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

[Kees Cook]

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 <andriy.shevche...@linux.intel.com>
> ---
>  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