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From: Josh Triplett <>
To: "Theodore Y. Ts'o" <>
Cc: "Darrick J. Wong" <>,
	Linus Torvalds <>,
	Andreas Dilger <>,
	Jan Kara <>,
	Linux Kernel Mailing List <>,
Subject: Re: ext4 regression in v5.9-rc2 from e7bfb5c9bb3d on ro fs with overlapped bitmaps
Date: Mon, 5 Oct 2020 23:03:27 -0700	[thread overview]
Message-ID: <20201006060327.GA9227@localhost> (raw)
In-Reply-To: <20201006050306.GA8098@localhost>

On Mon, Oct 05, 2020 at 10:03:13PM -0700, Josh Triplett wrote:
> On Mon, Oct 05, 2020 at 11:18:34PM -0400, Theodore Y. Ts'o wrote:
> > What Josh is proposing I'm pretty sure would also break "e2fsck -E
> > unshare_blocks", so that's another reason not to accept this as a
> > valid format change.
> The kernel already accepted this as a valid mountable filesystem format,
> without a single error or warning of any kind, and has done so stably
> for years.
> > As far as I'm concerned, contrib/e2fsdroid is the canonical definition
> > of how to create valid file systems with shared_blocks.
> I'm not trying to create a problem here; I'm trying to address a whole
> family of problems. I was generally under the impression that mounting
> existing root filesystems fell under the scope of the kernel<->userspace
> or kernel<->existing-system boundary, as defined by what the kernel
> accepts and existing userspace has used successfully, and that upgrading
> the kernel should work with existing userspace and systems. If there's
> some other rule that applies for filesystems, I'm not aware of that.
> (I'm also not trying to suggest that every random corner case of what
> the kernel *could* accept needs to be the format definition, but rather,
> cases that correspond to existing userspace.)
> It wouldn't be *impossible* to work around this, this time; it may be
> possible to adapt the existing userspace to work on the new and old
> kernels. My concern is, if a filesystem format accepted by previous
> kernels can be rejected by future kernels, what stops a future kernel
> from further changing the format definition or its strictness
> (co-evolving with one specific userspace) and causing further
> regressions?
> I don't *want* to rely on what apparently turned out to be an
> undocumented bug in the kernel's validator. That's why I was trying to
> fix the issue in what seemed like the right way, by detecting the
> situation and turning off the validator. That seemed like it would fully
> address the issue. If it would help, I could also supply a tiny filesystem
> image for regression testing.
> I'm trying to figure out what solution you'd like to see here, as long
> as it isn't "any userspace that isn't e2fsdroid can be broken at will".
> I'd be willing to work to adapt the userspace bits I have to work around
> the regression, but I'd like to get this on the radar so this doesn't
> happen again.

To clarify something further: I'm genuinely not looking to push hard on
the limits or corners of the kernel/userspace boundary here, nor do I
want to create an imposition on development. I'm happy to attempt to be
a little more flexible than most userspace. I'm trying to make
substantial, non-trivial use of the userspace side of a kernel/userspace
boundary, and within reason, I need to rely on the kernel's stability
guarantees. I'm relying on the combination of
Documentation/filesystems/ext4 and fs/ext4 as the format documentation.
The first time I discovered this issue was in doing some "there's about
to be a new kernel release" regression testing for 5.9, in which it
created a debugging adventure to track down what the problem was. I'd
like to find a good way to report and handle this kind of thing going
forward, if another issue like this arises.

- Josh Triplett

  reply	other threads:[~2020-10-06  6:03 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 [this message]
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
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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