Re: [Qemu-devel] [PATCH] linux-user: manage SOCK_PACKET socket type.

[Laurent Vivier <laurent@vivier.eu>]

Le 26/10/2015 15:40, Peter Maydell a écrit :
> On 6 October 2015 at 18:11, Laurent Vivier <laurent@vivier.eu> wrote:
>> This is obsolete, but if we want to use dhcp with some distros (like debian
>> ppc 8.2 jessie), we need it.
>>
>> At the bind level, we are not able to know the socket type so we try to
>> guess it by analyzing the name. We manage only the case "ethX",
>> "ethX" in spk_device is similar to set htons(0x6574) in sll_protocol in the
>> normal case, and as this protocol does not exist, it's ok.
>>
>> SOCK_PACKET uses network endian to encode protocol in socket()
>>
>> in PACKET(7) :
>>                                  protocol is the  IEEE  802.3  protocol
>> number in network order.  See the <linux/if_ether.h> include file for a
>> list of allowed protocols.  When protocol is  set  to  htons(ETH_P_ALL)
>> then all protocols are received.  All incoming packets of that protocol
>> type will be passed to the packet socket before they are passed to  the
>> protocols implemented in the kernel.
>>
>> Signed-off-by: Laurent Vivier <laurent@vivier.eu>
>> ---
>> This patch is a remix of an old patch sent in 2012:
>> https://patchwork.ozlabs.org/patch/208892/
>>
>>  linux-user/syscall.c | 33 +++++++++++++++++++++++++++------
>>  1 file changed, 27 insertions(+), 6 deletions(-)
>>
>> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
>> index 64be431..71cc1e2 100644
>> --- a/linux-user/syscall.c
>> +++ b/linux-user/syscall.c
>> @@ -111,6 +111,7 @@ int __clone2(int (*fn)(void *), void *child_stack_base,
>>  #include <linux/route.h>
>>  #include <linux/filter.h>
>>  #include <linux/blkpg.h>
>> +#include <linux/if_packet.h>
>>  #include "linux_loop.h"
>>  #include "uname.h"
>>
>> @@ -1198,11 +1199,20 @@ static inline abi_long target_to_host_sockaddr(struct sockaddr *addr,
>>      memcpy(addr, target_saddr, len);
>>      addr->sa_family = sa_family;
>>      if (sa_family == AF_PACKET) {
>> -       struct target_sockaddr_ll *lladdr;
>> +        /* Manage an obsolete case :
>> +         * if socket type is SOCK_PACKET, bind by name otherwise by index
>> +         * but we are not able to know socket type, so check if the name
>> +         * is usable...
>> +         * see linux/net/packet/af_packet.c: packet_bind_spkt()
>> +         */
>> +        if (strncmp((char *)((struct sockaddr_pkt *)addr)->spkt_device,
>> +                    "eth", 3) != 0) {
>> +            struct target_sockaddr_ll *lladdr;
> 
> This confuses me. The packet(7) manpage suggests there are two flavours
> of packet socket:
>  (1) legacy AF_INET + SOCK_PACKET
>  (2) new style AF_PACKET + SOCK_RAW / SOCK_DGRAM
> 
> but this comment suggests it's trying to handle AF_PACKET + SOCK_PACKET ?

In fact, I've started not from the man page, but from a non working dhcp
client, originally with a m68k target and etch-m68k distro, and I've met
again this problem on a ppc target and jessie distro.

> 
> If AF_PACKET was introduced as the new way of doing things, it's not
> clear why it would be the family type used in the legacy approach's
> sockaddr_pkt (though there seems to be code around that does this).
> I suspect that 2.0 kernels just didn't check af_family here at all.

by digging into the code, I have found:

$ apt-get source isc-dhcp-client
$ vi isc-dhcp-4.3.1/common/lpf.c

...
 72 int if_register_lpf (info)
 73         struct interface_info *info;
 74 {
...
 79         if ((sock = socket(PF_PACKET, SOCK_PACKET,
 80                            htons((short)ETH_P_ALL))) < 0) {
...

So we can see socket() is used with domain PF_PACKET and type SOCK_PACKET.

Next, the interface name is put into the sa_data of sockaddr, and bind()
is used with AF_PACKET:

 94         /* Bind to the interface name */
 95         memset (&sa, 0, sizeof sa);
 96         sa.sa_family = AF_PACKET;
 97         strncpy (sa.sa_data, (const char *)info -> ifp, sizeof
sa.sa_data);
 98         if (bind (sock, &sa, sizeof sa)) {

ifp is initialized from a list of all discovered interfaces in
common/discover.c:
...
 238 int
 239 begin_iface_scan(struct iface_conf_list *ifaces) {
...
 283         if (ioctl(ifaces->sock, SIOCGLIFCONF, &ifaces->conf) < 0) {
...
 918 void
 919 discover_interfaces(int state) {
...
 924         struct interface_info *tmp;
...
 939         if (!begin_iface_scan(&ifaces)) {
...
 955         while (next_iface(&info, &err, &ifaces)) {
 956
 957                 /* See if we've seen an interface that matches this
one. */
 958                 for (tmp = interfaces; tmp; tmp = tmp->next) {
 959                         if (!strcmp(tmp->name, info.name))
 960                                 break;
 961                 }

