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:

quoted

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...

-Kees

-- 
Kees Cook