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Date: Mon, 23 Jun 2014 19:52:55 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: Recommended way to probe for bcrypt support?
On Tue, Jun 24, 2014 at 03:17:35AM +0400, Solar Designer wrote:
> On Mon, Jun 23, 2014 at 06:33:39PM -0400, Rich Felker wrote:
> > On Mon, Jun 23, 2014 at 02:53:58PM -0700, Isaac Dunham wrote:
> > > I'm wondering if there's a recommended way to probe for bcrypt support;
> > > it would be nice to add this to toybox so mkpasswd could use blowfish on
> > > musl or OWL systems.
> >
> > The best way to do this is with runtime detection: simply attempt to
> > use crypt or crypt_r with a setting string that requests bcrypt and
> > see if it works.
>
> Sure. This works for ./configure when we're fine with static
> compile-time detection.
Yes; I rather frown upon such compile-time detection though because it
precludes cross-compiling, and because such _behaviors_ (as opposed to
interfaces) tend to be things that change between versions. In the
case of libc supporting bcrypt this is not going to change, but in
principle it's a bad policy. Especially when presence/absence of a
feature might depend on kernel, and running on an older kernel than
the one used while compiling is likely to happen.
> Unfortunately, at runtime detecting bcrypt in
> this way is a bit slow since the minimum cost setting is 4 (meaning 16
> iterations of the eksBlowfish loop). For mkpasswd it is acceptable -
> so do it - but e.g. in phpass I am reluctant to do it that way.
I'm not clear why it would be necessary to probe for it when not
actually attempting to use it, except in cases like providing a list
of supported hashes (e.g. --help or similar). The normal usage case
for "runtime probe" seems to be "try to use it, and report failure if
it's not available".
Rich
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