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Re: Largeobject Access Controls (r2460)

From: KaiGai Kohei <kaigai(at)kaigai(dot)gr(dot)jp>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com>, Takahiro Itagaki <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Jaime Casanova <jcasanov(at)systemguards(dot)com(dot)ec>, Greg Smith <greg(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Largeobject Access Controls (r2460)
Date: 2009-12-11 15:59:04
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Bruce Momjian さんは書きました:
> KaiGai Kohei wrote:
>> Takahiro Itagaki wrote:
>>> KaiGai Kohei <kaigai(at)ak(dot)jp(dot)nec(dot)com> wrote:
>>>> Tom Lane wrote:
>>>>> Takahiro Itagaki <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp> writes:
>>>>>>    <structname>pg_largeobject</structname> should not be readable by the
>>>>>>    public, since the catalog contains data in large objects of all users.
>>>>> This is going to be a problem, because it will break applications that
>>>>> expect to be able to read pg_largeobject.  Like, say, pg_dump.
>>>> Is it a right behavior, even if we have permission checks on large objects?
>>> Can we use column-level access control here?
>>>    REVOKE ALL ON pg_largeobject FROM PUBLIC;
>>> => GRANT SELECT (loid) ON pg_largeobject TO PUBLIC;
>> Indeed, it seems to me reasonable.
>>> We use "SELECT loid FROM pg_largeobject LIMIT 1" in pg_dump. We could
>>> replace pg_largeobject_metadata instead if we try to fix only pg_dump,
>>> but it's no wonder that any other user applications use such queries.
>>> I think to allow reading loid is a balanced solution.
>> Right, I also remind this query has to be fixed up by other reason right now.
>> If all the large objects are empty, this query can return nothing, even if
>> large object entries are in pg_largeobject_metadata.
> "metadata" seems very vague.  Can't we come up with a more descriptive
> name?

What about "property"?
The "metadata" was the suggested name from Robert Haas at the last
commit fest, because we may store any other properties of a large
object in this catalog future.

> Also, how will this affect pg_migrator?  pg_migrator copies
> pg_largeobject and its index from the old to the new server.  Is the
> format inside pg_largeobject changed by this patch?

The format of pg_largeobject was not touched.

> What happens when
> there is no entry in pg_largeobject_metadata for a specific row?

In this case, these rows become orphan.
So, I think we need to create an empty large object with same LOID on
pg_migrator. It makes an entry on pg_largeobject_metadata without
writing anything to the pg_largeobject.
I guess rest of migrations are not difference. Correct?


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