linux-kernel - Re: [PATCH v2 2/2] r8169: Avoid pointer aliasing

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Message-ID: <143a8f022c73c71f170eb04dc0b763f9ae83f6b5.camel@perches.com>
Date:   Tue, 05 Feb 2019 11:19:04 -0800
From:   Joe Perches <joe@...ches.com>
To:     David Miller <davem@...emloft.net>
Cc:     thierry.reding@...il.com, hkallweit1@...il.com, andrew@...n.ch,
        nic_swsd@...ltek.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] r8169: Avoid pointer aliasing

On Tue, 2019-02-05 at 11:14 -0800, David Miller wrote:
> From: Joe Perches <joe@...ches.com>
> Date: Tue, 05 Feb 2019 10:42:54 -0800
> 
> > On Mon, 2019-02-04 at 19:20 -0800, David Miller wrote:
> >> From: Thierry Reding <thierry.reding@...il.com>
> >> Date: Mon,  4 Feb 2019 17:42:13 +0100
> >> 
> >> > @@ -7316,7 +7325,7 @@ static int rtl_get_ether_clk(struct rtl8169_private *tp)
> >> >  static int rtl_init_one(struct pci_dev *pdev, const struct pci_device_id *ent)
> >> >  {
> >> >       const struct rtl_cfg_info *cfg = rtl_cfg_infos + ent->driver_data;
> >> > -     u8 mac_addr[ETH_ALEN] __aligned(4) = {};
> >> > +     u8 mac_addr[ETH_ALEN] = {};
> >> >       struct rtl8169_private *tp;
> >> 
> >> I agree with Heiner, you have to provide at least 2 byte alignment for this
> >> buffer due to the reasons he stated.
> > 
> > It's declared after a pointer so it is already is 2 byte aligned.
> > 
> > A lot of drivers wouldn't work otherwise.
> 
> That's assuming a lot about what the compiler will do when nit allocates
> local variables to the stack.

It's also assuming what a compiler will do when
it defines a struct.

> I want it _explicit_.

Your choice, but there are a _lot_ of existing uses
and I think requiring it is as senseless as requiring
void * arithmetic to be cast to char * as gcc and
clang already do not add padding after a pointer.