Re: [PATCH] netlink: fix compat recvmsg

[Eric Dumazet <eric.dumazet@gmail.com>]

Le lundi 16 août 2010 à 09:20 +0200, Johannes Berg a écrit :
> From: Johannes Berg <johannes.berg@intel.com>
> 
> Since
> commit 1dacc76d0014a034b8aca14237c127d7c19d7726
> Author: Johannes Berg <johannes@sipsolutions.net>
> Date:   Wed Jul 1 11:26:02 2009 +0000
> 
>     net/compat/wext: send different messages to compat tasks
> 
> we had a race condition when setting and then
> restoring frag_list. Eric attempted to fix it,
> but the fix created even worse problems.
> 
> However, the original motivation I had when I
> added the code that turned out to be racy is
> no longer clear to me, since we only copy up
> to skb->len to userspace, which doesn't include
> the frag_list length. As a result, not doing
> any frag_list clearing and restoring avoids
> the race condition, while not introducing any
> other problems.
> 
> Additionally, while preparing this patch I found
> that since none of the remaining netlink code is
> really aware of the frag_list, we need to use the
> original skb's information for packet information
> and credentials. This fixes, for example, the
> group information received by compat tasks.
> 
> Cc: Eric Dumazet <eric.dumazet@gmail.com>
> Cc: stable@kernel.org [2.6.31+, for 2.6.35 revert 1235f504aa]
> Signed-off-by: Johannes Berg <johannes.berg@intel.com>
> ---

As a side effect, it might fix the 

	if (nlk->flags & NETLINK_RECV_PKTINFO)
		netlink_cmsg_recv_pktinfo(msg, skb);

That tried to use pktinfo from the slave skb. After this
patch, skb points to master skb.

Acked-by: Eric Dumazet <eric.dumazet@gmail.com>

Thanks !