Date: Tue, 13 Nov 2018 23:52:18 -0800
From: Noah Misch <noah@leadboat.com>
To: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: DSM segment handle generation in background workers
Message-ID: <20181114075218.GE1096408@rfd.leadboat.com>
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On Wed, Nov 14, 2018 at 08:22:42PM +1300, Thomas Munro wrote:
> On Wed, Nov 14, 2018 at 6:34 PM Noah Misch <noah@leadboat.com> wrote:
> > On Wed, Nov 14, 2018 at 05:50:26PM +1300, Thomas Munro wrote:
> > > On Wed, Nov 14, 2018 at 3:24 PM Noah Misch <noah@leadboat.com> wrote:
> > > > What counts is the ease of predicting a complete seed.  HEAD's algorithm has
> > > > ~13 trivially-predictable bits, and the algorithm that stood in BackendRun()
> > > > from 98c5065 until 197e4af had no such bits.  You're right that the other 19
> > > > bits are harder to predict than any given 19 bits under the old algorithm, but
> > > > the complete seed remains more predictable than it was before 197e4af.
> > >
> > > However we mix them, given that the source code is well known, isn't
> > > an attacker's job really to predict the time and pid, two not
> > > especially well guarded secrets?
> >
> > True.  Better to frame the issue as uniform distribution of seed, not
> > unpredictability of seed selection.
> 
> What do you think about the attached?

You mentioned that you rewrote the algorithm because the new function had a
TimestampTz.  But the BackendRun() code, which it replaced, also had a
TimestampTz.  You can reuse the exact algorithm.  Is there a reason to change?