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Re: Editfiles Considered Harmful (was: Re: Complex Editfiles Examples)


From: Mark Burgess
Subject: Re: Editfiles Considered Harmful (was: Re: Complex Editfiles Examples)
Date: Thu, 4 Dec 2003 09:08:00 +0100 (MET)

I can agree with the general semtiment that "great power" yields
"great power to destroy". But I think you are unfairly pinning
this on one thing in a chain of responsibility.

In my experience, people are not used to thinking in the
cfengine way and make mistakes in trying to get around what
they perceive as limitations. There is no tool that can prevent
a user from writing nonsense over any file if they have the
privileges. So you argument is fine, but it is not really related
to editfiles or to cfengine.

"Be afraid. Be very afraid."

Mark

On  3 Dec, Eric Sorenson wrote:
> On Thu, 4 Dec 2003, Jamie Wilkinson wrote:
> 
>> This one time, at band camp, Russell Adams wrote:
>> >Egad man!! I'm impressed. ;]
>> >
>> >I setup a similar construct last night to edit Gentoo's
>> >/etc/make.conf, but its not nearly as complex.
>> 
>> My best editfiles is the one for /etc/ssh/sshd_config, most of my
>> other configuration file edits are based on that.
> 
> It really terrifies me when I see stuff like this.
> 
> Not that your editfiles won't work (necessarily); it's just scary to
> think that as a general practice, people let tools generate syntax
> which have no idea themselves whether the syntax is valid.
> 
> This is what I meant the other day by std_editfiles_rant.h, but I
> really would like to know if I'm completely off-base on this so I'll
> write a bit about where I'm coming from.
> 
> In my first cfengine site install, I never touched the editfiles
> functionality because I simply didn't need it. I was mostly shipping
> binaries around to keep a consistent gnu-tools environment on solaris
> in a production ISP environment that eschewed NFS and NIS.
> 
> For the password file distribution problem, I wrote a bit of a perl
> frontend to build up the appropriate passwd/shadow files and place
> them in the repository for clients to pick up via 'copy:' when they
> updated.. (this lives on in all its cobwebby glory at
> http://explosive.net/opensource/cfpasswd ) This worked pretty well and
> I didn't get bit too badly by any bugs -- even at cfengine 1.5.x!
> 
> On to my current site, where I wasn't directly involved in the
> first-gen cfengine deployment. It's an internal corporate network and
> we were butting up against NIS limits but had NFS available everwhere,
> so the main purpose for cfengine was to build up passwd, shadow, and
> group from fragments over NFS.
> 
> The implementor used editfiles for this task, and we started seeing
> weird edge cases like zero-length shadow files, corrupted entries,
> etc. Some of these were parse bugs or things not working as expected,
> some were implementation problems in our configs -- the blame for
> which I still ultimately pin on editfiles' strange set of features and
> commands, as has been amply discussed on this list.
> 
> But as I was trying to hone an alternative to this for our new
> next-gen cfengine2-based setup, I had to pin down exactly what bugged
> me about the editfiles problem in order to convince the rest of the
> team that we really needed to eliminate it from our vocabulary as an
> operating pragma, and instead copy whole files from a
> version-controlled repository.  This was the Right Thing to Do, I felt
> it in my gut, but it wasn't easy to articulate.  I think I've got it
> pretty well sussed, and it turns out there's really two issues: one 
> has to do with systemic behavior and the other is about management
> complexity.
> 
> First, let's look at a typical goal of an editfiles task. Usually
> you're updating a system configuration file from its default state, in
> order to bring the behavior of a daemon/subsystem in line with policy.
> After the first time it runs, the editfiles stanza ought to be a
> no-op; this is the stated purpose of commands like
> AppendIfNoLineMatching. The problem with this approach is that the
> product of the original file state run through editfiles actions 
> cannot be guaranteed to be equivalent to the desired end state, if
> there's no semantic rigor imposed on the actions performed. This is
> the idea behind Alva Couch's "conduits", and in no sense are the
> regexp description and editfiles commands a conduit for any of the
> things editfiles's potentially editing.
> 
> Put simply, there's nothing inherent in editfiles which prevents you
> from accidentally scribbling nonsense all over a live file on the
> target machine. This is incredibly dangerous for anything of
> significance! I've seen problems where 'AppendIfNoLinesMatching'
> didn't match when it should have, causing broken-record repetition.
> Substring matches for 'LocateLineMatching' may match too early or not
> at all (usually depending on whether or not you use ^ and $ anchors
> correctly).  Worse than all of this, the baseline behavior of the
> subsystem in question may have changed and your edits do not produce
> the intended consequences -- even though editfiles is doing exactly
> what you've told it to do!
> 
> Contrast this with editing or generating a config file, testing that
> file on a non-production system, and then "promoting" it into
> production by checking it in the repository. Unlike a series of adds,
> deletes and edits to the contents of the file, I have a high degree of
> certainty what the target system will look like (and by extension, how
> it will behave) at the end of the cfengine run.
> 
> In a related sense, the second problem shows iteself when someone else
> tries to modify a modification you've made, or when the policy changes,
> necessitating a change in the system's behavior. Now you have to
> backtrack from the configuration files as they exist on the production
> systems, into the guts of your editfiles cleverness (which by now has
> descended to mere unreadability) and reverse-engineer the adds,
> deletes, and edits, in order to add a new directive to the top of the
> file.
> 
> Why not just edit the file with an editor and check it in to the
> repository? Let cfengine copy the new version over and restart the
> deamon.  I'd argue that this is the end result that you want -- a new
> version of sshd_config on all machines of a particular class (or
> whatever), so why not just do it? Obviously there's been a lot of work
> put into making the editfiles command set very rich, but it's never
> going to be able to handle all the cases we run into without becoming
> (even more) hopelessly tangled, syntax-wise; regardless, it'll never
> be as smart as a human with a text editor.
> 
> The final blow to the editfiles methodology should be clear in the
> not-so-hypothetical situation where someone asks you to show them what
> changed between last week and this week in the ssh subsystems of your
> machines.  This is, and should be, a text diff between two revisions
> of the configuration file.
> 
> It should NOT be a text diff between two revisions of your editfiles
> section, where you have to pull out the whole stanza from each version
> *and the original configuration file* and compare the results of
> applying each stanza to a copy of the config... That's crazy-talk,
> absurd on its face.
> 
> Use CVS. Make a directory tree that looks like a filesystem, and
> populate each leaf directory with the files you want to manage. Either
> make separate trees per host-class or use class-keyed filename
> extensions in a unified tree, it really doesn't matter. Use copy: with
> checksums, not date-stamps, so machines which get installed after your
> latest config change still get the authoritative version.  Use a
> define=className in your copy directive to specify triggers for 
> restarting the affected service daemon (remembering to pre-declare
> className with AddInstallables in your control: section).
> 
> Voila, all your editfiles problems just disappeared.
> 
> Seriously though, I would like to hear any criticism or feedback on
> the above -- if there's something that just doesn't fit into this
> model, or maybe some loon^H^H^H^Hfine person who feels as strongly
> Pro-editfiles as I do Anti-, I'd welcome the dialog.
> 

-- 


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Work: +47 22453272            Email:  address@hidden
Fax : +47 22453205            WWW  :  http://www.iu.hio.no/~mark
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