Re: [PATCH 05/13] credential: gate new fields on capability

From: Patrick Steinhardt <ps@pks.im>
To: "brian m. carlson" <sandals@crustytoothpaste.net>,
	git@vger.kernel.org, Junio C Hamano <gitster@pobox.com>,
	Matthew John Cheetham <mjcheetham@outlook.com>,
	M Hickford <mirth.hickford@gmail.com>
Subject: Re: [PATCH 05/13] credential: gate new fields on capability
Date: Tue, 2 Apr 2024 12:04:31 +0200	[thread overview]
Message-ID: <ZgvYLxfNwBcOB_s1@tanuki> (raw)
In-Reply-To: <ZgSQr7uQdIA8oVNn@tapette.crustytoothpaste.net>

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On Wed, Mar 27, 2024 at 09:33:35PM +0000, brian m. carlson wrote:
> On 2024-03-27 at 08:02:39, Patrick Steinhardt wrote:
> > On Sun, Mar 24, 2024 at 01:12:53AM +0000, brian m. carlson wrote:
> > > +static int credential_has_capability(const struct credential_capability *capa, int op_type)
> > > +{
> > > +	/*
> > > +	 * We're checking here if each previous step indicated that we had the
> > > +	 * capability.  If it did, then we want to pass it along; conversely, if
> > > +	 * it did not, we don't want to report that to our caller.
> > > +	 */
> > > +	switch (op_type) {
> > > +	case CREDENTIAL_OP_HELPER:
> > > +		return capa->request_initial;
> > > +	case CREDENTIAL_OP_RESPONSE:
> > > +		return capa->request_initial && capa->request_helper;
> > > +	default:
> > > +		return 0;
> > > +	}
> > > +}
> > 
> > I think I'm missing the bigger picture here, so please bear with me.
> > 
> > What you provide here is simply an `op_type` that indicates the phase we
> > are currently in and thus allows us to check whether all of the
> > preceding phases had the capability set. But to me it seems like a phase
> > and the actual capability should be different things. So why is it that
> > the capability seems to be a mere boolean value instead of something
> > like a bitfield indicating whether a specific capability is supported or
> > not? Or is all of this infra really only to support a single capability,
> > namely the credential capability?
> > 
> > I'm mostly coming from the angle of how capabilities work with remote
> > helpers. When asked, the helper will announce a set of capabilities that
> > it supports, e.g. "capabilities stateless-connect". So from thereon the
> > client of the helper knows that it can assume "stateless-connect" to be
> > understood by the helper.
> > 
> > I would have expected capabilities to work similarly for the credential
> > helper, where it announces "I know to handle pre-encoded credentials".
> > But given that I have basically no clue at all for how the credential
> > helper works there may very well be good reasons why things work so
> > differently here.
> 
> Let me explain a little bit.  There are two possible flows that we can
> have for a credential request:
> 
>   git-credential input -> credential.c -> helper -> credential.c -> git-credential output
> 
>   git-http-backend -> credential.c -> helper -> credential.c -> git-http-backend
> 
> Let's look at the first one (which might, say, come from Git LFS or
> another external tool), but the second one works similarly.  When we get
> a request from `git credential fill`, we need to know first whether the
> requester supports the capability.  If we're using an external tool from
> last decade, it's not going to do so.
> 
> If it _does_ support that, then we want to pass that along to the
> helper, but if it doesn't, we don't.  That's because if the caller
> doesn't support `credential` and `authtype`, the helper might
> legitimately want to provide a username and password (or token) instead,
> knowing that that's more likely to work.
> 
> Similarly, in the final response, we want to indicate to the external
> caller whether the capability was in fact supported.  That's useful to
> know in case we want to pass the response back to `git credential
> store`, and it also discloses functionality about what the credential
> helper in use supports.
> 
> We can't assume that the helper does or doesn't support the same
> capabilities as Git because it might come from outside Git (e.g., Git
> Credential Manager Core, or a site-specific credential helper) or it
> just might not be capable of storing or handling that kind of
> credential.  By not making the assumption that the helper is implicitly
> capable, we allow users to continue to use very simple shell scripts as
> credential helpers.

The intent of this is quite clear to me, but thanks for re-explaining
the bigger picture :)

> As to why this functionality exists, it exists to support the two new
> capabilities in this series, `credential` and `state`.  A pie in the sky
> goal for the future is to support additional request signing
> functionality, so it might learn things like method, URI, and TLS
> channel binding info, which would be an additional capability.  (I might
> implement that, or I might not.)  All of those are boolean: they either
> are supported, or not.  If folks in the future want to expose
> non-boolean capabilities, I don't think that should be a problem.

I think you misunderstood my confusion. I didn't meant to say that there
should be non-boolean capabilities. I was rather missing the picture of
how exactly you can advertise multiple capabilities with the infra that
currently exists, and why the infra supports per-phase capabilities.

Basically, what I would have expected is a protocol where both Git and
the credential helper initially did a single "handshake" that also
announces capabilities. So something like:

    HELPER: capability foobar
    HELPER: capability barfoo
       GIT: capability foobar

Git would only acknowledge capabilities that it both understands and
that have been announced by the helper. So at the end of this both have
agreed on a single capability "foobar".

This is roughly how the remote helper capability subsystem works. What
this patch is introducing seems quite a bit more complicated than that
though because we have "staged" capabilities. I assume there is good
reason for this complexity, but I didn't yet manage to figure out the
reasoning behind it.

To ask more specifically: why would one side ever announce a capability
in phase 1, but not in phase 2? Is the reason that capabilities are in
fact tied to credentials?

Patrick

> > > +/*
> > > + * These values define the kind of operation we're performing and the
> > > + * capabilities at each stage.  The first is either an external request (via git
> > > + * credential fill) or an internal request (e.g., via the HTTP) code.  The
> > > + * second is the call to the credential helper, and the third is the response
> > > + * we're providing.
> > > + *
> > > + * At each stage, we will emit the capability only if the previous stage
> > > + * supported it.
> > > + */
> > > +#define CREDENTIAL_OP_INITIAL  1
> > > +#define CREDENTIAL_OP_HELPER   2
> > > +#define CREDENTIAL_OP_RESPONSE 3
> > 
> > Is there any specific reason why you're using defines instead of an enum
> > here? I think the latter would be more self-explanatory when you see
> > that functions take `enum credential_op` as input instead of an `int`.
> 
> I think an enum would be a nice improvement.  I'll include that in a
> reroll.
> -- 
> brian m. carlson (they/them or he/him)
> Toronto, Ontario, CA

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