linux-kernel - Re: {painfully BISECTED} Please revert f25c80a4b2: arch/um/drivers: remove duplicate structure field initialization

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Message-ID: <20100928211106.GE19804@ZenIV.linux.org.uk>
Date:	Tue, 28 Sep 2010 22:11:06 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Boaz Harrosh <bharrosh@...asas.com>, Julia Lawall <julia@...u.dk>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	uml-devel <user-mode-linux-devel@...ts.sourceforge.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: {painfully BISECTED} Please revert f25c80a4b2: arch/um/drivers:
 remove duplicate structure field initialization

On Tue, Sep 28, 2010 at 01:24:40PM -0700, Linus Torvalds wrote:
> It looks like that commit is indeed very misleading. The commit message says:
> 
>   "arch/um/drivers: remove duplicate structure field initialization"
> 
> but it is in fact not duplicate: there's two field initializations,
> but they are _different_. Looking at the patch, it has:
> 
>         .ndo_set_mac_address    = uml_net_set_mac,
> -       .ndo_set_mac_address    = eth_mac_addr,
> 
> so it removes the later one, but it is not at all clear which one the
> compiler actually used. My guess is that it used to use the later one
> (the standard eth_mac_addr function), and the patch made it suddenly
> use the uml_net_set_mac function.

C99 6.7.8p19:

	The initialization shall occur in initializer list order, each
initializer provided for a particular subobject overriding any previously
listed initializer for the same subobject[*]; all subobjects that are not
initialized explicitly shall be initialized implicitly the same as objects
that have static storage duration.

[*] Any initializer for subobject which is overridden and so not used to
initialize that subobject might not be evaluated at all.


IOW, it _must_ use the last one in such cases.

As for the driver, I smell an interface change (in eth_mac_addr() arguments)
that has been missed...  FWIW, grep through the tree shows one more instance
of eth_mac_addr() called with such argument and it's also in net_kern.c; there
we simply want memcpy() instead, since device is definitely not running at
that point and we'd done the validity checks earlier.

Not sure if we need lp->lock around that eth_mac_addr() call - not familiar
with the driver in question.  If we don't, we should switch to eth_mac_addr
for the method, indeed...
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