[PATCH] Fix ifaddrs error handling

[PATCH] Fix ifaddrs error handling

[Jakub Jelinek]

Hi!

netlink apparently only allows one pending dumper for one netlink
connection:
        /* A dump is in progress... */
        spin_lock(&nlk->cb_lock);
        if (nlk->cb) {
                spin_unlock(&nlk->cb_lock);
                netlink_destroy_callback(cb);
                sock_put(sk);
                return -EBUSY;
        }
        nlk->cb = cb;
        spin_unlock(&nlk->cb_lock);
If some box has too many interfaces and the 4K default buffer isn't
sufficient, some messages can be truncated (with MSG_TRUNC set in flags).
glibc in this case increments the sequence counter, resends the request
and ignores all responses with older sequence numbers.
If the old responses all were into the message that got truncated, maybe
with NLMSG_DONE alone in a next message, then this will work just fine,
but if MSG_TRUNC happens say on the 1st out of 3 response messages,
when we reissue the request a NLMSG_ERR -EBUSY response is queued.

The following patch fixes that by retrying with a new socket (should be
very rare, most of the people don't have so many interfaces and even if they
have so many, the incremented buffer size is remembered within the
application, so further getifaddrs etc. calls will start with a big enough
buffer).  Alternatively, perhaps we could when we see MSG_TRUNC recvmsg
in a loop until we see NLMSG_DONE with that seq number (though, I'm not
sure if we have a guarantee NLMSG_DONE was not in a MSG_TRUNC message).

This is reproduceable on ia64 with ~ 80 interfaces, or e.g. on x86_64
with 80 interfaces too, if initial buf_size in __netlink_request is
artificially lowered to say 250.

2007-03-02  Jakub Jelinek  <jakub@redhat.com>

	* sysdeps/unix/sysv/linux/ifaddrs.c (__netlink_request): Retry with
	a new netlink socket if NLMSG_ERR -EBUSY is seen after some MSG_TRUNC
	message.

--- libc/sysdeps/unix/sysv/linux/ifaddrs.c.jj	2007-03-02 14:52:11.000000000 +0100
+++ libc/sysdeps/unix/sysv/linux/ifaddrs.c	2007-03-02 15:14:22.000000000 +0100
@@ -135,6 +135,7 @@ __netlink_request (struct netlink_handle
     return -1;
 
   size_t this_buf_size = buf_size;
+  size_t orig_this_buf_size = this_buf_size;
   if (__libc_use_alloca (this_buf_size))
     buf = alloca (this_buf_size);
   else
@@ -236,6 +237,36 @@ __netlink_request (struct netlink_handle
 	      struct nlmsgerr *nlerr = (struct nlmsgerr *) NLMSG_DATA (nlmh);
 	      if (nlmh->nlmsg_len < NLMSG_LENGTH (sizeof (struct nlmsgerr)))
 		errno = EIO;
+	      else if (nlerr->error == -EBUSY
+		       && orig_this_buf_size != this_buf_size)
+		{
+		  /* If EBUSY and MSG_TRUNC was seen, try again with a new
+		     netlink socket.  */
+		  struct netlink_handle hold = *h;
+		  if (__netlink_open (h) < 0)
+		    {
+		      *h = hold;
+		      goto out_fail;
+		    }
+		  __netlink_close (&hold);
+		  orig_this_buf_size = this_buf_size;
+		  nlm_next = *new_nlm_list;
+		  while (nlm_next != NULL)
+		    {
+		      struct netlink_res *tmpptr;
+
+		      tmpptr = nlm_next->next;
+		      free (nlm_next);
+		      nlm_next = tmpptr;
+		    }
+		  *new_nlm_list = NULL;
+		  count = 0;
+		  h->seq++;
+
+		  if (__netlink_sendreq (h, type) < 0)
+		    goto out_fail;
+		  break;
+		}
 	      else
 		errno = -nlerr->error;
 	      goto out_fail;

	Jakub