linux-ext4 - Re: [RFC PATCH 0/6] Allow setting file birth time with utimensat()

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Date:   Sun, 17 Feb 2019 12:40:09 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Adam Borowski <kilobyte@...band.pl>
Cc:     Boaz Harrosh <openosd@...il.com>,
        Dave Chinner <david@...morbit.com>,
        Omar Sandoval <osandov@...ndov.com>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        kernel-team <kernel-team@...com>,
        Linux API <linux-api@...r.kernel.org>,
        Linux btrfs Developers List <linux-btrfs@...r.kernel.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org
Subject: Re: [RFC PATCH 0/6] Allow setting file birth time with utimensat()

On Sun, Feb 17, 2019 at 9:55 AM Adam Borowski <kilobyte@...band.pl> wrote:
>
> On Sun, Feb 17, 2019 at 06:35:25PM +0200, Boaz Harrosh wrote:
> > On 15/02/19 00:06, Dave Chinner wrote:
> > > So you're adding an interface that allows users to change the create
> > > time of files without needing any privileges?
>
> > > Inode create time is forensic metadata in XFS  - information we use
> > > for sequence of event and inode lifetime analysis during examination
> > > of broken filesystem images and systems that have been broken into.
>
> > I think the difference in opinion here is that there are two totally
> > different BTIme out in the world. For two somewhat opposite motivations
> > and it seems they both try to be crammed into the same on disk space.
> >
> > One - Author creation time
> > Two - Local creation time
>
> > So it looks like both sides are correct trying to preserve their own guy?
>
> I'd say that [2] is too easily gameable to be worth the effort.  You can
> just change it on the disk.  That right now it'd take some skill to find the
> right place to edit doesn't matter -- a tool to update the btime against
> your wishes would need to be written just once.  Unlike btrfs, XFS doesn't
> even have a chain of checksums all the way to the root.
>
> On the other hand, [1] has a lot of uses.  It can also be preserved in
> backups and version control (svnt and git-restore-mtime could be easily
> extended).
>
> I'd thus go with [2] -- any uses for [1] are better delegated to filesystem
> specific tools.
>

I started out in the Windows world, and I found it quite handy to
right-click a file and see when it was created.  When I started using
Linux, I saw things like this:

Access: 2019-02-16 22:19:32.024284060 -0800
Modify: 2016-02-02 19:26:47.901766778 -0800
Change: 2016-02-02 19:26:47.907766649 -0800

and my mind boggled a bit.  Modify makes sense.  Change?  What's that?
 Why do I care?

Adding "birth" makes sense, and I think that filesystem-agnostic
backup tools *should* be able to write it.

So I'm highly in favor of this patch.  If XFS wants to disallow
writing the birth time, fine, but I think that behavior should be
overridable.