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

From: Geert Uytterhoeven
Date: Tue Nov 03 2015 - 08:52:51 EST

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;


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
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at