Subject: current_role of caller of a DEFINER function

From: Dominique Devienne <[email-masked]>
Date: Wed, 26 Jun 2024 10:35:49 +0200

Hi. I was led to believe (by an hallucination...) that I could know
the current_role of the caller of a DEFINER function, but after actual
experimentation, turns out it shows the OWNER of the function, and not
the current_role of the caller.

I foolishly thought curent_role != current_user inside the DEFINER
function, but reading back the doc, it's clear current_role =
current_user = user, thus that was wishful thinking. Only session_user
is representative of the caller, and reliable (modulo SUPERUSER and
SET AUTHORIZATION, but that's a different story and kinda normal)

So I have two questions:
1) Is there any way to know the current_role of the caller of a
DEFINER function. I fear the answer is no, but better be sure from
experts here.
2) Why isn't there a way to obtain the above? What harm would it be?

Obviously for #2, distinguishing current_role from current_user inside
DEFINER is a no-go, for backward compatibility. But could a new
variable be invented for that? What obvious technical reason I'm
missing would make that harmful or difficult?

As to the use-case, now that I have a queue mechanism for tasks to be
delegated to services, I need to make sure the poster of the task has
the right privileges, so I wanted to capture session_user and
current_role of the "poster", and the service would use that info,
reliably captured (inside the DEFINER function to post a task), using
ROLEs and GRANTs and other app-specific permission data.

LOGIN users have different persona in the system, so the current_role
matters, to determine whether the connection posting the task to be
processed asynchronously by a service is allowed or not. I'd think I'm
not the only one that would need something like this, no?

Thanks, --DD

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From: Laurenz Albe <[email-masked]>
Date: Wed, 26 Jun 2024 11:08:16 +0200

> 2) Why isn't there a way to obtain the above? What harm would it be?

Just te be certain, let me ask a question back:

If a SECURITY DEFINER function calls another SECURITY DEFINER function,
which role would you like to get:

1. the invoker that called the first function

2. the owner of the first function (which is the user that called the
   second function)

Yours,
Laurenz Albe

---

From: [email-masked]
Date: Wed, 26 Jun 2024 12:11:26 +0200

> DEFINER function. I fear the answer is no, but better be sure from

You can do something like this:

CREATE DOMAIN current_user_only AS text
   CONSTRAINT current_user_only CHECK (VALUE = CURRENT_USER);

CREATE FUNCTION func(
   calling_user current_user_only DEFAULT CURRENT_USER
) ... SECURITY DEFINER;

The default value will be evaluated in the context of the calling user, 
the constraint forces it to never be set explicitly to something else.

Thus you can use calling_user inside your function.

Best,

Wolfgang