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

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Message-ID: <20200420170131.GD1080594@mit.edu>
Date:   Mon, 20 Apr 2020 13:01:31 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Peter Maydell <peter.maydell@...aro.org>
Cc:     Eric Blake <eblake@...hat.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Linux API <linux-api@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>,
        Florian Weimer <fw@...eb.enyo.de>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Andy Lutomirski <luto@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] fcntl: Add 32bit filesystem mode

On Mon, Apr 20, 2020 at 04:29:32PM +0100, Peter Maydell wrote:
> On Mon, 20 Apr 2020 at 16:24, Eric Blake <eblake@...hat.com> wrote:
> > 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.
> 
> For instance, a quick grep shows 4 instances of this in QEMU :-)

Fortunately, most applications aren't going to be interested in
forcing 32-bit mode for 64-bit applications, QEMU being the notable
exception.  We do need to make sure that for 32-bit applications, we
either make FD_32BIT_MODE a no-op (don't set the bit, and ignore the
bit).  We could allow the bit to be visible for 32-bit applications,
but we would want to disallow clearing the the bit for 32-bit
applications if it was visible.

If we did that, then blind assignments of fcntl(fd, F_SETFD, 1) should
be mostly harmless with respect to the FD_32BIT_MODE bit.

   	      	 	  	       - Ted