Re: Sparse not supporting kzalloc_obj() and friends

Hi Linus

On Mon, 23 Feb 2026 at 17:36, Linus Torvalds
<[email protected]> wrote:
>
> On Mon, 23 Feb 2026 at 08:19, Linus Torvalds
> <[email protected]> wrote:
> >
> > Argh. The culprit is the use of __VA_OPT__, which apparently sparse
> > never got taught.
> >    [...]
> > Ho humm. I guess I should look at the sparse pre-processor code,
> > unless somebody already implemented __VA_OPT__ somewhere?
>
> Actually, it looks like Al at least *looked* at doing __VA_OPT__ back
> at the end of last year. In fact, just a couple of months ago I said
> "I'm not sure it buys us much", but that was when I thought it was
> just a standard versions of the gcc ## thing, and before I realized
> how much more convenient it actually is to use.
>
> Al? You know the pre-process.c file a *lot* better than I do, even
> ignoring the fact that it's been fifteen years since I touched it. And
> you presumably already had some thoughts on doing __VA_OPT__?
>
> That said, I can also just make default_gfp() use ## after all. I
> _had_ grepped for __VA_OPT__ in the kernel and decided "we already use
> it", but clearly it's causing some issues.
>
> So instead of <linux/gfp.h> doing
>
>   #define __default_gfp(a,...) a
>   #define default_gfp(...) __default_gfp(__VA_ARGS__ __VA_OPT__(,) GFP_KERNEL)
>
> which I think is fairly straightforward and the "natural" way to do
> things, the kernel can just do
>
>   #define __default_gfp(a,b,...) b
>   #define default_gfp(...) __default_gfp(,##__VA_ARGS__,GFP_KERNEL)
>
> instead, with a dummy empty argument to make the gcc ## trick work.
> It's smaller and denser, so it's actually superior in that sense, even
> if it feels a bit less natural than the __VA_OPT__ model.
>
> Ricardo, can you verify that that trick works for you?

I tried both smatch and sparse and both work fine with your trick.

I also tried to boot it with virtme... That also worked :)

I will try to retrofit your hack into media-ci until it lands in rc.

Thanks


>
> Al, comments?
>
>              Linus



-- 
Ricardo Ribalda