Re: [PATCH 02/22] util: Helper for formatting MAC addresses

[Stefano Brivio <sbrivio@redhat.com>]

On Fri, 16 Aug 2024 15:39:43 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:

> There are a couple of places where we somewhat messily open code formatting
> an Ethernet like MAC address for display.  Add an eth_ntop() helper for
> this.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  conf.c |  7 +++----
>  dhcp.c |  5 ++---
>  util.c | 19 +++++++++++++++++++
>  util.h |  3 +++
>  4 files changed, 27 insertions(+), 7 deletions(-)
> 
> diff --git a/conf.c b/conf.c
> index ed097bdc..830f91a6 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -921,7 +921,8 @@ pasta_opts:
>   */
>  static void conf_print(const struct ctx *c)
>  {
> -	char buf4[INET_ADDRSTRLEN], buf6[INET6_ADDRSTRLEN], ifn[IFNAMSIZ];
> +	char buf4[INET_ADDRSTRLEN], buf6[INET6_ADDRSTRLEN];
> +	char bufmac[ETH_ADDRSTRLEN], ifn[IFNAMSIZ];
>  	int i;
>  
>  	info("Template interface: %s%s%s%s%s",
> @@ -955,9 +956,7 @@ static void conf_print(const struct ctx *c)
>  		info("Namespace interface: %s", c->pasta_ifn);
>  
>  	info("MAC:");
> -	info("    host: %02x:%02x:%02x:%02x:%02x:%02x",
> -	     c->mac[0], c->mac[1], c->mac[2],
> -	     c->mac[3], c->mac[4], c->mac[5]);
> +	info("    host: %s", eth_ntop(c->mac, bufmac, sizeof(bufmac)));
>  
>  	if (c->ifi4) {
>  		if (!c->no_dhcp) {
> diff --git a/dhcp.c b/dhcp.c
> index aa9f59da..acc5b03e 100644
> --- a/dhcp.c
> +++ b/dhcp.c
> @@ -276,6 +276,7 @@ static void opt_set_dns_search(const struct ctx *c, size_t max_len)
>  int dhcp(const struct ctx *c, const struct pool *p)
>  {
>  	size_t mlen, dlen, offset = 0, opt_len, opt_off = 0;
> +	char macstr[ETH_ADDRSTRLEN];
>  	const struct ethhdr *eh;
>  	const struct iphdr *iph;
>  	const struct udphdr *uh;
> @@ -340,9 +341,7 @@ int dhcp(const struct ctx *c, const struct pool *p)
>  		return -1;
>  	}
>  
> -	info("    from %02x:%02x:%02x:%02x:%02x:%02x",
> -	     m->chaddr[0], m->chaddr[1], m->chaddr[2],
> -	     m->chaddr[3], m->chaddr[4], m->chaddr[5]);
> +	info("    from %s", eth_ntop(m->chaddr, macstr, sizeof(macstr)));
>  
>  	m->yiaddr = c->ip4.addr;
>  	mask.s_addr = htonl(0xffffffff << (32 - c->ip4.prefix_len));
> diff --git a/util.c b/util.c
> index 0b414045..892358b1 100644
> --- a/util.c
> +++ b/util.c
> @@ -676,6 +676,25 @@ const char *sockaddr_ntop(const void *sa, char *dst, socklen_t size)
>  	return dst;
>  }
>  
> +/** eth_ntop() - Convert an Ethernet MAC address to text format
> + * @mac:	MAC address
> + * @dst:	output buffer, minimum ETH_ADDRSTRLEN bytes
> + * @size:	size of buffer at @dst

Nit: s/output/Output, s/size/Size

> + *
> + * Return: On success, a non-null pointer to @dst, NULL on failure
> + */
> +const char *eth_ntop(const unsigned char *mac, char *dst, size_t size)
> +{
> +	int len;
> +
> +	len = snprintf(dst, size, "%02x:%02x:%02x:%02x:%02x:%02x",
> +		       mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
> +	if (len < 0 || (size_t)len >= size)
> +		return NULL;
> +
> +	return dst;
> +}
> +
>  /** str_ee_origin() - Convert socket extended error origin to a string
>   * @ee:		Socket extended error structure
>   *
> diff --git a/util.h b/util.h
> index cb4d181c..c1748074 100644
> --- a/util.h
> +++ b/util.h
> @@ -215,9 +215,12 @@ static inline const char *af_name(sa_family_t af)
>  
>  #define SOCKADDR_STRLEN		MAX(SOCKADDR_INET_STRLEN, SOCKADDR_INET6_STRLEN)
>  
> +#define ETH_ADDRSTRLEN	(ETH_ALEN * 3)

The fact that this includes two digits plus separator for all non-last
octets of a MAC address, and two digits plus NULL terminator for the
last octet, looks a bit subtle to me.

Defining this as sizeof("00:11:22:33:44:55") wouldn't scream
"off-by-one" as much, to me. Not a strong preference.

-- 
Stefano