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

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Message-ID: <CALCETrUJn4cV+o3scP1EzYS=JA--5KUXvjuxZDbsnhXra1vrwA@mail.gmail.com>
Date:   Sat, 29 Dec 2018 08:49:19 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Peter Maydell <peter.maydell@...aro.org>,
        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 <libc-alpha@...rceware.org>,
        Arnd Bergmann <arnd@...db.de>,
        Eric Van Hensbergen <ericvh@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>,
        Ron Minnich <rminnich@...dia.gov>,
        V9FS Developers <v9fs-developer@...ts.sourceforge.net>
Subject: Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation

> On Dec 28, 2018, at 6:54 PM, Matthew Wilcox <willy@...radead.org> wrote:
>
>> 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?


How would this work?  We already have the separate
COMPAT_DEFINE_SYSCALL entries *and* in_compat_syscall(). Now we’d have
a third degree of freedom.

Either the arches people care about should add reasonable ways to
issue 32-bit syscalls from 64-bit mode or there should be an explicit
way to ask for the 32-bit directory offsets.