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

From: Kees Cook <[email-masked]>
Date: Wed, 28 Feb 2024 13:46:10 -0800

On Wed, Feb 28, 2024 at 10:41:37PM +0200, Andy Shevchenko wrote:
> We have two new helpers struct_size_with_data() and struct_data_pointer()
> that we can utilize in alloc_netdev_mqs() and netdev_priv(). Do it so.
> 
> Signed-off-by: Andy Shevchenko <[email-masked]>
> ---
>  include/linux/netdevice.h |  3 ++-
>  net/core/dev.c            | 10 +++++-----
>  2 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index c41019f34179..d046dca18854 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -25,6 +25,7 @@
>  #include <linux/bug.h>
>  #include <linux/delay.h>
>  #include <linux/atomic.h>
> +#include <linux/overflow.h>
>  #include <linux/prefetch.h>
>  #include <asm/cache.h>
>  #include <asm/byteorder.h>
> @@ -2668,7 +2669,7 @@ 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);
> +	return struct_data_pointer(dev, NETDEV_ALIGN);
>  }

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);
 };
 #define to_net_dev(d) container_of(d, struct net_device, dev)
 
@@ -2665,7 +2667,7 @@ 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);
+	return dev->priv_data;
 }
 
 /* Set the sysfs physical device reference for the network logical device
diff --git a/net/core/dev.c b/net/core/dev.c
index cb2dab0feee0..afaaa3224656 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10814,18 +10814,14 @@ struct net_device *alloc_netdev_mqs(int sizeof_priv, const char *name,
 		return NULL;
 	}
 
-	alloc_size = sizeof(struct net_device);
-	if (sizeof_priv) {
-		/* ensure 32-byte alignment of private area */
-		alloc_size = ALIGN(alloc_size, NETDEV_ALIGN);
-		alloc_size += sizeof_priv;
-	}
+	alloc_size = struct_size(p, priv_data, sizeof_priv);
 	/* ensure 32-byte alignment of whole construct */
 	alloc_size += NETDEV_ALIGN - 1;
 
 	p = kvzalloc(alloc_size, GFP_KERNEL_ACCOUNT | __GFP_RETRY_MAYFAIL);
 	if (!p)
 		return NULL;
+	p->priv_size = sizeof_priv;
 
 	dev = PTR_ALIGN(p, NETDEV_ALIGN);
 	dev->padded = (char *)dev - (char *)p;

-- 
Kees Cook