Re: BUG #17847: Unaligned memory access in ltree_gist

Hi,
19.03.2023 20:08, Alexander Korotkov wrote:
> On Thu, Mar 16, 2023 at 10:35 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> What I'm inclined to do about this is add a restriction that the siglen
>> value be a multiple of MAXALIGN. It doesn't look like the reloption
>> mechanism has a way to specify that declaratively, but we could probably
>> get close enough by just making LTREE_GET_SIGLEN throw an error if it's
>> wrong. That's not ideal because you could probably get through making
>> an empty index without hitting the error, but I don't offhand see a
>> way to make it better.
> Sorry for missing this.
>
> Please, note that there are infrastructure of reltoption validators.
> I think this is the most appropriate place to check for alignment of
> siglen. That works even for empty indexes. See the attached patch.

Thanks for the fix! It works for me.

Maybe it's worth to reflect this restriction in the documentation too?
     <literal>gist_ltree_ops</literal> GiST opclass approximates a set of
     path labels as a bitmap signature.  Its optional integer parameter
     <literal>siglen</literal> determines the
     signature length in bytes.  The default signature length is 8 bytes.
     Valid values of signature length are between 1 and 2024 bytes.

How about "The length must be a multiple of <type>int</type> alignment between 4 and 2024."?
(There is a wording "<type>int</type> alignment (4 bytes on most machines)" in catalogs.sgml.)

Also maybe change the error message a little:
s/siglen value must be integer-alignment/siglen value must be integer-aligned/
or "int-aligned"? (this spelling can be found in src/)

(There is also a detail message, that probably should be corrected too:
DETAIL:  Valid values are between "1" and "2024".
->
DETAIL:  Valid values are int-aligned positive integers less than 2024.
?)

Best regards,
Alexander