linux-ext4 - Re: [PATCH] fcntl: Add 32bit filesystem mode

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Date:   Mon, 20 Apr 2020 10:23:46 -0500
From:   Eric Blake <eblake@...hat.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>,
        Linus Walleij <linus.walleij@...aro.org>
Cc:     Peter Maydell <peter.maydell@...aro.org>,
        linux-api@...r.kernel.org, qemu-devel@...gnu.org,
        Florian Weimer <fw@...eb.enyo.de>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Andy Lutomirski <luto@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] fcntl: Add 32bit filesystem mode

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