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

From: Jarod Wilson
Date: Tue Nov 03 2015 - 11:09:52 EST

Nikolay Aleksandrov wrote:
On 11/03/2015 02:57 PM, Jarod Wilson wrote:
Geert Uytterhoeven wrote:
On Tue, Nov 3, 2015 at 11:03 AM, Nikolay Aleksandrov
<nikolay@xxxxxxxxxxxxxxxxxxx> wrote:
On 11/03/2015 03:55 AM, Jarod Wilson wrote:
+#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.
Bah, crap, I was still staring at the code not seeing it, thank you for the detailed cluebat. I'll fix that up right now.

Yeah, sorry for not elaborating, I wrote it in a hurry. :-)
Thanks Geert!

By the way since you'll be changing this code, I don't know if it's okay to
declare caller-visible hidden local variables in a macro like this, at the very
least please consider renaming it to something that's much less common, I can see
"bit" being used here and there. IMO either try to find a way to avoid it
altogether or add another argument to the macro so it's explicitly passed.

Just posted a follow-up that removes the macro-internal use of bit and doesn't botch up assigning feature. It's not as pretty, but it works correctly with multiple feature bits.

Jarod Wilson

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