netdev - Re: [PATCH 2/2] move unneeded data to initdata section

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Date:	Thu, 15 Nov 2007 07:32:40 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Denis V. Lunev" <den@...nvz.org>
Cc:	davem@...emloft.net, containers@...ts.osdl.org,
	netdev@...r.kernel.org, clg@...ibm.com, benjamin.thery@...l.net
Subject: Re: [PATCH 2/2] move unneeded data to initdata section

"Denis V. Lunev" <den@...nvz.org> writes:

> This patch reverts Eric's commit 2b008b0a8e96b726c603c5e1a5a7a509b5f61e35
>
> It diets .text & .data section of the kernel if CONFIG_NET_NS is not set.
> This is safe after list operations cleanup.

Ok.  This patch is technically safe because none of the touched
code can live in a module and so we never touch the exit code path.

However in the general case and as a code idiom this __net_initdata
on struct pernet_operations is fundamentally horribly broken.

Look at what happens if we use this idiom in module.  There
is only one definition of __initdata ".init.data".  The module
loader places all sections that begin with .init in a region of
memory that will be discarded after module initialization.  

So in register_pernet_operations we pass in the a pointer to struct
pernet_operations and call the init method.  Later when we remove the
module we again pass in the pointer to struct pernet_operations which
lived in an init section so it has been discarded.  We dereference
that pointer to find the exit method and KABOOM!!!!

So I'm still opposed to __net_initdata on the grounds that at best
it is like putting our head under a guillotine and reaching up and
sawing at the row that holds the blade up with a pocket knife.  It is
a think rope and a puny knife so you are safe for a while....

Eric
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