linux-kernel - Re: [PATCH net-next v2] netdevice: define and allocate &net_device _properly

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Message-ID: <279c95ad-a716-415b-a050-b323e21bec31@intel.com>
Date: Thu, 11 Jul 2024 14:54:06 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Eric Dumazet <edumazet@...gle.com>
CC: Breno Leitao <leitao@...ian.org>, "David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Kees Cook
	<kees@...nel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>,
	<keescook@...omium.org>, <horms@...nel.org>,
	<nex.sw.ncis.osdt.itp.upstreaming@...el.com>,
	<linux-hardening@...r.kernel.org>, Przemek Kitszel
	<przemyslaw.kitszel@...el.com>, Jiri Pirko <jiri@...nulli.us>, "Sebastian
 Andrzej Siewior" <bigeasy@...utronix.de>, Daniel Borkmann
	<daniel@...earbox.net>, Lorenzo Bianconi <lorenzo@...nel.org>, Johannes Berg
	<johannes.berg@...el.com>, Heiner Kallweit <hkallweit1@...il.com>, "open
 list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>, open list
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v2] netdevice: define and allocate &net_device
 _properly_

From: Eric Dumazet <edumazet@...gle.com>
Date: Wed, 10 Jul 2024 10:04:39 -0700

> On Wed, Jul 10, 2024 at 4:19 AM Breno Leitao <leitao@...ian.org> wrote:
>>
>> Hello Eric,
>>
>> On Tue, Jul 09, 2024 at 08:27:45AM -0700, Eric Dumazet wrote:
>>> On Tue, Jul 9, 2024 at 5:54 AM Breno Leitao <leitao@...ian.org> wrote:
>>
>>>> @@ -2596,7 +2599,7 @@ 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 (void *)dev->priv;
>>>
>>> Minor remark : the cast is not needed, but this is fine.
>>
>> In fact, the compiler is not very happy if I remove the cast:
>>
>>         ./include/linux/netdevice.h:2603:9: error: returning 'const u8[]' (aka 'const unsigned char[]') from a function with result type 'void *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
>>          2603 |         return dev->priv;
>>               |                ^~~~~~~~~
> 
> This is because of the ‘const’ qualifier of the parameter.
> 
> This could be solved with _Generic() later, if we want to keep the
> const qualifier.

I tried _Generic() when I was working on this patch and it seems like
lots of drivers need to be fixed first. They pass a const &net_device,
but assign the result to a non-const variable and modify fields there.
That's why I bet up on this and just casted to (void *) for now.

Thanks,
Olek