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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Mon, 12 Apr 2021 13:26:00 +0200
Message-ID: <CAK8P3a1JZ=JerasdkntzX_ApaCF7C29ZS1E31aPQATOts0ZiLw@mail.gmail.com>
Subject: Re: [PATCH 5/5] compat: consolidate the compat_flock{,64} definition
To:     David Laight <David.Laight@aculab.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
        Helge Deller <deller@gmx.de>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        "David S. Miller" <davem@davemloft.net>,
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On Mon, Apr 12, 2021 at 12:54 PM David Laight <David.Laight@aculab.com> wrote:
> From: David Laight > Sent: 12 April 2021 10:37
> ...
> > I'm guessing that compat_pid_t is 16 bits?
> > So the native 32bit version has an unnamed 2 byte structure pad.
> > The 'packed' removes this pad from the compat structure.
> >
> > AFAICT (apart from mips) the __ARCH_COMPAT_FLOCK_PAD is just
> > adding an explicit pad for the implicit pad the compiler
> > would generate because compat_pid_t is 16 bits.
>
> I've just looked at the header.
> compat_pid_t is 32 bits.
> So Linux must have gained 32bit pids at some earlier time.
> (Historically Unix pids were 16 bit - even on 32bit systems.)
>
> Which makes the explicit pad in 'sparc' rather 'interesting'.

I saw it was there since the sparc kernel support got merged in
linux-1.3, possibly copied from an older sunos version.

> oh - compat_loff_t is only used in a couple of other places.
> neither care in any way about the alignment.
> (Provided get_user() doesn't fault on a 8n+4 aligned address.)

Ah right, I also see that after this series it's only used in to other
places:  compat_resume_swap_area, which could also lose the
__packed annotation, and in the declaration of
compat_sys_sendfile64, where it makes no difference.

      Arnd