Re: RFC: Additional Directory for Extensions

Re: David E. Wheeler
> > Yes, I like the suggestion to make it require a restart, which lets the sysadmin control it and not limited to whatever the person who compiled it thought would make sense.
> 
> Here’s a revision of the Debian patch that requires a server start.

Thanks for bringing this up, I should have submitted this years ago.
(The patch is originally from September 2020.)

I designed the patch to require a superuser to set it, so it doesn't
matter very much by which mechanism it gets updated. There should be
little reason to vary it at run-time, so I'd be fine with requiring a
restart, but otoh, why restrict the superuser from reloading it if
they know what they are doing?

> However, in studying the patch, it appears that the `extension_directory` is searched for *all* shared libraries, not just those being loaded for an extension. Am I reading the `expand_dynamic_library_name()` function right?
> 
> If so, this seems like a good way for a bad actor to muck with things, by putting an exploited libpgtypes library into the extension directory, where it would be loaded in preference to the core libpgtypes library, if they couldn’t exploit the original.
> 
> I’m thinking it would be better to have the dynamic library lookup for extension libraries (and LOAD libraries?) separate, so that the `extension_directory` would not be used for core libraries.

I'm not sure the concept of "core libraries" exists. PG happens to
dlopen things at run time, and it doesn't know/care if they were
installed by users or by the original PG server. Also, an exploited
libpgtypes library is not worse than any other untrusted "user"
library, so you really don't want to allow users to provide their own
.so files, no matter by what mechanism.

> This would also allow the lookup of extension libraries prefixed by the directory field from the control file, which would enable much tidier extension installation: The control file, SQL scripts, and DSOs could all be in a single directory for an extension.

Nice idea, but that would mean installing .so files into PGSHAREDIR.
Perhaps the whole extension stuff would have to move to PKGLIBDIR
instead.


Fwiw, I wrote this patch to solve the problem of testing extensions at
build-time where the build process does not have write access to
PGSHAREDIR. It solves that problem quite well, almost all PG
extensions have build-time test coverage now (where there was
basically 0 before).

Security is not a concern at this point as everything is running as
the same user, and the test cluster will be wiped right after the
test. I figured marking the setting as "super user" only was enough
security at that point, but I would recommend another audit before
using it together with "trusted" extensions and other things in
production.

Christoph