netlink: make nlmsg_end() and genlmsg_end() void

Contrary to common expectations for an "int" return, these functions
return only a positive value -- if used correctly they cannot even
return 0 because the message header will necessarily be in the skb.

This makes the very common pattern of

  if (genlmsg_end(...) < 0) { ... }

be a whole bunch of dead code. Many places also simply do

  return nlmsg_end(...);

and the caller is expected to deal with it.

This also commonly (at least for me) causes errors, because it is very
common to write

  if (my_function(...))
    /* error condition */

and if my_function() does "return nlmsg_end()" this is of course wrong.

Additionally, there's not a single place in the kernel that actually
needs the message length returned, and if anyone needs it later then
it'll be very easy to just use skb->len there.

Remove this, and make the functions void. This removes a bunch of dead
code as described above. The patch adds lines because I did

-	return nlmsg_end(...);
+	nlmsg_end(...);
+	return 0;

I could have preserved all the function's return values by returning
skb->len, but instead I've audited all the places calling the affected
functions and found that none cared. A few places actually compared
the return value with <= 0 in dump functionality, but that could just
be changed to < 0 with no change in behaviour, so I opted for the more
efficient version.

One instance of the error I've made numerous times now is also present
in net/phonet/pn_netlink.c in the route_dumpit() function - it didn't
check for <0 or <=0 and thus broke out of the loop every single time.
I've preserved this since it will (I think) have caused the messages to
userspace to be formatted differently with just a single message for
every SKB returned to userspace. It's possible that this isn't needed
for the tools that actually use this, but I don't even know what they
are so couldn't test that changing this behaviour would be acceptable.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 6 insertions, 3 deletions

diff --git a/net/netlink/diag.c b/net/netlink/diag.c
index bb59a7ed0859..3ee63a3cff30 100644
--- a/net/netlink/diag.c
+++ b/net/netlink/diag.c
@@ -91,7 +91,8 @@ static int sk_diag_fill(struct sock *sk, struct sk_buff *skb,
             sk_diag_put_rings_cfg(sk, skb))
                 goto out_nlmsg_trim;
 
-        return nlmsg_end(skb, nlh);
+        nlmsg_end(skb, nlh);
+        return 0;
 
 out_nlmsg_trim:
         nlmsg_cancel(skb, nlh);
diff --git a/net/netlink/genetlink.c b/net/netlink/genetlink.c
index 2e11061ef885..f52a7d5734cd 100644
--- a/net/netlink/genetlink.c
+++ b/net/netlink/genetlink.c
@@ -756,7 +756,8 @@ static int ctrl_fill_info(struct genl_family *family, u32 portid, u32 seq,
                 nla_nest_end(skb, nla_grps);
         }
 
-        return genlmsg_end(skb, hdr);
+        genlmsg_end(skb, hdr);
+        return 0;
 
 nla_put_failure:
         genlmsg_cancel(skb, hdr);
@@ -796,7 +797,8 @@ static int ctrl_fill_mcgrp_info(struct genl_family *family,
         nla_nest_end(skb, nest);
         nla_nest_end(skb, nla_grps);
 
-        return genlmsg_end(skb, hdr);
+        genlmsg_end(skb, hdr);
+        return 0;
 
 nla_put_failure:
         genlmsg_cancel(skb, hdr);