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From: Josh Triplett <josh@joshtriplett.org>
To: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: "Theodore Y. Ts'o" <tytso@mit.edu>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	Jan Kara <jack@suse.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-ext4@vger.kernel.org
Subject: Re: ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with overlapped bitmaps
Date: Fri, 9 Oct 2020 12:08:48 -0700	[thread overview]
Message-ID: <20201009190848.GB4649@localhost> (raw)
In-Reply-To: <20201009025409.GB6532@magnolia>

On Thu, Oct 08, 2020 at 07:54:09PM -0700, Darrick J. Wong wrote:
> On Thu, Oct 08, 2020 at 03:38:58PM -0700, Josh Triplett wrote:
> > On Thu, Oct 08, 2020 at 10:54:48AM -0700, Darrick J. Wong wrote:
> > > > > the head", and continued with the notion that anything other than
> > > > > e2fsprogs making something to be mounted by mount(2) and handled by
> > > > > fs/ext4 is being "inflicted", and if the goal didn't still seem to be
> > > > > "how do we make it go away so that only e2fsprogs and the kernel ever
> > > > > touch ext4". I started this thread because I'd written some userspace
> > > > > code, a new version of the kernel made that userspace code stop working,
> > > > > so I wanted to report that the moment I'd discovered that, along with a
> > > > > potential way to address it with as little disrupton to ext4 as
> > > > > possible.
> > > 
> > > Ted: I don't think it's a good idea to make SHARED_BLOCKS disable block
> > > validity checking by default.  You could someday enable users to write
> > > to block-sharing files by teaching ext4 how to COW, at which point you'd
> > > need correct bitmaps and block validity to prevent accidental overwrite
> > > of critical metadata.  At that point you'd either be stuck with the
> > > precedent that SHARED_BLOCKS implies noblock_validity, or you'd end up
> > > breaking Josh's images again.
> > 
> > That's a fair point (though I think a writable COW version of
> > SHARED_BLOCKS would need a different flag). It'd make more sense to key
> > this logic off of RO_COMPAT_READONLY or similar. But even better:
> 
> It wouldn't require a new flag -- "rocompat" features bits mean that
> "it's safe to allow userspace to read files off the disk if software
> doesn't recognize this feature bit".

Sure; I was more thinking that if that involved adding some data
structures to track shared blocks and the need to COW, whatever
mechanism that used might potentially need an incompat flag.

> If someone /did/ add the code to write to files safely in the presence
> of shared blocks, all they'd have to do to light up the functionality is
> add it to the _SUPP define.  Also, it's strange that the kernel source
> doesn't even know of SHARED_BLOCKS, but that's also on Ted...

It would be nice if the kernel's ext4.h header and e2fsprogs were fully
in sync, yes.

- Josh Triplett

  reply	other threads:[~2020-10-09 19:09 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-04 23:17 Linux 5.9-rc8 Linus Torvalds
2020-10-05  8:14 ` ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with overlapped bitmaps Josh Triplett
2020-10-05  9:46   ` Jan Kara
2020-10-05 10:16     ` Josh Triplett
2020-10-05 16:19       ` Jan Kara
2020-10-05 16:20   ` Jan Kara
2020-10-05 17:36   ` Darrick J. Wong
2020-10-06  0:04     ` Theodore Y. Ts'o
2020-10-06  0:32     ` Josh Triplett
2020-10-06  2:51       ` Darrick J. Wong
2020-10-06  3:18         ` Theodore Y. Ts'o
2020-10-06  5:03           ` Josh Triplett
2020-10-06  6:03             ` Josh Triplett
2020-10-06 13:35             ` Theodore Y. Ts'o
2020-10-07  8:03               ` Josh Triplett
2020-10-07 14:32                 ` Theodore Y. Ts'o
2020-10-07 20:14                   ` Josh Triplett
2020-10-08  2:10                     ` Theodore Y. Ts'o
2020-10-08 17:54                       ` Darrick J. Wong
2020-10-08 22:38                         ` Josh Triplett
2020-10-09  2:54                           ` Darrick J. Wong
2020-10-09 19:08                             ` Josh Triplett [this message]
2020-10-08 22:22                       ` Josh Triplett
2020-10-09 14:37                         ` Theodore Y. Ts'o
2020-10-09 20:30                           ` Josh Triplett
2021-01-10 18:41                           ` Malicious fs images was " Pavel Machek
2021-01-11 18:51                             ` Darrick J. Wong
2021-01-11 19:39                               ` Eric Biggers
2021-01-12 21:43                             ` Theodore Ts'o
2021-01-12 22:28                               ` Pavel Machek
2021-01-13  5:09                                 ` Theodore Ts'o
2020-10-08  2:57                     ` Andreas Dilger
2020-10-08 19:12                       ` Josh Triplett
2020-10-08 19:25                         ` Andreas Dilger
2020-10-08 22:28                           ` Josh Triplett

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