netdev - Re: [PATCH] sctp: Clean up type-punning in sctp_cmd

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Message-Id: <20121029.150444.905377521761602639.davem@davemloft.net>
Date:	Mon, 29 Oct 2012 15:04:44 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	nhorman@...driver.com
Cc:	David.Laight@...LAB.COM, vyasevich@...il.com,
	netdev@...r.kernel.org, linux-sctp@...r.kernel.org
Subject: Re: [PATCH] sctp: Clean up type-punning in sctp_cmd_t union

From: Neil Horman <nhorman@...driver.com>
Date: Mon, 29 Oct 2012 14:59:32 -0400

> I don't think thats whats going on at all.  The problem, as commit
> 19c7e9eef503dc1ae926f3d26c56f88bee568d7b describes it, is that ia64 was
> speculatively loading a value from memory of type sctp_arg_t.  If the
> initialization step set the value of one of the smaller members of the union
> (say a short), then the load stage may still load a larger amount of data (given
> that the union is larger than its smallest members).  That results in part of
> the load being uninitalized, and ia64 declaring it Not A Thing, and consequently
> trapping out.

Agreed.

> The fix previously was to just initazlie the unsigned long member zero in that
> union before setting the actuall type member that was requried so as to ensure
> all the data was initalized.  That works well enough, but it presumes that
> unsigned long is the largest member of the union, which is risky.  Its better to
> memset the union to 0 for sizeof bytes to ensure future proofing.

Also agreed.
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