linux-ext4 - Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation

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Date:   Fri, 28 Dec 2018 17:54:54 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Maydell <peter.maydell@...aro.org>
Cc:     Andreas Dilger <adilger@...ger.ca>,
        Florian Weimer <fw@...eb.enyo.de>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        Latchesar Ionkov <lucho@...kov.net>, libc-alpha@...rceware.org,
        Arnd Bergmann <arnd@...db.de>,
        Eric Van Hensbergen <ericvh@...il.com>, hpa@...or.com,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>, rminnich@...dia.gov,
        v9fs-developer@...ts.sourceforge.net
Subject: Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation

On Sat, Dec 29, 2018 at 12:12:27AM +0000, Peter Maydell wrote:
> On Fri, 28 Dec 2018 at 23:16, Andreas Dilger <adilger@...ger.ca> wrot
> > On Dec 28, 2018, at 4:18 AM, Peter Maydell <peter.maydell@...aro.org> wrote:
> > > The problem is that there is no 32-bit API in some cases
> > > (unless I have misunderstood the kernel code) -- not all
> > > host architectures implement compat syscalls or allow them
> > > to be called from 64-bit processes or implement all the older
> > > syscall variants that had smaller offets. If there was a guaranteed
> > > "this syscall always exists and always gives me 32-bit offsets"
> > > we could use it.
> >
> > The "32bitapi" mount option would use 32-bit hash for seekdir
> > and telldir, regardless of what kernel API was used.  That would
> > just set the FMODE_32BITHASH flag in the file->f_mode for all files.
> 
> A mount option wouldn't be much use to QEMU -- we can't tell
> our users how to mount their filesystems, which they're
> often doing lots of other things with besides running QEMU.
> (Otherwise we could just tell them "don't use ext4", which
> would also solve the problem :-)) We need something we can
> use at the individual-syscall level.

Could you use a prctl to set whether you were running in 32 or 64 bit
mode?  Or do you change which kind of task you're emulating too often
to make this a good idea?