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From:   Andy Lutomirski <luto@kernel.org>
Date:   Sat, 29 Dec 2018 08:49:19 -0800
Message-ID: <CALCETrUJn4cV+o3scP1EzYS=JA--5KUXvjuxZDbsnhXra1vrwA@mail.gmail.com>
Subject: Re: [Qemu-devel] d_off field in struct dirent and 32-on-64 emulation
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Peter Maydell <peter.maydell@linaro.org>,
        Andreas Dilger <adilger@dilger.ca>,
        Florian Weimer <fw@deneb.enyo.de>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Latchesar Ionkov <lucho@ionkov.net>,
        libc-alpha <libc-alpha@sourceware.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Eric Van Hensbergen <ericvh@gmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>,
        QEMU Developers <qemu-devel@nongnu.org>,
        Ron Minnich <rminnich@sandia.gov>,
        V9FS Developers <v9fs-developer@lists.sourceforge.net>
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> On Dec 28, 2018, at 6:54 PM, Matthew Wilcox <willy@infradead.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@dilger.ca> wrot
>>> On Dec 28, 2018, at 4:18 AM, Peter Maydell <peter.maydell@linaro.org> w=
rote:
>>>> 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=E2=80=99d h=
ave
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.