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Date:   Wed, 21 Apr 2021 07:50:28 +0200
From:   "hch@lst.de" <hch@lst.de>
To:     Arnd Bergmann <arnd@kernel.org>
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        Matthew Wilcox <willy@infradead.org>,
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Subject: Re: [PATCH 1/2] mm: Fix struct page layout on 32-bit systems
Message-ID: <20210421055028.GA28910@lst.de>
References: <20210416230724.2519198-1-willy@infradead.org> <20210416230724.2519198-2-willy@infradead.org> <20210417024522.GP2531743@casper.infradead.org> <9f99b0a0-f1c1-f3b0-5f84-3a4bfc711725@synopsys.com> <20210420031029.GI2531743@casper.infradead.org> <CAK8P3a0KUwf1Z0bHiUaHC2nHztevkxg5_FBSzHddNeSsBayWUA@mail.gmail.com> <8d0fce1c-be7c-1c9b-bf5c-0c531db496ac@synopsys.com> <CAK8P3a3rzz1gfNLoGC8aZJiAC-tgZYD6P8pQsoEfgCAmQK=FAw@mail.gmail.com>
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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.