Re: [PATCH v4 7/8] net-device: Use new helpers from overflow.h in netdevice APIs

[Gustavo A. R. Silva]

On 2/28/24 18:01, Kees Cook wrote:
> On Wed, Feb 28, 2024 at 02:41:48PM -0800, Jakub Kicinski wrote:
> > On Wed, 28 Feb 2024 13:46:10 -0800 Kees Cook wrote:
> > > I really don't like hiding these trailing allocations from the compiler.
> > > Why can't something like this be done (totally untested):
> > >
> > >
> > > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > > index 118c40258d07..dae6df4fb177 100644
> > > --- a/include/linux/netdevice.h
> > > +++ b/include/linux/netdevice.h
> > > @@ -2475,6 +2475,8 @@ struct net_device {
> > >   	/** @page_pools: page pools created for this netdevice */
> > >   	struct hlist_head	page_pools;
> > >   #endif
> > > +	u32			priv_size;
> > > +	u8			priv_data[] __counted_by(priv_size) __aligned(NETDEV_ALIGN);
> >
> > I like, FWIW, please submit! :)
>
> So, I found several cases where struct net_device is included in the
> middle of another structure, which makes my proposal more awkward. But I
> also don't understand why it's in the _middle_. Shouldn't it always be
> at the beginning (with priv stuff following it?)
> Quick search and examined manually: git grep 'struct net_device [a-z0-9_]*;'
>
> struct rtw89_dev
> struct ath10k
> etc.
>
> Some even have two included (?)
>
> But I still like the idea -- Gustavo has been solving these cases with
> having two structs, e.g.:
>
> struct net_device {
> 	...unchanged...
> };
>
> struct net_device_alloc {
> 	struct net_device	dev;
> 	u32			priv_size;
> 	u8			priv_data[] __counted_by(priv_size) __aligned(NETDEV_ALIGN);
> };
>
> And internals can use struct net_device_alloc...

Yep, we should really consider going with the above, otherwise we would
have to do something like the following, to avoid having the flexible-array
member nested in the middle of other structs:

struct net_device {
	struct_group_tagged(net_device_hdr, hdr,
		...
		u32			priv_size;
	);
	u8			priv_data[] __counted_by(priv_size) __aligned(NETDEV_ALIGN);
}

We are grouping together the members in `struct net_device`, except the
flexible-array member, into a tagged `struct net_device_hdr`. This allows
us to exclude the flex array from its inclusion in any other struct
that contains `struct net_device` as a member without having to create
a completely separate struct definition.

And let's take as example `struct hfi1_netdev_rx`, where `struct net_device` is
included in the beginning:

drivers/infiniband/hw/hfi1/netdev.h:
struct hfi1_netdev_rx {

-	struct net_device rx_napi;
+       struct net_device_hdr rx_napi;


        struct hfi1_devdata *dd;
        struct hfi1_netdev_rxq *rxq;
        int num_rx_q;
        int rmt_start;
        struct xarray dev_tbl;
        /* count of enabled napi polls */
        atomic_t enabled;
        /* count of netdevs on top */
        atomic_t netdevs;
};

Of course we would also have to update the code that access `struct net_device`
members through `rx_napi` in `struct hfi1_netdev_rx`.

I'm currently working on the above solution for all the cases where having two
separate structs is not currently feasible. And with that we are looking to enable
`-Wflex-array-member-not-at-end`

So, if we can prevent this from the beginning it'd be really great. :)

--
Gustavo