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

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Date:	Wed, 03 Feb 2016 13:47:52 -0800
From:	Joe Perches <joe@...ches.com>
To:	"Jason A. Donenfeld" <Jason@...c4.com>,
	Daniel Borkmann <daniel@...earbox.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux@...musvillemoes.dk, andriy.shevchenko@...ux.intel.com,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vsprintf: do not append unset Scope ID to IPv6

On Wed, 2016-02-03 at 22:14 +0100, Jason A. Donenfeld wrote:
> The idea here is to be able to printk a sockaddr_in6, and have it show
> something that looks like what the user would naturally pass to
> getaddrinfo(3), which is entirely complete.
> 
> However, I could be convinced that this kind of behavior belongs in
> it's own flag. Maybe I'll cook up a flag for that instead.

I think that'd be best.

Maybe using something like %pISG for this that
would optionally show these flow and scope values
only when non-zero.

Something like:
---
 lib/vsprintf.c | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 6dc4288..2003c6f 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1147,7 +1147,8 @@ static noinline_for_stack
 char *ip6_addr_string_sa(char *buf, char *end, const struct sockaddr_in6 *sa,
 			 struct printf_spec spec, const char *fmt)
 {
-	bool have_p = false, have_s = false, have_f = false, have_c = false;
+	u8 show_p = 0, show_s = 0, show_f = 0;
+	bool use_c = false;
 	char ip6_addr[sizeof("[xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:255.255.255.255]") +
 		      sizeof(":12345") + sizeof("/123456789") +
 		      sizeof("%1234567890")];
@@ -1160,43 +1161,50 @@ char *ip6_addr_string_sa(char *buf, char *end, const struct sockaddr_in6 *sa,
 	while (isalpha(*++fmt)) {
 		switch (*fmt) {
 		case 'p':
-			have_p = true;
+			show_p = 1;
 			break;
 		case 'f':
-			have_f = true;
+			show_f = 1;
 			break;
 		case 's':
-			have_s = true;
+			show_s = 1;
+			break;
+		case 'G':
+			show_p = 2;
+			show_f = 2;
+			show_s = 2;
 			break;
 		case 'c':
-			have_c = true;
+			use_c = true;
 			break;
 		}
 	}
 
-	if (have_p || have_s || have_f) {
+	if (show_p || show_s || show_f) {
 		*p = '[';
 		off = 1;
 	}
 
-	if (fmt6[0] == 'I' && have_c)
+	if (fmt6[0] == 'I' && use_c)
 		p = ip6_compressed_string(ip6_addr + off, addr);
 	else
 		p = ip6_string(ip6_addr + off, addr, fmt6);
 
-	if (have_p || have_s || have_f)
+	if (show_p || show_s || show_f)
 		*p++ = ']';
 
-	if (have_p) {
+	if (show_p) {
 		*p++ = ':';
 		p = number(p, pend, ntohs(sa->sin6_port), spec);
 	}
-	if (have_f) {
+	if (show_f == 1 ||
+	    (show_f == 2 && (sa->sin6_flowinfo & IPV6_FLOWINFO_MASK))) {
 		*p++ = '/';
 		p = number(p, pend, ntohl(sa->sin6_flowinfo &
 					  IPV6_FLOWINFO_MASK), spec);
 	}
-	if (have_s) {
+	if (show_s == 1 ||
+	    (show_s == 2 && sa->sin6_scope_id)) {
 		*p++ = '%';
 		p = number(p, pend, sa->sin6_scope_id, spec);
 	}