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

[Natanael Copa <ncopa@alpinelinux.org>]

On Thu, 4 Sep 2014 20:11:29 +0200
Szabolcs Nagy <nsz@port70.net> wrote:

> From 1a068a048b64999f97add01ce8f5013a83b0e916 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 dn_expand empty name handling and offsets to 0
> 
> Empty name was rejected in dn_expand since commit
> 56b57f37a46dab432247bf29d96fcb11fbd02a6d
> which is a regression as reported by Natanael Copa.
> 
> Furthermore if an offset pointer in a compressed name
> pointed to a terminating 0 byte (instead of a label)
> the returned name was not null terminated.
> ---
>  src/network/dn_expand.c |   14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/src/network/dn_expand.c b/src/network/dn_expand.c
> index 849df19..d1ebebf 100644
> --- a/src/network/dn_expand.c
> +++ b/src/network/dn_expand.c
> @@ -5,8 +5,8 @@ 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;
> +	int len = -1, i, j, first = 1;

How about, instead of adding int first, we do:
	char *dest_start = dest;

> +	if (p==end || dest==dend) 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;
> +			if (!first) *dest++ = '.';
> +			first = 0;

and instead of the 2 above:
			if (dest != dest_start) *dest++ = '.';

> +			j = *p++;
> +			if (j >= end-p || j >= dend-dest) return -1;
> +			while (j--) *dest++ = *p++;
>  		} else {
> +			*dest = 0;
>  			if (len < 0) len = p+1-src;
>  			return len;
>  		}

-nc