Re: Row Level Security − leakproof-ness and performance implications

On Wednesday, February 20, 2019 5:24:17 PM CET Tom Lane wrote:
> Pierre Ducroquet  writes:
> > For simple functions like enum_eq/enum_ne, marking them leakproof is an
> > obvious fix (patch attached to this email, including also textin/textout).
> 
> This is not nearly as "obvious" as you think.  See prior discussions,
> notably
> https://www.postgresql.org/message-id/flat/31042.1546194242%40sss.pgh.pa.us
> 
> Up to now we've taken a very strict definition of what leakproofness
> means; as Noah stated, if a function can throw errors for some inputs,
> it's not considered leakproof, even if those inputs should never be
> encountered in practice.  Most of the things we've been willing to
> mark leakproof are straight-line code that demonstrably can't throw
> any errors at all.
> 
> The previous thread seemed to have consensus that the hazards in
> text_cmp and friends were narrow enough that nobody had a practical
> problem with marking them leakproof --- but we couldn't define an
> objective policy statement that would allow making such decisions,
> so nothing's been changed as yet.  I think it is important to have
> an articulable policy about this, not just a seat-of-the-pants
> conclusion about the risk level in a particular function.
> 
> 			regards, tom lane


I undestand these decisions, but it makes RLS quite fragile, with numerous un-
documented side-effects. In order to save difficulties from future users, I 
wrote this patch to the documentation, listing the biggest restrictions I hit 
with RLS so far.

Regards

 Pierre