How to ensure that a stored function always returns TRUE or FALSE?

On 3/2/16, Alexander Farber <alexander.farber@gmail.com> wrote:

Good morning,

with the following stored function I would like to validate user data:

CREATE OR REPLACE FUNCTION check_user(
in_social integer,
in_sid varchar(255),
in_auth varchar(32))
RETURNS boolean AS
$func$
SELECT MD5('secret word' || in_social || in_sid) = in_auth;
$func$ LANGUAGE sql IMMUTABLE;

I am going to call it while looping through a JSON array of objects in
another stored functions - and will RAISE EXCEPTION if it returns FALSE for
any of the JSON objects (and thus rollback the whole transaction).

I have prepared 3 simple test functions below -

<overquoting>

CREATE OR REPLACE FUNCTION test3() RETURNS void AS
$func$
BEGIN
IF NOT check_user(42, 'user1', NULL) THEN
RAISE NOTICE 'invalid user';
ELSE
RAISE NOTICE 'valid user';
END IF;
END
$func$ LANGUAGE plpgsql;

The 3rd function does NOT work as expected and prints "valid user".

This happens because check_user() returns NULL instead of a boolean value.

I guess it is enough to swap blocks inside of IF statement and reverse
its condition:

CREATE OR REPLACE FUNCTION test3() RETURNS void AS
$func$
BEGIN
IF check_user(42, 'user1', NULL) THEN
RAISE NOTICE 'valid user';
ELSE
RAISE NOTICE 'invalid user';
END IF;
END
$func$ LANGUAGE plpgsql;

would give "invalid user". NULL works as FALSE at the top of IF expressions.
For more information see[1]https://en.wikipedia.org/wiki/Null_(SQL)#Comparisons_with_NULL_and_the_three-valued_logic_.283VL.29 -- Best regards, Vitaly Burovoy.

COALESCE could be wrapped around the check_user() call in the
IF-statement... but is there maybe a nicer way to solve this problem?

Thank you
Alex

[1]: https://en.wikipedia.org/wiki/Null_(SQL)#Comparisons_with_NULL_and_the_three-valued_logic_.283VL.29 -- Best regards, Vitaly Burovoy
--
Best regards,
Vitaly Burovoy

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Thanks Vitaly, but instead of inverting the IF-condition I would prefer to
make my function more robust, since it is kind of security-related and I
might forget about the special IF-condition later when using it elsewhere...

On Wed, Mar 2, 2016 at 11:09 AM, Vitaly Burovoy <vitaly.burovoy@gmail.com>
wrote:

On Wed, Mar 2, 2016 at 3:45 AM, Alexander Farber
<alexander.farber@gmail.com> wrote:

Good morning,

with the following stored function I would like to validate user data:

CREATE OR REPLACE FUNCTION check_user(
in_social integer,
in_sid varchar(255),
in_auth varchar(32))
RETURNS boolean AS
$func$
SELECT MD5('secret word' || in_social || in_sid) = in_auth;
$func$ LANGUAGE sql IMMUTABLE;

I am going to call it while looping through a JSON array of objects in
another stored functions - and will RAISE EXCEPTION if it returns FALSE for
any of the JSON objects (and thus rollback the whole transaction).

Personally I would write the check like this:
SELECT MD5('secret word' || in_social || in_sid) IS NOT DISTINCT FROM in_auth;

...for better handling of NULLS within the input arguments. It is
definitely write for this function to be sql, not plpgsql, because it
is a good candidate for inlining.

Also, I tend to wrap RAISE NOTICE with a function:

CREATE OR REPLACE FUNCTION Notice(TEXT) RETURNS VOID AS
$$
BEGIN
RAISE NOTICE '%', $1;
END;
$$ LANGUAGE PLPGSQL;

CREATE OR REPLACE FUNCTION Exception(TEXT) RETURNS VOID AS
$$
BEGIN
RAISE NOTICE '%', $1;
END;
$$ LANGUAGE PLPGSQL;

CREATE OR REPLACE FUNCTION NoticeValue(anyelement) RETURNS anyelement AS
$$
SELECT $1 FROM (SELECT NOTICE($1::TEXT)) q;
$$ LANGUAGE SQL IMMUTABLE;

Then you can write a checker function like this:

CREATE OR REPLACE FUNCTION test4() RETURNS void AS
$func$
BEGIN
PERFORM Exception('invalid user') WHERE NOT
check_user(42, 'user1', NULL);
END
$func$ LANGUAGE plpgsql;

"NoticeValue()" Is a wonderful debugging tool for pl/pgsql. It allows
you to quickly virtually anything in a query without rewriting the
entire query.

SELECT NoticeValue(foo) FROM bar;

merlin

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On 3/2/16, Alexander Farber <alexander.farber@gmail.com> wrote:

On Wed, Mar 2, 2016 at 11:09 AM, Vitaly Burovoy <vitaly.burovoy@gmail.com>
wrote:

On 3/2/16, Alexander Farber <alexander.farber@gmail.com> wrote:

CREATE OR REPLACE FUNCTION check_user(
in_social integer,
in_sid varchar(255),
in_auth varchar(32))
RETURNS boolean AS
$func$
SELECT MD5('secret word' || in_social || in_sid) =

in_auth;

$func$ LANGUAGE sql IMMUTABLE;

CREATE OR REPLACE FUNCTION test3() RETURNS void AS
$func$
BEGIN
IF NOT check_user(42, 'user1', NULL) THEN
RAISE NOTICE 'invalid user';
ELSE
RAISE NOTICE 'valid user';
END IF;
END
$func$ LANGUAGE plpgsql;

The 3rd function does NOT work as expected and prints "valid user".

