linux-kernel - Re: [PATCH 02/20] 32-bit ABI: introduce ARCH_32BIT_OFF

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Message-ID: <20170614123121.GA44176@MBP.local>
Date:   Wed, 14 Jun 2017 13:31:22 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Yury Norov <ynorov@...iumnetworks.com>
Cc:     linux-doc@...r.kernel.org,
        Andrew Pinski <Andrew.Pinski@...iumnetworks.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Chris Metcalf <cmetcalf@...hip.com>,
        philipp.tomsich@...obroma-systems.com,
        Joseph Myers <joseph@...esourcery.com>,
        zhouchengming1@...wei.com,
        Steve Ellcey <sellcey@...iumnetworks.com>,
        Prasun.Kapoor@...iumnetworks.com, Andreas Schwab <schwab@...e.de>,
        agraf@...e.de, szabolcs.nagy@....com, geert@...ux-m68k.org,
        Adam Borowski <kilobyte@...band.pl>,
        manuel.montezelo@...il.com, Chris Metcalf <cmetcalf@...lanox.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Pinski <pinskia@...il.com>, linyongting@...wei.com,
        klimov.linux@...il.com, broonie@...nel.org,
        Bamvor Zhangjian <bamvor.zhangjian@...wei.com>,
        linux-arm-kernel@...ts.infradead.org,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Florian Weimer <fweimer@...hat.com>, Nathan_Lynch@...tor.com,
        linux-kernel@...r.kernel.org,
        Ramana Radhakrishnan <ramana.gcc@...glemail.com>,
        schwidefsky@...ibm.com, davem@...emloft.net,
        christoph.muellner@...obroma-systems.com
Subject: Re: [PATCH 02/20] 32-bit ABI: introduce ARCH_32BIT_OFF_T config
 option

On Tue, Jun 13, 2017 at 02:04:11PM +0300, Yury Norov wrote:
> On Thu, Jun 08, 2017 at 04:09:50PM +0100, Catalin Marinas wrote:
> > On Sun, Jun 04, 2017 at 02:59:51PM +0300, Yury Norov wrote:
> > > All new 32-bit architectures should have 64-bit off_t type, but existing
> > > architectures has 32-bit ones.
> > > 
> > > To handle it, new config option is added to arch/Kconfig that defaults
> > > ARCH_32BIT_OFF_T to be disabled for non-64 bit architectures. All existing
> > > 32-bit architectures enable it explicitly here.
> > > 
> > > New option affects force_o_largefile() behaviour. Namely, if off_t is
> > > 64-bits long, we have no reason to reject user to open big files.
> > > 
> > > Note that even if architectures has only 64-bit off_t in the kernel
> > > (arc, c6x, h8300, hexagon, metag, nios2, openrisc, tile32 and unicore32),
> > > a libc may use 32-bit off_t, and therefore want to limit the file size
> > > to 4GB unless specified differently in the open flags.
> > > 
> > > Signed-off-by: Yury Norov <ynorov@...iumnetworks.com>
> > > Acked-by: Arnd Bergmann <arnd@...db.de>
> > [...]
> > > diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
> > > index 1b48d9c9a561..297993c92490 100644
> > > --- a/include/linux/fcntl.h
> > > +++ b/include/linux/fcntl.h
> > > @@ -11,7 +11,7 @@
> > >  	 O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE)
> > >  
> > >  #ifndef force_o_largefile
> > > -#define force_o_largefile() (BITS_PER_LONG != 32)
> > > +#define force_o_largefile() (!IS_ENABLED(CONFIG_ARCH_32BIT_OFF_T))
> > >  #endif
> > 
> > I may have confused myself with which off_t is 64-bit here for new
> > 32-bit architectures. Are we referring to the glibc definition, the
> > kernel one or simply that force_o_largefile() is true by default.
> > Because the type off_t for 32-bit kernel builds is still, well, 32-bit.
> > 
> > Otherwise it seems that the first paragraph in the description above
> > should read "all new 32-bit ABIs on a 64-bit kernel..." but then
> > AArch64/ILP32 is no longer the same as a new, pure 32-bit architecture.
>  
> This is all about userspace off_t types, like Arnd told in the comment
> to patch 13. I'll underline it in the comment to the patch. If it's
> not enough, I can also rename the config option to
> CONFIG_ARCH_32BIT_USER_OFF_T or similar. For me it's too much, but if
> you find it reasonable, I'll do it. Just let me know.

Thanks for clarification. I had the impression that it should match the
kernel's off_t (which is exported in the kernel headers as 32-bit) but
compiling with -mabi=ilp32 indeed shows sizeof(off_t) == 8. So that's
just a user decision to use loff_t instead and such port shouldn't use
any of the syscalls that pass the kernel's off_t.

I would rather see the comment in the arch/Kconfig help entry in this
patch for future reference.

-- 
Catalin