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

[Geert Uytterhoeven]

On Tue, Nov 3, 2015 at 11:03 AM, Nikolay Aleksandrov
<nikolay@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx

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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