[PATCH 0/6] File Sealing & memfd_create()

[Alex Elsayed]

Colin Walters wrote:

> On Thu, Apr 10, 2014 at 3:15 PM, Andy Lutomirski <luto at amacapital.net>
> wrote:
>> 
>> 
>> COW links can do this already, I think.  Of course, you'll have to
>> use a
>> filesystem that supports them.
> 
> COW is nice if the filesystem supports them, but my userspace code
> needs to be filesystem agnostic.  Because of that, the design for
> userspace simply doesn't allow arbitrary writes.
> 
> Instead, I have to painfully audit every rpm %post/dpkg postinst type
> script to ensure they break hardlinks, and furthermore only allow
> executing scripts that are known to do so.
> 
> But I think even in a btrfs world it'd still be useful to mark files as
> content-immutable.

If you create each tree as a subvolume and when it's complete put it in 
place with btrfs subvolume snapshot -r FOO_inprogress /ostree/repo/FOO,
you get exactly that.

You can even use the new(ish) btrfs out-of-band dedup functionality to 
deduplicate read-only snapshots safely.