netdev - Re: [PATCH v2 net-next] net/core: generic support for disabling netdev features down stack

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Message-ID: <CAMuHMdXrs2fkHw+DGgutM1y71Y=MTsxSqJ+J78e_Q7oZjaNesA@mail.gmail.com>
Date:	Tue, 3 Nov 2015 14:52:44 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Nikolay Aleksandrov <nikolay@...ulusnetworks.com>
Cc:	Jarod Wilson <jarod@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jay Vosburgh <j.vosburgh@...il.com>,
	Veaceslav Falico <vfalico@...il.com>,
	Andy Gospodarek <gospo@...ulusnetworks.com>,
	Jiri Pirko <jiri@...nulli.us>,
	Nikolay Aleksandrov <razor@...ckwall.org>,
	Michal Kubecek <mkubecek@...e.cz>,
	Alexander Duyck <alexander.duyck@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 net-next] net/core: generic support for disabling
 netdev features down stack

On Tue, Nov 3, 2015 at 11:03 AM, Nikolay Aleksandrov
<nikolay@...ulusnetworks.com> wrote:
> On 11/03/2015 03:55 AM, Jarod Wilson wrote:
> [snip]
>>
>> +#define for_each_netdev_feature(mask_addr, feature)                          \
>> +     int bit;                                                                \
>> +     for_each_set_bit(bit, (unsigned long *)mask_addr, NETDEV_FEATURE_COUNT) \
>> +             feature = __NETIF_F_BIT(bit);
>> +
> ^
> This is broken, it will not work for more than a single feature.

Indeed it is.

This is used as:

        for_each_netdev_feature(&upper_disables, feature) {
        ...
        }

which expands to:

        int bit;
        for_each_set_bit(bit, (unsigned long *)mask_addr, NETDEV_FEATURE_COUNT)
                feature = __NETIF_F_BIT(bit);
        {
                ...
        }

Note the assignment to "feature" happens outside the {}-delimited block.
And the block is always executed once.

Interestingly, arm-unknown-linux-gnueabi-gcc 4.9.0 warns about this in
some of my configs, but not in all of them:

net/core/dev.c: In function '__netdev_update_features':
net/core/dev.c:6302:16: warning: 'feature' may be used uninitialized
in this function [-Wmaybe-uninitialized]
    features &= ~feature;
                ^
net/core/dev.c:6295:20: note: 'feature' was declared here
  netdev_features_t feature;
                    ^
Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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