linux-kernel - Re: [git patches] libata updates, GPG signed (but see admin notes)

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Date:	Thu, 10 Nov 2011 10:23:05 -0500
From:	Marc Branchaud <marcnarc@...link.com>
To:	David Woodhouse <dwmw2@...radead.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jochen Striepe <jochen@...ot.escape.de>,
	Shawn Pearce <spearce@...arce.org>,
	Junio C Hamano <gitster@...ox.com>, git@...r.kernel.org,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Jeff Garzik <jeff@...zik.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ide@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [git patches] libata updates, GPG signed (but see admin notes)

On 11-11-10 08:51 AM, David Woodhouse wrote:
> On Wed, 2011-11-02 at 21:13 -0700, Linus Torvalds wrote:
>> No, my main objection to saving the data is that it's ugly and it's
>> redundant. Sure, in practice you can check the signatures later fine
>> (with the rare exceptions you mention), but even when you can do it,
>> what's the big upside? 
> 
> Another objection (although it may not be insurmountable) is that it's
> not necessarily *entirely* clear what's being signed.

I think this is a non-issue as far as the implementation is concerned.  That
is, the question exists regardless of what actual bits get (hashed and)
encrypted by a private key.  Furthermore, the answer will depend on who's
using the signatures and in what context, and it's not appropriate for the
git tool to make assumptions about those things.

> In the simple case where I clone your tree, make a few commits with my
> Signed-off-by:, sign a tag and then ask you to pull, that's easy enough.
> I'm vouching for what I committed, and not for everything that was in
> your tree beforehand.
> 
> But what if I'm working on top of someone else's published git tree?
> Does a signed tag at the top of *my* work imply that I'm vouching for
> all of theirs too?

<philosophy>

It all depends on what you mean by "vouch for".

You obviously thought that the 3rd-party repo was good for something,
otherwise why did you base your work on it in the first place?  So maybe
you're just vouching for the 3rd-party repo being good enough for what you're
trying to do.

Or, maybe you've done a thorough analysis of the 3rd-party code and are ready
to certify it as completely memory-leak-free or something.

Or or, maybe you're only making a statement about the commits that you've
authored yourself.  (You probably want to individually sign each of those
commits in this case.)

These sorts of issues have been debated on PKI mailing lists ad nauseum.  I
think the best approach is that if you want your signature to have a
particular meaning, then put that into some text that's part of what's being
signed.  Let other humans read that text and make their own decisions.

</philosophy>

And whatever the case, the software that makes and validates the signatures
shouldn't make any assertions about how to interpret good or bad signatures.
 (Yes, other software could interpret meanings according to some criteria,
and that software could exist alongside or be incorporated into the basic
digital signature software, but the interpretation software is doing a
different job.)

		M.
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