Re: [PATCH] fix handling of zero length domain names in dn_expand

[Szabolcs Nagy <nsz@port70.net>]

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* Natanael Copa <ncopa@alpinelinux.org> [2014-09-04 09:50:39 +0200]:
> On Wed, 13 Aug 2014 10:21:41 +0200
> Natanael Copa <ncopa@alpinelinux.org> wrote:
> 
> > Copy a zero length string instead of returning error when trying to
> > expand a zero lentgh domain name (null terminator).
> > 
> Rich pointed out to me on IRC that this will not write terminating '\0'
> when domain name length is zero. I'll send new patch.
> 

here is a patch

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From 49b96b160fdde9218bcaffa196ca9e6d5a233094 Mon Sep 17 00:00:00 2001
From: Szabolcs Nagy <nsz@port70.net>
Date: Thu, 4 Sep 2014 18:29:16 +0200
Subject: [PATCH] fix empty name handling in dn_expand

Empty name was rejected in dn_expand since commit
56b57f37a46dab432247bf29d96fcb11fbd02a6d
which is a regression as reported by Natanael Copa.

But it turns out only an "uncompressed" empty name was rejected,
if an offset pointer is used to represent an empty name then
dn_expand failed to null terminate the returned name.

this fix makes dn_expand accept and return empty names correctly.
---
 src/network/dn_expand.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/network/dn_expand.c b/src/network/dn_expand.c
index 849df19..4fa6af8 100644
--- a/src/network/dn_expand.c
+++ b/src/network/dn_expand.c
@@ -6,7 +6,7 @@ int __dn_expand(const unsigned char *base, const unsigned char *end, const unsig
 	const unsigned char *p = src;
 	char *dend = dest + (space > 254 ? 254 : space);
 	int len = -1, i, j;
-	if (p==end || !*p) return -1;
+	if (p==end) return -1;
 	/* detect reference loop using an iteration counter */
 	for (i=0; i < end-base; i+=2) {
 		if (*p & 0xc0) {
@@ -16,11 +16,13 @@ int __dn_expand(const unsigned char *base, const unsigned char *end, const unsig
 			if (j >= end-base) return -1;
 			p = base+j;
 		} else if (*p) {
-			j = *p+1;
-			if (j>=end-p || j>dend-dest) return -1;
-			while (--j) *dest++ = *++p;
-			*dest++ = *++p ? '.' : 0;
+			j = *p++;
+			if (j >= end-p || j >= dend-dest) return -1;
+			while (j--) *dest++ = *p++;
+			if (*p) *dest++ = '.';
 		} else {
+			if (dest == dend) return -1;
+			*dest = 0;
 			if (len < 0) len = p+1-src;
 			return len;
 		}
-- 
1.7.10.4