Re: [Cbor] Robert Wilton's No Objection on draft-ietf-cbor-file-magic-11: (with COMMENT)

"Rob Wilton (rwilton)" <rwilton@cisco.com> Mon, 25 April 2022 09:36 UTC

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From: "Rob Wilton (rwilton)" <rwilton@cisco.com>
To: Carsten Bormann <cabo@tzi.org>
CC: Christian Amsüss <christian@amsuess.com>, "cbor@ietf.org" <cbor@ietf.org>, "cbor-chairs@ietf.org" <cbor-chairs@ietf.org>, The IESG <iesg@ietf.org>, "draft-ietf-cbor-file-magic@ietf.org" <draft-ietf-cbor-file-magic@ietf.org>
Thread-Topic: [Cbor] Robert Wilton's No Objection on draft-ietf-cbor-file-magic-11: (with COMMENT)
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Date: Mon, 25 Apr 2022 09:35:52 +0000
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Subject: Re: [Cbor] Robert Wilton's No Objection on draft-ietf-cbor-file-magic-11: (with COMMENT)
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Hi Carsten, all,

> -----Original Message-----
> From: iesg <iesg-bounces@ietf.org> On Behalf Of Carsten Bormann
> Sent: 22 April 2022 13:56
> To: Rob Wilton (rwilton) <rwilton=40cisco.com@dmarc.ietf.org>
> Cc: Christian Amsüss <christian@amsuess.com>; cbor@ietf.org; cbor-
> chairs@ietf.org; The IESG <iesg@ietf.org>; draft-ietf-cbor-file-magic@ietf.org
> Subject: Re: [Cbor] Robert Wilton's No Objection on draft-ietf-cbor-file-
> magic-11: (with COMMENT)
> 
> Hi Rob,
> 
> On 2022-04-22, at 11:14, Rob Wilton (rwilton)
> <rwilton=40cisco.com@dmarc.ietf.org> wrote:
> >
> > Hi Carsten,
> >
> > One other thought on this (when I was out running this morning) …
> 
> Thank you for spending some of your running cycles on this — this class of
> cycles is precious (I'm not a running type, so CBOR was almost entirely
> designed under the shower).

:-)

> 
> > if C often does use strcpy (or even strncpy), then avoiding the zero byte
> isn't the only problem, it also has the risk of capturing arbitrary extra bytes
> which would presumably make the magic number check less reliable?
> 
> That is easily avoided by using strncpy(dst, buf, 8).
> But that is advice we maybe don’t want to give (see below).

My thought was that if a prefix of those bytes were fixed with the real magic number and the remainder were arbitrary then the magic number for a given file isn't fixed since it is potentially picking up contents that could change with the definition of the file.  But perhaps the algorithm for checking against magic numbers is designed to cope with this, e.g., find the longest prefix match.


> 
> > My instinct here is that we keep the algorithm simple, don't avoid
> embedded zero bytes, don't zero terminate, and just state that robust
> implementations should not assume that the magic number is a null
> terminated string.
> 
> We were assuming that some people would want to *store* it in a null-
> terminated string (after obtaining that using the strncpy processing above).

I read this as "We are assuming that some people would want to do it wrong" :-).

I just think that it is somewhat dangerous to say that this isn't a string but it is designed that it might work okay if you treat it as such, as long as you remember to null terminate it.  Why not just say that applications should treat it as a sequence/array of bytes (which is effectively what it really is)?

There are various other parts of treating this as a string that isn't great:
 - it might contain values that are interpreted as control codes.
 - it might or might not be a valid UTF-8 string.

If applications store this as a string, then they may also try and display it as a string, and that doesn't seem wise.


> 
> > Specifically, I'm assuming that C using strncpy isn't a security issue (because
> they don't know whether a file contains a magic number at all), just a
> reliability one, and that implementations will hopefully realize and fix this
> over time.  But this is just my opinion/suggestion.
> 
> I would love to hear more opinions out of the WG here.
> 
> My current state of mind is that we maybe fell into an analogon of the Postel
> Principle trap [1].
> By anticipating how people will want to be lazy when implementing this, we
> may be *too* “conservative in what we send”.

Yes, I think that this is where my thoughts lie.


> 
> But then, making life easy for implementers is important, and so making the
> first 8 bytes compatible with being stored in a C string may also be an
> appropriate accommodation.

I agree with making implementors lives easy, but I don't really see why treating this an opaque sequence/array of bytes is harder than treating it as a string.


> 
> Grüße, Carsten
> 
> [1]: https://datatracker.ietf.org/doc/draft-iab-protocol-maintenance/
> 
> PS.: I wrote
> 
> >> This is easy to fix by using
> >>
> >> 0x63740000 + (ct / 255 + 1) * 256 + ct % 255 + 1
> >>
> >> as the tag number (*).
> 
> More readable version (and entirely obvious in this form):
> 
> 0x63740101 + (ct / 255) * 256 + ct % 255
> where the content-format number ct < 255*255.

Thanks,
Rob