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Subject: Re: [PATCH] fcntl: Add 32bit filesystem mode
To:     "Theodore Y. Ts'o" <tytso@mit.edu>,
        Linus Walleij <linus.walleij@linaro.org>
Cc:     Peter Maydell <peter.maydell@linaro.org>,
        linux-api@vger.kernel.org, qemu-devel@nongnu.org,
        Florian Weimer <fw@deneb.enyo.de>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Andy Lutomirski <luto@kernel.org>,
        linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org
References: <20200331133536.3328-1-linus.walleij@linaro.org>
 <20200420151344.GC1080594@mit.edu>
From:   Eric Blake <eblake@redhat.com>
Organization: Red Hat, Inc.
Message-ID: <d3fb73a3-ecf6-6371-783f-24a94eb66c59@redhat.com>
Date:   Mon, 20 Apr 2020 10:23:46 -0500
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On 4/20/20 10:13 AM, Theodore Y. Ts'o wrote:
> On Tue, Mar 31, 2020 at 03:35:36PM +0200, Linus Walleij wrote:
>> It was brought to my attention that this bug from 2018 was
>> still unresolved: 32 bit emulators like QEMU were given
>> 64 bit hashes when running 32 bit emulation on 64 bit systems.
>>
>> This adds a fcntl() operation to set the underlying filesystem
>> into 32bit mode even if the file hanle was opened using 64bit
>> mode without the compat syscalls.
> 
> s/hanle/handle/
> 
> The API that you've proposed as a way to set the 32-bit mode, but
> there is no way to clear the 32-bit mode, nor there is a way to get
> the current status mode.
> 
> My suggestion is to add a flag bit for F_GETFD and F_SETFD (set and
> get file descriptor flags).  Currently the only file descriptor flag
> is FD_CLOEXEC, so why not add a FD_32BIT_MODE bit?

Also, POSIX is proposing standardizing FD_CLOFORK, which would be 
another file descriptor flag worth considering in Linux (Solaris and BSD 
already have it):

https://www.austingroupbugs.net/view.php?id=1318

It will be interesting to find how much code (wrongly) assumes it can 
use a blind assignment of fcntl(fd, F_SETFD, 1) and thereby accidentally 
wipes out other existing flags, when it should have instead been doing a 
read-modify-write to protect flags other than FD_CLOEXEC.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org