code: plan9front

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idn: fix crash with long domain names

When a domain name gets long enough, we can end
up passing a remaining length of 0 to seprint.
When seprint has no buffer space at all, it returns
nil, which then crashes our DNS server.

This checks for a nil return from seprint and returns
an error

--- a/sys/src/libc/9sys/idn.c

+++ b/sys/src/libc/9sys/idn.c

@@ -211,7 +211,8 @@

 		if(cistrncmp(cp, "xn--", 4) == 0)

 			if((nr = punydecode(nc-4, cp+4, nelem(rb), rb)) < 0)

 				return -1;

-		dp = seprint(dp, de, "%.*S", nr, rb);

+		if((dp = seprint(dp, de, "%.*S", nr, rb)) == nil)

+			return -1;

 		if(dp >= de)

 			return -1;

 		if(cp[nc] == 0)

@@ -251,10 +252,12 @@

 			rb[nr++] = r;

 			nc += n;

 		}

-		if(nc == nr)

-			dp = seprint(dp, de, "%.*s", nc, cp);

-		else {

-			dp = seprint(dp, de, "xn--");

+		if(nc == nr){

+			if((dp = seprint(dp, de, "%.*s", nc, cp)) == nil)

+				return -1;

+		}else{

+			if((dp = seprint(dp, de, "xn--")) == nil)

+				return -1;

 			if((n = punyencode(nr, rb, de - dp, dp)) < 0)

 				return -1;

 			dp += n;