Removing build metadata, for reproducible kernel builds

[Ian Lepore]

On Thu, 2015-12-03 at 12:53 -0700, Warner Losh wrote:
> On Thu, Dec 3, 2015 at 12:55 AM, Ed Maste <emaste at freebsd.org> wrote:
> 
> > On 3 December 2015 at 05:51, Warner Losh <imp at bsdimp.com> wrote:
> > > 
> > > I noted in the review that I don’t like the default being no.
> > > 
> > > I also don’t like that we’re growing lots of different knobs that need
> > > to be set to get a repeatable build. Let’s have one, or barring that,
> > > let’s have one that sets all the sub-knobs.
> > 
> > My hope is that we'll have a reproducible build by default, and that
> > *no* knobs need to be set. That's what I intend with my patch. I can
> > rename the knob to WITH_/WITHOUT_REPRODUCIBLE_BUILD though if that's
> > generally desired. If there's a consensus to default to including the
> > metadata I'm fine with setting it in make release.
> 
> 
> I think this an unwise decision in the current form suggested. The kernel
> metadata has saved my butt enough times I really don't want to see it
> go by default. But see below for a reasonable (imho) middle ground that
> would be a good default.
> 

I'm curious why anyone wants this enabled by default, like... are we
missing something?  Does it improve freebsd-update behavior maybe?

If it's just for some general "reproducibility is good" philosophy then
I would counter with "information is even better, so don't throw it
away without a good reason."

Reproducibility is good for some people, and completely useless for
others, and the people who need it aren't going to mind turning on a
knob or two to get what they want.

> 
> > > I think that host and path are more worthless than date and time
> > > in many environments. Who builds it likewise. Those are all things
> > > that are likely to change between builds, yet change the kernel
> > > image. I’d rather see it all gone when this option is in effect.
> > 
> > I don't follow -- other than the build iteration number (which I
> > indeed missed), it is all gone.
> > 
> 
> Yea I was reading things backwards.
> 
> In the review, I suggested that if you've modified the tree (which the SCM
> will tell you), then do the old format to preserve useful metadata that's
> really really needed and if not to use the shorter version. When you've
> modified the tree, reproducible builds aren't a concern at all.
> 

How are you going to determine what consitutes a modified tree?  What
you think of as modifications may be what I call my baseline version.

-- Ian