Message-ID: <ff2b9ee994eb490a10fd0ade4d6ff5d3c8ee6bfe.camel@cybertec.at>
Subject: Re: equalPolicy() doesn't compare the permissive flag
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Andreas Lind <andreaslindpetersen@gmail.com>,
	pgsql-hackers@postgresql.org
Date: Fri, 10 Jul 2026 08:58:22 +0200
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On Thu, 2026-07-09 at 16:28 +0200, Andreas Lind wrote:
> While working on an unrelated RLS patch, I noticed that equalPolicy() in
> relcache.c doesn't compare the permissive field of RowSecurityPolicy. It
> compares polcmd, hassublinks, policy_name, roles, qual, and
> with_check_qual, but not permissive, so two policies that are identical
> in every other respect except their PERMISSIVE/RESTRICTIVE designation
> are reported as equal.
>=20
> equalPolicy() is used (via equalRSDesc()) by RelationRebuildRelation()
> to decide whether an open relation's existing row-security descriptor
> can be kept as-is across a relcache rebuild, or must be treated as
> changed. ALTER POLICY has no way to flip a policy's
> PERMISSIVE/RESTRICTIVE designation, but DROP POLICY followed by CREATE
> POLICY of the same name, roles, command, and quals, differing only in AS
> PERMISSIVE/RESTRICTIVE, hits this exactly: the stale descriptor would be
> kept, leaving the relcache out of sync with how the policy should now
> combine with others (PERMISSIVE policies are ORed together; RESTRICTIVE
> policies are ANDed with the rest).
>=20
> I wasn't able to construct a simple SQL reproduction:
> RelationRebuildRelation() (and thus this comparison) is only reached
> while the relation is still open (refcount > 0) at the moment its
> invalidation is processed. Closing and reopening the relation between
> statements -- the usual behavior -- sidesteps the bug, and a second
> session can't hold the relation open across the DROP/CREATE either,
> since both need AccessExclusiveLock. This looks analogous to
> ab6d1cd26eb, which fixed the same kind of omission in this function for
> the USING qual and likewise shipped without a test.

I don't know if the omission to check for identical PERMISSIVE or
RESTRICTIVE settings can lead to real problems, but I feel that it
violates the promise of the function.  So I am +1 on this change.

Yours,
Laurenz Albe