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

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Date:   Fri, 22 Feb 2019 11:00:48 -0800
From:   Omar Sandoval <osandov@...ndov.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     Andreas Dilger <adilger@...ger.ca>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>, kernel-team@...com,
        Linux API <linux-api@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        linux-xfs@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Steve French <sfrench@...ba.org>
Subject: Re: [RFC PATCH 0/6] Allow setting file birth time with utimensat()

On Tue, Feb 19, 2019 at 09:18:20AM +1100, Dave Chinner wrote:
> On Sat, Feb 16, 2019 at 06:57:45PM -0700, Andreas Dilger wrote:
> > While it may be a bit of a stretch to call this "forensic evidence", making
> 
> We do forensic analysis of corrupt filesystems looking for evidence
> of what went wrong, not just looking for evidence of what happened
> on systems that have been broken into.
> 
> > it hard to change from except via total root compromise by a skilled hacker
> > is very useful.
> 
> *nod*.
> 
> > If this were to go in (which I'm not in favour of), then there would need to
> > be a CONFIG and/or runtime knob to turn it off (or better to only turn it on),
> > similar to how FIPS and other security options can only go in one direction.
> 
> The problem here is that "inode birth time" is being conflated with
> "user document creation time". These two things are very different.
> 
> i.e. One is filesystem internal information and is not related to
> when the original copy of the data in the file was created, the
> other is user specified metadata that is related to the file data
> contents and needs to travel with the data, not the filesystem.
> 
> IMO, trying to make one on-disk field hold two different types of
> information defeats one or the other purpose, and nobody knows which
> one the field stores for any given file.
> 
> I'd suggest that "authored date" should be a generic system xattr so
> most filesystems support it, not just those that have a birth time
> field on disk. Sure, modify it through utimesat() and expose it
> through statx() (as authored time, not birth time), but store it a
> system xattr rather than an internal filesystem metadata field that
> requires was never intended to be user modifiable.

It seems that this is the general consensus, so I'll look into
implementing this functionality as an xattr.