...
 987                         strcpy(tmp->name, info.name);
...
1050         }
...
1063         for (tmp = interfaces ; tmp != NULL ; tmp = tmp->next) {
1064                 if (tmp->ifp == NULL) {
1065                         struct ifreq *tif;
1066
1067                         tif = (struct ifreq *)dmalloc(sizeof(struct
ifreq),
1068                                                       MDL);
1069                         if (tif == NULL)
1070                                 log_fatal("no space for ifp mockup.");
1071                         strcpy(tif->ifr_name, tmp->name);
1072                         tmp->ifp = tif;
1073                 }
1074         }

> 
> The code in the kernel's packet_recvmsg fills in the spkt_family
> field with the netdevice's type field, which is an ARPHRD_* constant
> as far as I can tell (I could well be wrong here).

kernel 4.3.0-rc3, net/packet/af_packet.c:

   2961
   2962 static int packet_bind_spkt(struct socket *sock, struct sockaddr
*uaddr,
   2963                             int addr_len)
   2964 {
   2965         struct sock *sk = sock->sk;
   2966         char name[15];
   2967         struct net_device *dev;
   2968         int err = -ENODEV;
   2969
   2970         /*
   2971          *      Check legality
   2972          */
   2973
   2974         if (addr_len != sizeof(struct sockaddr))
   2975                 return -EINVAL;
   2976         strlcpy(name, uaddr->sa_data, sizeof(name));
   2977
   2978         dev = dev_get_by_name(sock_net(sk), name);
   2979         if (dev)
   2980                 err = packet_do_bind(sk, dev, pkt_sk(sk)->num);
   2981         return err;
   2982 }
...
   4246 static const struct proto_ops packet_ops_spkt = {
   4247         .family =       PF_PACKET,
...
   4250         .bind =         packet_bind_spkt,
...
   3022
   3023 static int packet_create(struct net *net, struct socket *sock,
int protocol,
   3024                          int kern)
...
   3045         if (sock->type == SOCK_PACKET)
   3046                 sock->ops = &packet_ops_spkt;
...

> Odd to have code in target_to_host_sockaddr and not in
> host_to_target_sockaddr.

In the case of host_to_target_sockaddr(), there is no "if (sa_family ==
AF_PACKET) {" as it was in target_to_host_sockaddr(), it's why I didn't
add it (and I don't like to add code I don't test).

> 
>> -       lladdr = (struct target_sockaddr_ll *)addr;
>> -       lladdr->sll_ifindex = tswap32(lladdr->sll_ifindex);
>> -       lladdr->sll_hatype = tswap16(lladdr->sll_hatype);
>> +            lladdr = (struct target_sockaddr_ll *)addr;
>> +            lladdr->sll_ifindex = tswap32(lladdr->sll_ifindex);
>> +            lladdr->sll_hatype = tswap16(lladdr->sll_hatype);
>> +        }
>>      }
>>      unlock_user(target_saddr, target_addr, 0);
>>
>> @@ -2509,7 +2519,12 @@ static abi_long do_socketcall(int num, abi_ulong vptr)
>>      /* now when we have the args, actually handle the call */
>>      switch (num) {
>>      case SOCKOP_socket: /* domain, type, protocol */
>> -        return do_socket(a[0], a[1], a[2]);
>> +        if (a[0] == AF_PACKET ||
>> +            a[1] == TARGET_SOCK_PACKET) {
>> +            return do_socket(a[0], a[1], tswap16(a[2]));
>> +        } else {
>> +            return do_socket(a[0], a[1], a[2]);
>> +        }
>>      case SOCKOP_bind: /* sockfd, addr, addrlen */
>>          return do_bind(a[0], a[1], a[2]);
>>      case SOCKOP_connect: /* sockfd, addr, addrlen */
>> @@ -7500,7 +7515,13 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
>>  #endif
>>  #ifdef TARGET_NR_socket
>>      case TARGET_NR_socket:
>> -        ret = do_socket(arg1, arg2, arg3);
>> +        if (arg1 == AF_PACKET ||
>> +            arg2 == TARGET_SOCK_PACKET) {
>> +            /* in this case, socket() needs a network endian short */
>> +            ret = do_socket(arg1, arg2, tswap16(arg3));
>> +        } else {
>> +            ret = do_socket(arg1, arg2, arg3);
>> +        }
> 
> This doesn't make sense to me. The argument to socket()
> is passed in via a register; so if the guest code correctly
> passes the protocol value as htons(whatever) then that will
> be the value in arg3 and we do not need to swap anything.
> 
> I see we had this discussion about the previous version of the
> patch too... and my argument that we don't need the tswap16
> in the socketcall code path either still makes sense to me.

I agree with you, I think I have confused socketcall() that passes
parameters by memory, and socket() that passes parameters by registers.
I will remove this part and resend a patch.

> 
>>          fd_trans_unregister(ret);
>>          break;
>>  #endif
>> --
>> 2.4.3
> 
> thanks
> -- PMM
> 

Thank you for your comments,
Laurent