af_unix: implement socket filter

Linux Socket Filters can already be successfully attached and detached on unix
sockets with setsockopt(sockfd, SOL_SOCKET, SO_{ATTACH,DETACH}_FILTER, ...).
See: Documentation/networking/filter.txt

But the filter was never used in the unix socket code so it did not work. This
patch uses sk_filter() to filter buffers before delivery.

This short program demonstrates the problem on SOCK_DGRAM.

int main(void) {
  int i, j, ret;
  int sv[2];
  struct pollfd fds[2];
  char *message = "Hello world!";
  char buffer[64];
  struct sock_filter ins[32] = {{0,},};
  struct sock_fprog filter;

  socketpair(AF_UNIX, SOCK_DGRAM, 0, sv);

  for (i = 0 ; i < 2 ; i++) {
    fds[i].fd = sv[i];
    fds[i].events = POLLIN;
    fds[i].revents = 0;
  }

  for(j = 1 ; j < 13 ; j++) {

    /* Set a socket filter to truncate the message */
    memset(ins, 0, sizeof(ins));
    ins[0].code = BPF_RET|BPF_K;
    ins[0].k = j;
    filter.len = 1;
    filter.filter = ins;
    setsockopt(sv[1], SOL_SOCKET, SO_ATTACH_FILTER, &filter, sizeof(filter));

    /* send a message */
    send(sv[0], message, strlen(message) + 1, 0);

    /* The filter should let the message pass but truncated. */
    poll(fds, 2, 0);

    /* Receive the truncated message*/
    ret = recv(sv[1], buffer, 64, 0);
    printf("received %d bytes, expected %d\n", ret, j);
  }

    for (i = 0 ; i < 2 ; i++)
      close(sv[i]);

  return 0;
}

Signed-off-by: Alban Crequy <alban.crequy@collabora.co.uk>
Reviewed-by: Ian Molton <ian.molton@collabora.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 0 deletions

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index dd419d286204..8d9bbba345a4 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1475,6 +1475,12 @@ restart:
                         goto out_free;
         }
 
+        if (sk_filter(other, skb) < 0) {
+                /* Toss the packet but do not return any error to the sender */
+                err = len;
+                goto out_free;
+        }
+
         unix_state_lock(other);
         err = -EPERM;
         if (!unix_may_send(sk, other))