Re: Misplaced superuser check in pg_log_backend_memory_contexts()

On Sun, Jun 06, 2021 at 03:53:10PM +0900, Michael Paquier wrote:
> 
> While reading the code of pg_log_backend_memory_contexts(), I have
> been surprised to see that the code would attempt to look at a PROC
> entry based on the given input PID *before* checking if the function
> has been called by a superuser.  This does not strike me as a good
> idea as this allows any users to call this function and to take
> ProcArrayLock in shared mode, freely.

It doesn't seem like a huge problem as at least GetSnapshotData also acquires
ProcArrayLock in shared mode.  Knowing if a specific pid is a postgres backend
or not isn't privileged information either, and anyone can check that using
pg_stat_activity as an unprivileged user (which will also acquire ProcArrayLock
in shared mode).
> 
> It seems to me that we had better check for a superuser at the
> beginning of the function, like in the attached.

However +1 for the patch, as it seems more consistent to always get a
permission failure if you're not a superuser.