linux-kernel - Re: [PATCH 5/5] compat: consolidate the compat

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Date:   Mon, 12 Apr 2021 13:26:00 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     David Laight <David.Laight@...lab.com>
Cc:     Christoph Hellwig <hch@....de>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "David S. Miller" <davem@...emloft.net>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 5/5] compat: consolidate the compat_flock{,64} definition

On Mon, Apr 12, 2021 at 12:54 PM David Laight <David.Laight@...lab.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