netdev - Re: [RFC] xfrm: x86_64 CONFIG

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Message-ID: <20100205195409.GB23792@Chamillionaire.breakpoint.cc>
Date:	Fri, 5 Feb 2010 20:54:09 +0100
From:	Florian Westphal <fw@...len.de>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [RFC] xfrm: x86_64 CONFIG_COMPAT support

David Miller <davem@...emloft.net> wrote:
> From: Florian Westphal <fw@...len.de>
> Date: Fri, 5 Feb 2010 02:47:44 +0100
> 
> > Unfortunately, the MSG_COMPAT flag seems to get lost along
> > the way, so this uses x86_64 is_compat_task() for now.
> 
> You can't use is_compat_task() for this.  The current
> execution context is not always the same as who is
> going to get the message.

Thank you, this saved me a lot of trouble.

I had a look at the wireless compat layer you mentioned;
when sending data to userspace one just prepares two
skbs (one native, one compat); netlink_recvmsg() then decides
which one to use for a particular receiver.

However, I believe I can use is_compat_task() on
the userspace -> kernel side to figure out how to interpret
the incoming data.

Is that correct or did I misunderstand?

Another way would be to first try native decode,
and then re-try compat format on error, but I consider that ugly.

Thanks for your help,
       Florian
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