This happens because check_user() returns NULL instead of a boolean

value.

I guess it is enough to swap blocks inside of IF statement and reverse
its condition:

CREATE OR REPLACE FUNCTION test3() RETURNS void AS
$func$
BEGIN
IF check_user(42, 'user1', NULL) THEN
RAISE NOTICE 'valid user';
ELSE
RAISE NOTICE 'invalid user';
END IF;
END
$func$ LANGUAGE plpgsql;

would give "invalid user". NULL works as FALSE at the top of IF
expressions.

[1]https://en.wikipedia.org/wiki/Null_(SQL)#Comparisons_with_NULL_and_the_three-valued_logic_.283VL.29

Thanks Vitaly, but instead of inverting the IF-condition I would prefer to
make my function more robust, since it is kind of security-related and I
might forget about the special IF-condition later when using it
elsewhere...

As Merlin Moncure mentioned[2]/messages/by-id/CAHyXU0xdFQ--0aTm3Md7d1x5ZnfBJDe0eMJHnVtCZdBJuFDqew@mail.gmail.com -- Best regards, Vitaly Burovoy the best way is to replace "=" by "IS
NOT DISTINCT FROM" in the "check_user" function.
But if you want to change only IF statement in "testX" functions it is
enough to replace the condition "IF NOT check_user(42, 'user1', NULL)
THEN" by "IF check_user(42, 'user1', NULL) IS NOT TRUE THEN". See the
example below:

postgres=# SELECT var, var IS NOT TRUE AS result
postgres-# FROM unnest(ARRAY[TRUE, FALSE, NULL]::bool[])as var;
var | result
-----+--------
t | f
f | t
| t
(3 rows)

P.S.: please, don't top post.

[2]: /messages/by-id/CAHyXU0xdFQ--0aTm3Md7d1x5ZnfBJDe0eMJHnVtCZdBJuFDqew@mail.gmail.com -- Best regards, Vitaly Burovoy
--
Best regards,
Vitaly Burovoy

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On Wed, Mar 2, 2016 at 5:39 AM, Alexander Farber <alexander.farber@gmail.com

wrote:

Thanks Vitaly, but instead of inverting the IF-condition I would prefer to
make my function more robust, since it is kind of security-related and I
might forget about the special IF-condition later when using it elsewhere...

​Merlin's point about inlining and SQL language functions not withstanding:​

You should consider writing a variation of the check_user function that
returns VOID or raises an exception and use is like an assertion.

​I would consider raising an exception if in_auth is NULL as I'd
potentially consider such a situation to represent mis-usage of the
function which should gets it own error instead of simply indicating that
the validation failed.
​
​David J.​

Thank you all for the valuable replies.

I've also got suggestions to use IS NOT DISTINCT FROM or STRICT at
http://stackoverflow.com/questions/35742865/how-to-ensure-that-a-stored-function-always-returns-true-or-false

but the former has the edge case of NULL=NULL returning TRUE
and with the latter I would have to be careful with the way I call my
function -
and I am worried I might forget it later and this is a security related...

So I will probably use this function:

CREATE OR REPLACE FUNCTION check_user(in_social integer,
in_sid varchar(255),
in_auth varchar(32))
RETURNS boolean AS
$func$
SELECT CASE
WHEN in_social IS NULL THEN FALSE
WHEN in_sid IS NULL THEN FALSE
WHEN in_auth IS NULL THEN FALSE
ELSE (MD5('secret word' || in_social || in_sid) = in_auth)
END;

$func$ LANGUAGE sql IMMUTABLE;

Regards
Alex

Hi

2016-03-02 19:31 GMT+01:00 Alexander Farber <alexander.farber@gmail.com>:

Thank you all for the valuable replies.

I've also got suggestions to use IS NOT DISTINCT FROM or STRICT at
http://stackoverflow.com/questions/35742865/how-to-ensure-that-a-stored-function-always-returns-true-or-false

but the former has the edge case of NULL=NULL returning TRUE
and with the latter I would have to be careful with the way I call my
function -
and I am worried I might forget it later and this is a security related...

So I will probably use this function:

CREATE OR REPLACE FUNCTION check_user(in_social integer,
in_sid varchar(255),
in_auth varchar(32))
RETURNS boolean AS
$func$
SELECT CASE
WHEN in_social IS NULL THEN FALSE
WHEN in_sid IS NULL THEN FALSE
WHEN in_auth IS NULL THEN FALSE
ELSE (MD5('secret word' || in_social || in_sid) = in_auth)
END;

$func$ LANGUAGE sql IMMUTABLE;

this solution is ilustrative, but probably slower

I hope so function

REATE OR REPLACE FUNCTION check_user(in_social integer,
in_sid varchar(255),
in_auth varchar(32))
RETURNS boolean AS
$func$
SELECT COALESCE(MD5('secret word' || in_social || in_sid) =
in_auth, FALSE)
$func$ LANGUAGE sql IMMUTABLE;

should to return same result quckly.

Regards

Pavel