netdev - Re: [PATCH 5/5] net: sock_aio_write: set CMSG_MSG_COMPAT flag if is_compat

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Message-Id: <20100226.011646.257642830.davem@davemloft.net>
Date:	Fri, 26 Feb 2010 01:16:46 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	fwestphal@...aro.com
Cc:	netdev@...r.kernel.org, johannes@...solutions.net
Subject: Re: [PATCH 5/5] net: sock_aio_write: set CMSG_MSG_COMPAT flag if
 is_compat_task

From: Florian Westphal <fwestphal@...aro.com>
Date: Fri, 19 Feb 2010 13:41:47 +0100

> some programs (e.g. pluto ike daemon), send netlink data to the
> kernel via write().
> 
> So far, the CMSG_MSG_COMPAT flag is not set in this case, which
> results in the kernel interpreting the netlink data the wrong way.
> 
> An alternative solution is to switch userspace to sendmsg() when
> using netlink sockets.
> 
> Cc: Johannes Berg <johannes@...solutions.net>
> Signed-off-by: Florian Westphal <fwestphal@...aro.com>

I was about to apply this entire patch set, but thinking about this
one patch we have to do it differently:

> @@ -837,6 +837,10 @@ static ssize_t do_sock_write(struct msghdr *msg, struct kiocb *iocb,
>  	msg->msg_iov = (struct iovec *)iov;
>  	msg->msg_iovlen = nr_segs;
>  	msg->msg_flags = (file->f_flags & O_NONBLOCK) ? MSG_DONTWAIT : 0;
> +#ifdef CONFIG_COMPAT
> +	if (is_compat_task())
> +		msg->msg_flags |= MSG_CMSG_COMPAT;
> +#endif
>  	if (sock->type == SOCK_SEQPACKET)
>  		msg->msg_flags |= MSG_EOR;
>  

What if the kernel itself does a socket write, say from a software
interrupt or to generate events in response to a user event,
and the current process happens to be a compat task?

I think this could even occur for NFS perhaps?

Anyways, that isn't going to work properly.

We might need to add ->compat_write to file operations to do
it properly.
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