Re: WAL's listing in pg_xlog by some sql query

On Mon, Jun 6, 2016 at 9:51 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
> * Vik Fearing (vik(at)2ndquadrant(dot)fr) wrote:
>> On 03/06/16 04:32, Michael Paquier wrote:
>> > On Fri, Jun 3, 2016 at 11:23 AM, Sameer Kumar <sameer(dot)kumar(at)ashnik(dot)com> wrote:
>> >> On Fri, Jun 3, 2016 at 4:30 AM Stephen Frost <sfrost(at)snowman(dot)net> wrote:
>> >>> Given the usefulness of this specific query and that it could be used
>> >>> without risk of the user being able to gain superuser access through it,
>> >>> I'd like to see a new function added which does not have the superuser
>> >>> check, but is not allowed to be called by public initially either.
>>
>> CREATE FUNCTION ls_dir(text)
>> RETURNS SETOF text
>> LANGUAGE sql
>> SECURITY DEFINER
>> AS 'select * from pg_ls_dir($1)';
>
> This isn't a good idea as it allows access to a great deal more than
> just the number of xlogs. Further, as described above, it gives that
> access to everyone and not just to specific roles.

Sure, because it is possible to a path string at will. In the context
of this use case, Alex could just hardcode pg_xlog and nothing else
than the list of files in this path would be leaked to a user who has
the execution grant right of this function.

> This is a great example of why we should provide an explicit function
> which is documented (both in our documentation and in the documentation
> of tools like check_postgres.pl) that users can use and can GRANT access
> to for their monitoring systems which gives access to only the
> information needed- that is, the number of xlog segments.

I have been wondering for some time now about the possibility to have
at SQL level a representation of the shared memory structure
XLogCtlData instead, though there is no tracking of what is the newest
segment that has been recycled ahead, but it could be a reason to
track that as well.
--
Michael