "unset" scope.

It would be principle of least surprise -- I could imagine if not dumped,
then a developer would check the source code to find out whether he did a
mistake or it's expected behavior, just to make sure. That's what I would
do probably.

Is this causing a _real_ issue somewhere (I couldn't parse one out of your
commit message other than 'it would be nice to have')?

> So, this patch only prints the Scope ID when one exists, and does not
> print a Scope ID when one does not exist.
>
> It turns out, too, that this is what developers want. The most common
> purpose of %pISpfsc is to show "what is in that sockaddr so that I can
> have some idea of how to connect to it?"
>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>   lib/vsprintf.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 48ff9c3..1b1b1c8 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -1194,7 +1194,7 @@ char *ip6_addr_string_sa(char *buf, char *end, const struct sockaddr_in6 *sa,
>   		p = number(p, pend, ntohl(sa->sin6_flowinfo &
>   					  IPV6_FLOWINFO_MASK), spec);
>   	}
> -	if (have_s) {
> +	if (have_s && sa->sin6_scope_id) {
>   		*p++ = '%';
>   		p = number(p, pend, sa->sin6_scope_id, spec);
>   	}
>