Re: PATCH: Configurable file mode mask

From: Chapman Flack <chap(at)anastigmatix(dot)net>
To: Stephen Frost <sfrost(at)snowman(dot)net>
Cc: Michael Paquier <michael(at)paquier(dot)xyz>, David Steele <david(at)pgmasters(dot)net>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Peter Eisentraut <peter(dot)eisentraut(at)2ndquadrant(dot)com>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Adam Brightwell <adam(dot)brightwell(at)crunchydata(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, "Tsunakawa, Takayuki" <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: PATCH: Configurable file mode mask
Date: 2018-03-13 18:26:41
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On 03/13/2018 01:50 PM, Stephen Frost wrote:

> Yes, that's true, but PG has never paid attention to POSIX ACLs or Linux
> capabilities. Changing it to do so is quite a bit beyond the scope...

I think we're largely in agreement here, as my aim was not to advocate
that PG should work harder to understand the subtleties of every system
it could be installed on, but rather that it should work less hard at
pretending to understand them when it doesn't, and thus avoid
obstructing the admin, who presumably does.

> I'll point out that PG does run on quite a few other OS's beyond Linux.

I'll second that, as I think it is making my argument. When I can
supply three or four examples of semantic subtleties that undermine
PG's assumptions under Linux alone, the picture when broadened to
those quite-a-few other platforms as well certainly doesn't become

>> but then sooner or later it will still end up making assumptions
>> that aren't true under, say, SELinux, so there's another #ifdef,
>> and where does it end?
> I agree with this general concern.

:) That's probably where it became clear that I'm not advocating
an add-#ifdefs-for-everything approach.

> PG will stat PGDATA and conclude that the system is saying that group
> access has been granted on PGDATA and will do the same for subsequent
> files created later on. ... The only issue that remains is that PG
> doesn't understand or work with POSIX ACLs or Linux capabilities,
> but that's not anything new.

What's new is that it is now pretending even more extravagantly to
understand what it doesn't understand. Where it would previously draw
in incorrect conclusion and refuse to start, which is annoying but
not very difficult to work around if need be, it would now draw the
same incorrect conclusion and then actively go about making the real
world embody the incorrect conclusion, contrary to the admin's efforts.

>> umask inherited by the postmaster. A --permission-transparency option
>> would simply use open and mkdir in the traditional ways, allowing
>> the chosen umask, ACL defaults, SELinux contexts, etc., to do their
>> thing, and would avoid then stepping on the results with explicit
>> chmods (and of course skip the I-refuse-to-start-because-I-
>> misunderstand-your-setup check).
>> ...
> I'm a fan of this idea in general, but it's unclear how that
> --permission-transparency option would work in practice. Are you
> suggesting that it be a compile-time flag, which would certainly work
> but also then have to be debated among packagers as to the right setting
> and if there's any need to be concerned about users misunderstanding it,
> or a flag for each program,

I was thinking of a command-line option ...

> which hardly seems ideal as some invokations
> of programs might forget to specify that, leading to inconsistent
> permissions, or something else..?

... but I see how that gets complicated with the various other command-
line utilities included.

> .. we'd have to build in complete
> support for POSIX ACLs and Linux capabilities if we go down a route

I'm wary of an argument that we can't do better except by building
in complete support for POSIX ACLs, and capabilities (and NFSv4
ACLs, and SELinux, and AppArmor, and #ifdef and #ifdef and #ifdef).

It seems to me that, in most cases, the designers of these sorts of
extensions to old traditional Unix behavior take great pains to design
them such that they can still usefully function in the presence of
programs that "don't pay attention to or understand or use" them, as
long as those programs are in some sense well-behaved, and not going
out of their way with active steps that insist on or impose permissions
that aren't appropriate under the non-understood circumstances.

So my suggestion boils down to PG having at least an option, somehow,
to be well-behaved in that sense.


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