Subject: Re: [PATCH] netdev: Use flexible array for trailing private bytes

From: Jakub Kicinski <[email-masked]>
Date: Thu, 29 Feb 2024 22:59:10 -0800

On Thu, 29 Feb 2024 13:30:22 -0800 Kees Cook wrote:
> Introduce a new struct net_device_priv that contains struct net_device
> but also accounts for the commonly trailing bytes through the "size" and
> "data" members.

I'm a bit unclear on the benefit. Perhaps I'm unaccustomed to "safe C".

> As many dummy struct net_device instances exist still,
> it is non-trivial to but this flexible array inside struct net_device

put

Non-trivial, meaning what's the challenge?
We also do somewhat silly things with netdev lifetime, because we can't
assume netdev gets freed by netdev_free(). Cleaning up the "embedders"
would be beneficial for multiple reasons.

> itself. But we can add a sanity check in netdev_priv() to catch any
> attempts to access the private data of a dummy struct.
> 
> Adjust allocation logic to use the new full structure.
> 
> Signed-off-by: Kees Cook <[email-masked]>

> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 118c40258d07..b476809d0bae 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -1815,6 +1815,8 @@ enum netdev_stat_type {
>  	NETDEV_PCPU_STAT_DSTATS, /* struct pcpu_dstats */
>  };
>  
> +#define	NETDEV_ALIGN		32

Unless someone knows what this is for it should go.
Align priv to cacheline size.

>  /**
>   *	struct net_device - The DEVICE structure.
>   *

> @@ -2665,7 +2673,14 @@ void dev_net_set(struct net_device *dev, struct net *net)
>   */
>  static inline void *netdev_priv(const struct net_device *dev)
>  {
> -	return (char *)dev + ALIGN(sizeof(struct net_device), NETDEV_ALIGN);
> +	struct net_device_priv *priv;
> +
> +	/* Dummy struct net_device have no trailing data. */
> +	if (WARN_ON_ONCE(dev->reg_state == NETREG_DUMMY))
> +		return NULL;

This is a static inline with roughly 11,000 call sites, according to 
a quick grep. Aren't WARN_ONCE() in static inlines creating a "once"
object in every compilation unit where they get used?

> +	priv = container_of(dev, struct net_device_priv, dev);
> +	return (u8 *)priv->data;
>  }