Re: [PATCH net-next] net: switch to storing KCOV handle directly in sk_buff

[Willem de Bruijn]

On Thu, Nov 26, 2020 at 3:19 AM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Wed, 25 Nov 2020 at 21:43, Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
> >
> > On Wed, 25 Nov 2020 18:34:36 +0100 Marco Elver wrote:
> > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > > index ffe3dcc0ebea..070b1077d976 100644
> > > --- a/net/core/skbuff.c
> > > +++ b/net/core/skbuff.c
> > > @@ -233,6 +233,7 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
> > >       skb->end = skb->tail + size;
> > >       skb->mac_header = (typeof(skb->mac_header))~0U;
> > >       skb->transport_header = (typeof(skb->transport_header))~0U;
> > > +     skb_set_kcov_handle(skb, kcov_common_handle());
> > >
> > >       /* make sure we initialize shinfo sequentially */
> > >       shinfo = skb_shinfo(skb);
> > > @@ -249,9 +250,6 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
> > >
> > >               fclones->skb2.fclone = SKB_FCLONE_CLONE;
> > >       }
> > > -
> > > -     skb_set_kcov_handle(skb, kcov_common_handle());
> >
> > Why the move?
>
> v2 of the original series had it above. I frankly don't mind.
>
> 1. Group it with the other fields above?
>
> 2. Leave it at the end here?
>
> > >  out:
> > >       return skb;
> > >  nodata:
> > > @@ -285,8 +283,6 @@ static struct sk_buff *__build_skb_around(struct sk_buff *skb,
> > >       memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
> > >       atomic_set(&shinfo->dataref, 1);
> > >
> > > -     skb_set_kcov_handle(skb, kcov_common_handle());
> > > -
> > >       return skb;
> > >  }
> >
> > And why are we dropping this?
>
> It wasn't here originally.
>
> > If this was omitted in earlier versions it's just a independent bug,
> > I don't think build_skb() will call __alloc_skb(), so we need a to
> > set the handle here.
>
> Correct, that was an original omission.
>
> Will send v2.

Does it make more sense to revert the patch that added the extensions
and the follow-on fixes and add a separate new patch instead?

If adding a new field to the skb, even if only in debug builds,
please check with pahole how it affects struct layout if you
haven't yet.

The skb_extensions idea was mine. Apologies for steering
this into an apparently unsuccessful direction. Adding new fields
to skb is very rare because possibly problematic wrt allocation.