linux-kernel - Re: IRe: [PATCH] vsprintf: flowinfo in IPv6 is optional too

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Date:	Wed, 03 Feb 2016 22:13:10 +0100
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Joe Perches <joe@...ches.com>,
	"Jason A. Donenfeld" <Jason@...c4.com>, akpm@...ux-foundation.org,
	linux@...musvillemoes.dk, andriy.shevchenko@...ux.intel.com,
	linux-kernel@...r.kernel.org, dborkman@...hat.com
Subject: Re: IRe: [PATCH] vsprintf: flowinfo in IPv6 is optional too

On 02/03/2016 06:56 PM, Joe Perches wrote:
> On Wed, 2016-02-03 at 13:13 +0100, Jason A. Donenfeld wrote:
>> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
>> ---
>>   lib/vsprintf.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
>> index 1b1b1c8..85e6645 100644
>> --- a/lib/vsprintf.c
>> +++ b/lib/vsprintf.c
>> @@ -1189,7 +1189,7 @@ char *ip6_addr_string_sa(char *buf, char *end, const struct sockaddr_in6 *sa,
>>   		*p++ = ':';
>>   		p = number(p, pend, ntohs(sa->sin6_port), spec);
>>   	}
>> -	if (have_f) {
>> +	if (have_f && (sa->sin6_flowinfo & IPV6_FLOWINFO_MASK)) {
>>   		*p++ = '/';
>>   		p = number(p, pend, ntohl(sa->sin6_flowinfo &
>>   					  IPV6_FLOWINFO_MASK), spec);
>
> Why does this matter at all?
>
> The format string "%pIS[...]f" is not used currently in the kernel.
>
> If one were to call out this 'f' qualifier to %pIS, wouldn't it
> be better to show /0 than elide the / output completely?

+1

Another possibility also in regards to your other patch would be to have
a different format string char option instead of 'f'/'s' that would then
allow a developer for having both options to choose from. Dunno if it's
really worth it, but if you have a use case that definitely needs it, then
it'd be probably better. Less surprises during debugging, at least.