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From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: Eric Blake <eblake@redhat.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Linux API <linux-api@vger.kernel.org>,
	QEMU Developers <qemu-devel@nongnu.org>,
	Florian Weimer <fw@deneb.enyo.de>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	Andy Lutomirski <luto@kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Ext4 Developers List <linux-ext4@vger.kernel.org>
Subject: Re: [PATCH] fcntl: Add 32bit filesystem mode
Date: Mon, 20 Apr 2020 13:01:31 -0400	[thread overview]
Message-ID: <20200420170131.GD1080594@mit.edu> (raw)
In-Reply-To: <CAFEAcA9BQQah2vVfnwO4-3m4eHv9QtfvjvDpTdw+SmqicsDOMA@mail.gmail.com>

On Mon, Apr 20, 2020 at 04:29:32PM +0100, Peter Maydell wrote:
> On Mon, 20 Apr 2020 at 16:24, Eric Blake <eblake@redhat.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

WARNING: multiple messages have this Message-ID (diff)
From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: Linux API <linux-api@vger.kernel.org>,
	Linus Walleij <linus.walleij@linaro.org>,
	QEMU Developers <qemu-devel@nongnu.org>,
	Florian Weimer <fw@deneb.enyo.de>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	Andy Lutomirski <luto@kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Ext4 Developers List <linux-ext4@vger.kernel.org>
Subject: Re: [PATCH] fcntl: Add 32bit filesystem mode
Date: Mon, 20 Apr 2020 13:01:31 -0400	[thread overview]
Message-ID: <20200420170131.GD1080594@mit.edu> (raw)
In-Reply-To: <CAFEAcA9BQQah2vVfnwO4-3m4eHv9QtfvjvDpTdw+SmqicsDOMA@mail.gmail.com>

On Mon, Apr 20, 2020 at 04:29:32PM +0100, Peter Maydell wrote:
> On Mon, 20 Apr 2020 at 16:24, Eric Blake <eblake@redhat.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


  reply	other threads:[~2020-04-20 17:01 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-31 13:35 [PATCH] fcntl: Add 32bit filesystem mode Linus Walleij
2020-03-31 13:35 ` Linus Walleij
2020-04-20 11:19 ` Peter Maydell
2020-04-20 11:19   ` Peter Maydell
2020-04-20 11:23   ` Florian Weimer
2020-04-20 11:23     ` Florian Weimer
2020-04-20 11:38     ` Peter Maydell
2020-04-20 11:38       ` Peter Maydell
2020-04-20 14:16       ` Peter Maydell
2020-04-20 14:16         ` Peter Maydell
2020-04-20 23:51       ` Andreas Dilger
2020-04-20 23:51         ` Andreas Dilger
2020-04-21 13:02         ` Peter Maydell
2020-04-21 13:02           ` Peter Maydell
2020-04-20 15:13 ` Theodore Y. Ts'o
2020-04-20 15:13   ` Theodore Y. Ts'o
2020-04-20 15:23   ` Eric Blake
2020-04-20 15:29     ` Peter Maydell
2020-04-20 15:29       ` Peter Maydell
2020-04-20 17:01       ` Theodore Y. Ts'o [this message]
2020-04-20 17:01         ` Theodore Y. Ts'o

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