[PATCH 02/13] wifi: nl80211: send underlying multi-hardware channel capabilities to user space

[Johannes Berg]

On Thu, 2024-03-28 at 11:49 -0700, Jakub Kicinski wrote:
> > So in that sense, I prefer that, but I'm truly not sure how the (hand-
> > written) userspace code would deal with that.
> 
> I think the best way today would be two walks:
> 
> 	for_each_attr() {
> 		switch (type):
> 		case THE_A_ARRAY_1:
> 			cnt1++;
> 			break;
> 		case THE_A_ARRAY_2:
> 			cnt2++;
> 			break;
> 	}
> 
> 	if (cnt1)
> 		array_1 = calloc();
> 	cnt1 = 0; /* we'll use it as index in second loop */
> 	if (cnt2)
> 		array_2 = calloc();
> 	cnt2 = 0;
> 
> 	for_each_attr() {
> 		/* [ normal parsing, populating array_1[cnt1++] etc. ] */
> 	}

Yeah, that makes sense.

I'm not sure we even need the calloc() all the time, depends what we're
doing with it, of course.

> Compared to "indexed array" the only practical difference I think is
> the fact that all attrs are walked. I think you have to count them
> either way before parsing.

Right, generally the pattern would be something like

nla_for_each_nested(...)
	n++;

// alloc etc.

idx = 0;
nla_for_each_nested(...)
	array[idx++] = whatever(attr);

or something like that.

So I guess the only thing that changes really is that this now becomes

nla_for_each(...)
	if (type != DESIRED)
		continue;

vs.

nla_for_each_nested(...)


I suppose we could even define a

nla_for_each_type(..., type)

for that.

> I was wondering at some point whether we should require that all
> multi-attr attributes are grouped together. Or add an explicit "count"
> attribute. But couldn't convince myself that such extra rules will
> pay off sufficiently with perf and/or ease of use...

That doesn't seem likely, after all, you'll definitely want to double-
check all that ... Personally, unless you have something super perf
critical, I definitely prefer _not_ having a count like that in the API
because it encourages unsafe code that doesn't do the necessary bounds
checks and then crashes ...

johannes