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Thread: Re: BUG #17877: Referencing a system column in a foreign key leads to incorrect memory access

Re: BUG #17877: Referencing a system column in a foreign key leads to incorrect memory access

From
Ranier Vilela
Date:
Hi,

I think that commit f0d65c0 has an oversight.

Attnum == 0, is system column too, right?

All other places at tablecmds.c, has this test:

if (attnum <= 0)
    ereport(ERROR,

regards,
Ranier Vilela
Attachment

Re: BUG #17877: Referencing a system column in a foreign key leads to incorrect memory access

From
Tom Lane
Date:
Ranier Vilela <ranier.vf@gmail.com> writes:
> I think that commit f0d65c0
> <https://github.com/postgres/postgres/commit/f0d65c0eaf05d6acd3ae05cde4a31465eb3992b2>
> has an oversight.
> Attnum == 0, is system column too, right?

No, it's not valid in pg_attribute rows.

> All other places at tablecmds.c, has this test:
> if (attnum <= 0)
>     ereport(ERROR,

I was actually copying this code in indexcmds.c:

        if (attno < 0)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("index creation on system columns is not supported")));

There's really no reason to prefer one over the other in this context.

            regards, tom lane

Re: BUG #17877: Referencing a system column in a foreign key leads to incorrect memory access

From
Ranier Vilela
Date:
Em sex., 31 de mar. de 2023 às 16:25, Tom Lane <tgl@sss.pgh.pa.us> escreveu:
Ranier Vilela <ranier.vf@gmail.com> writes:
> I think that commit f0d65c0
> <https://github.com/postgres/postgres/commit/f0d65c0eaf05d6acd3ae05cde4a31465eb3992b2>
> has an oversight.
> Attnum == 0, is system column too, right?

No, it's not valid in pg_attribute rows.

> All other places at tablecmds.c, has this test:
> if (attnum <= 0)
>     ereport(ERROR,

I was actually copying this code in indexcmds.c:

        if (attno < 0)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("index creation on system columns is not supported")));

There's really no reason to prefer one over the other in this context.
I think the documentation is a bit confusing.
According to the current documentation:
/*
* attnum is the "attribute number" for the attribute: A value that
* uniquely identifies this attribute within its class. for user
* attributes, Attribute numbers are greater than 0 and not greater than
* the number of attributes in the class. i.e. if the Class pg_class says
* that Class XYZ has 10 attributes, then the user attribute numbers in
* Class pg_attribute must be 1-10.
*
* System attributes have attribute numbers less than 0 that are unique
* within the class, but not constrained to any particular range.
*
* Note that (attnum - 1) is often used as the index to an array.
Attributes equal to zero are in limbo.

IMO should be:
* System attributes have attribute numbers less or equal to 0 that are
* unique
* within the class, but not constrained to any particular range.

regards,
Ranier Vilela

Re: BUG #17877: Referencing a system column in a foreign key leads to incorrect memory access

From
Kyotaro Horiguchi
Date:
At Sat, 1 Apr 2023 10:56:08 -0300, Ranier Vilela <ranier.vf@gmail.com> wrote in 
> IMO should be:
> * System attributes have attribute numbers less or equal to 0 that are
> * unique
> * within the class, but not constrained to any particular range.

Attnum == 0 is invalid and doesn't belong to either user columns or
system columns. You're actually right that it's in limbo, but I
believe the change you suggested actually makes the correct comment
incorrect. In the condition you're asking about, I don't think we
really need to worry about an impossible case. If I wanted to pay more
attenstion to it, I would use an assertion, but I don't think it's
necessary here.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center