Re: Restricting Direct Access to a C Function in PostgreSQL

On 11/08/2024 12:41, Pavel Stehule wrote:
> ne 11. 8. 2024 v 9:23 odesílatel Ayush Vatsa <ayushvatsa1810(at)gmail(dot)com
> <mailto:ayushvatsa1810(at)gmail(dot)com>> napsal:
>
> Hi PostgreSQL Community,
>
> I have a scenario where I am working with two functions: one in SQL
> and another in C, where the SQL function is a wrapper around C
> function. Here’s an example:
>
> |CREATE OR REPLACE FUNCTION my_func(IN input text) RETURNS BIGINT AS
> $$ DECLARE result BIGINT; BEGIN SELECT col2 INTO result FROM
> my_func_extended(input); RETURN result; END; $$ LANGUAGE plpgsql;
> CREATE OR REPLACE FUNCTION my_func_extended( IN input text, OUT col1
> text, OUT col2 BIGINT ) RETURNS SETOF record AS 'MODULE_PATHNAME',
> 'my_func_extended' LANGUAGE C STRICT PARALLEL SAFE; |
>
> I need to prevent direct execution of |my_func_extended| from psql
> while still allowing it to be called from within the wrapper
> function |my_func|.
>
> I’m considering the following options:
>
> 1. Using GRANT/REVOKE in SQL to manage permissions.
> 2. Adding a check in the C function to allow execution only if
> |my_func| is in the call stack (previous parent or something),
> and otherwise throwing an error.
>
> Is there an existing approach to achieve this, or would you
> recommend a specific solution?
>
> You can use fmgr hook, and hold some variable as gate if your function
> my_func_extended can be called
>
> https://pgpedia.info/f/fmgr_hook.html
> <https://pgpedia.info/f/fmgr_hook.html>
>
> With this option, the execution of my_func_extended will be faster, but
> all other execution will be little bit slower (due overhead of hook).
> But the code probably will be more simpler than processing callback stack.
>
> plpgsql_check uses fmgr hook, and it is working well - just there can be
> some surprises, when the hook is activated in different order against
> function's execution, and then the FHET_END can be executed without
> related FHET_START.

Sounds complicated. I would go with the GRANT approach. Make my_func() a
SECURITY DEFINER function, and revoke access to my_func_extended() for
all other roles.

Another option to consider is to not expose my_func_extended() at the
SQL level in the first place, and rewrite my_func() in C. Dunno how
complicated the logic in my_func() is, if that makes sense.

--
Heikki Linnakangas
Neon (https://neon.tech)