linux-kernel - Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems

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Date:   Wed, 21 Apr 2021 07:50:28 +0200
From:   "hch@....de" <hch@....de>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        Matthew Wilcox <willy@...radead.org>,
        "grygorii.strashko@...com" <grygorii.strashko@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ilias.apalodimas@...aro.org" <ilias.apalodimas@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "brouer@...hat.com" <brouer@...hat.com>,
        "mcroce@...ux.microsoft.com" <mcroce@...ux.microsoft.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "hch@....de" <hch@....de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems

On Tue, Apr 20, 2021 at 11:20:19PM +0200, Arnd Bergmann wrote:
> In that case, there should be no problem for you.
> 
> The main issue is with system calls and ioctls that contain a misaligned
> struct member like
> 
> struct s {
>        u32 a;
>        u64 b;
> };
> 
> Passing this structure by reference from a 32-bit user space application
> to a 64-bit kernel with different alignment constraints means that the
> kernel has to convert the structure layout. See
> compat_ioctl_preallocate() in fs/ioctl.c for one such example.

We've also had this problem with some on-disk structures in the past,
but hopefully people desining those have learnt the lesson by now.