netdev - Re: [COMPAT]: non-cmsg data gets interpreted as cmsg data (sparc64)

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Date:	Thu, 17 Apr 2008 23:16:22 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	gerrit@....abdn.ac.uk
Cc:	netdev@...r.kernel.org
Subject: Re: [COMPAT]: non-cmsg data gets interpreted as cmsg data (sparc64)

From: Gerrit Renker <gerrit@....abdn.ac.uk>
Date: Thu, 17 Apr 2008 14:08:37 +0100

> The following code segment works without problems on IA-32:
> 
> 	uint32_t prio = 10;
> 	struct msghdr msg;
> 
> 	// fill in iov bits of msg
> 	// ...
> 	msg.msg_control	   = &prio;
> 	msg.msg_controllen = sizeof prio;
> 
> 	if (sendmsg(sockfd, &msg, 0))
> 		err(1, "sendmsg failed");

It works in non-compat situations only by accident.

You have to use proper CMSG encoding to send things into the kernel
vis the msg_control area.

Modify your interface such that it properly encapsulates your priority
value in a proper CMSG blob and all will work.
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