[PATCH] wrong ENETDOWN in af_packet?

[PATCH] wrong ENETDOWN in af_packet?

[Eric Lammerts]

Hi,
I have a program that goes like this (source code at end of mail):

open PF_PACKET socket
look up index of interface x
bind to that interface
bring interface x down (~IFF_UP)
bring interface x up (IFF_UP|IFF_RUNNING)
for(;;) {
	recvfrom()
}

Problem: the first recvfrom() always results in ENETDOWN.

The reason is that (in af_packet.c) packet_notifier(NETDEV_DOWN) sets
sk->err to ENETDOWN, but packet_notifier(NETDEV_UP) doesn't clear it.
Is this behaviour deliberate?

If not, I suggest the following patch:

diff -u -r1.1.1.1 af_packet.c
--- linux-2.4.19/net/packet/af_packet.c	10 Jan 2003 16:20:09 -0000	1.1.1.1
+++ linux-2.4.19/net/packet/af_packet.c	5 Mar 2003 11:04:33 -0000
@@ -1407,6 +1407,7 @@
 				dev_add_pack(&po->prot_hook);
 				sock_hold(sk);
 				po->running = 1;
+				sk->err = 0;
 			}
 			spin_unlock(&po->bind_lock);
 #ifdef CONFIG_PACKET_MULTICAST


Currently I work around the problem by doing a getsockopt(x, SOL_SOCKET,
SO_ERROR,...) to clear the error variable.

Eric




#include <stdio.h>
#include <string.h>
#include <net/ethernet.h>
#include <net/if.h>
#include <net/if_packet.h>
#include <netinet/in.h>
#include <netpacket/packet.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/types.h>

void modify_iface_flags(int sock, char *device_name, short set, short reset)
{
	struct ifreq ifr;

	strncpy(ifr.ifr_name, device_name, IFNAMSIZ);
	if(ioctl(sock, SIOCGIFFLAGS, &ifr) < 0) {
		perror("SIOCGIFFLAGS");
		exit(1);
	}

	ifr.ifr_flags |= set;
	ifr.ifr_flags &= ~reset;

	strncpy(ifr.ifr_name, device_name, IFNAMSIZ);
	if(ioctl(sock, SIOCSIFFLAGS, &ifr) < 0) {
		perror("SIOCSIFFLAGS");
		exit(1);
	}
}

void bind_to_iface(int sock, char *ifacename)
{
	struct ifreq ifr;
	struct sockaddr_ll sa;

	strncpy(ifr.ifr_name, ifacename, IFNAMSIZ);
	if(ioctl(sock, SIOCGIFINDEX, &ifr) < 0) {
		perror("ioctl SIOCGIFINDEX");
		exit(1);
	}

	sa.sll_family = AF_PACKET;
	sa.sll_ifindex = ifr.ifr_ifindex;
	if(bind(sock, (struct sockaddr *)&sa, sizeof(sa)) < 0) {
		perror("bind");
		exit(1);
	}
}

int main()
{
	int fd, sz;
	char iface[] = "eth0";
	unsigned char data[1518];
	struct sockaddr_ll sa;
	socklen_t salen;
	
	fd = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
	if(fd < 0) { perror("socket"); exit(1); }

	bind_to_iface(fd, iface);

	// bring it down
	modify_iface_flags(fd, iface, 0, IFF_UP);

	// bring it up
	modify_iface_flags(fd, iface, IFF_UP | IFF_RUNNING, 0);

	//receive packet	
	salen = sizeof(sa);
	sz = recvfrom(fd, data, sizeof(data), 0, (struct sockaddr *)&sa, &salen);
	if(sz == -1) {
		perror("recvfrom");
		exit(1);
	}
	return 0;
}

Re: [PATCH] wrong ENETDOWN in af_packet?

[David S. Miller]

   From: Eric Lammerts <eric@lammerts.org>
   Date: Wed, 5 Mar 2003 15:11:23 +0100
   
   The reason is that (in af_packet.c) packet_notifier(NETDEV_DOWN) sets
   sk->err to ENETDOWN, but packet_notifier(NETDEV_UP) doesn't clear it.
   Is this behaviour deliberate?

Yes the behavior is deliberate.  You want to be aware of the
event.  Just because the opposite event has occurred afterwards
doesn't mean the first event didn't happen :-)