Date: Mon, 20 Apr 2020 21:34:12 -0500 From: Kyle Evans <kevans@freebsd.org> Cc: Eric van Gyzen <eric@vangyzen.net>, Shawn Webb <shawn.webb@hardenedbsd.org>, Kristof Provost <kp@freebsd.org>, Ronald Klop <ronald-lists@klop.ws>, svn-src-head <svn-src-head@freebsd.org>, svn-src-all <svn-src-all@freebsd.org>, src-committers <src-committers@freebsd.org> Subject: Re: svn commit: r360068 - in head/sys: kern net sys Message-ID: <CACNAnaFLyhM8GjVuUgGtHJK=CmYPdwbVsHr5Ckfq3kcP9WFhzg@mail.gmail.com> In-Reply-To: <CACNAnaHVZozWLcNVT%2BZA4aptc5oSYD68OPpX7_sLvomZYia-fw@mail.gmail.com> References: <202004180750.03I7oUK6032898@repo.freebsd.org> <op.0jbpatnnkndu52@sjakie> <67B55A62-848B-4876-8367-DE0D32A8B7D4@FreeBSD.org> <20200419142327.nhotyboqubk3vl2l@mutt-hbsd> <523895fb-7f30-c4ce-78a7-d16b2ff4954c@vangyzen.net> <CACNAnaHVZozWLcNVT%2BZA4aptc5oSYD68OPpX7_sLvomZYia-fw@mail.gmail.com>
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On Mon, Apr 20, 2020 at 9:14 PM Kyle Evans <kevans@freebsd.org> wrote:
>
> On Mon, Apr 20, 2020 at 8:15 PM Eric van Gyzen <eric@vangyzen.net> wrote:
> >
> > >>>> + sz = asprintf(&buf, M_TEMP, "%s-%s-%s", uuid, if_name(ifp),
> > >>>> + jailname);
> > >>>> + if (sz < 0) {
> > >>>> + /* Fall back to a random mac address. */
> > >>>
> > >>>
> > >>> I was wondering if it would be valuable to give this fall back something
> > >>> like:
> > >>>
> > >>> printf("%s: unable to create fixed mac address; using random
> > >>> mac address", if_name(ifp));
> > >>>
> > >>> This will only be printed in rare circumstances. But in that case will
> > >>> provide valuable information.
> > >>>
> > >> That would potentially be valuable, yes. On the other hand, we traditionally
> > >> don???t sprinkle a lot of printf()s around in the kernel. This is extremely
> > >> unlikely to happen, and if it does odds are attaching the interface will
> > >> fail at an earlier or later point, you may struggle to pass packets and run
> > >> into any number of other issues.
> > >> It???s also possible to diagnose absent the printf(), because the MAC
> > >> address will be locally administered rather than within the FreeBSD OUI.
> > >>
> > >> So, in short: not a bad idea. You can argue it both ways, and I find myself
> > >> (weakly) on the opposite side.
> > >
> > > Would displaying the message only when verbose boot mode is enabled be
> > > a suitable compromise?
> >
> > We could completely avoid the problems of dynamic allocation by calling
> > SHA1Update three times, feeding each piece of data separately.
> >
> > For bonus points, use a single char[] to save stack space, too. Maybe
> > use a union, for legibility, and to ensure the proper size without ugly
> > assertions.
> >
>
> To be honest, I'd be more inclined to just revert this part of it and
> push it all back onto the stack. It's still < 512 bytes and pretty
> much always called in short paths because it's generally only used
> during initial creation of some ifnet; I found the concern about the
> stack usage here, specifically, a bit dubious in the first place, and
> this follow-up hasn't left me enjoying it any further.
>
Sorry, to clarify: I'm also pretty much OK with SHA1Update 3x if I'm
alone in the "don't really care about this particular stack usage"
camp, but I've found it useful that they're currently joined into a
single buffer as I've had occasion to dump it in the past to confirm
my understanding of the pedigree of the output, in case of, e.g.,
generated conflicts.
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