Remove explicit superuser checks in favor of ACLs

This removes the explicit superuser checks in the various file-access
functions in the backend, specifically pg_ls_dir(), pg_read_file(),
pg_read_binary_file(), and pg_stat_file().  Instead, EXECUTE is REVOKE'd
from public for these, meaning that only a superuser is able to run them
by default, but access to them can be GRANT'd to other roles.

Reviewed-By: Michael Paquier
Discussion: https://postgr.es/m/20171231191939.GR2416%40tamriel.snowman.net

src/backend/catalog/system_views.sql

@@ -1156,6 +1156,20 @@ REVOKE EXECUTE ON FUNCTION lo_export(oid, text) FROM public;
 REVOKE EXECUTE ON FUNCTION pg_ls_logdir() FROM public;
 REVOKE EXECUTE ON FUNCTION pg_ls_waldir() FROM public;
 
+REVOKE EXECUTE ON FUNCTION pg_read_file(text) FROM public;
+REVOKE EXECUTE ON FUNCTION pg_read_file(text,bigint,bigint) FROM public;
+REVOKE EXECUTE ON FUNCTION pg_read_file(text,bigint,bigint,boolean) FROM public;
+
+REVOKE EXECUTE ON FUNCTION pg_read_binary_file(text) FROM public;
+REVOKE EXECUTE ON FUNCTION pg_read_binary_file(text,bigint,bigint) FROM public;
+REVOKE EXECUTE ON FUNCTION pg_read_binary_file(text,bigint,bigint,boolean) FROM public;
+
+REVOKE EXECUTE ON FUNCTION pg_stat_file(text) FROM public;
+REVOKE EXECUTE ON FUNCTION pg_stat_file(text,boolean) FROM public;
+
+REVOKE EXECUTE ON FUNCTION pg_ls_dir(text) FROM public;
+REVOKE EXECUTE ON FUNCTION pg_ls_dir(text,boolean,boolean) FROM public;
+
 --
 -- We also set up some things as accessible to standard roles.
 --

src/backend/utils/adt/genfile.c

@@ -195,11 +195,6 @@ pg_read_file(PG_FUNCTION_ARGS)
 	char	   *filename;
 	text	   *result;
 
-	if (!superuser())
-		ereport(ERROR,
-				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-				 (errmsg("must be superuser to read files"))));
-
 	/* handle optional arguments */
 	if (PG_NARGS() >= 3)
 	{
@@ -236,11 +231,6 @@ pg_read_binary_file(PG_FUNCTION_ARGS)
 	char	   *filename;
 	bytea	   *result;
 
-	if (!superuser())
-		ereport(ERROR,
-				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-				 (errmsg("must be superuser to read files"))));
-
 	/* handle optional arguments */
 	if (PG_NARGS() >= 3)
 	{
@@ -313,11 +303,6 @@ pg_stat_file(PG_FUNCTION_ARGS)
 	TupleDesc	tupdesc;
 	bool		missing_ok = false;
 
-	if (!superuser())
-		ereport(ERROR,
-				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-				 (errmsg("must be superuser to get file information"))));
-
 	/* check the optional argument */
 	if (PG_NARGS() == 2)
 		missing_ok = PG_GETARG_BOOL(1);
@@ -399,11 +384,6 @@ pg_ls_dir(PG_FUNCTION_ARGS)
 	directory_fctx *fctx;
 	MemoryContext oldcontext;
 
-	if (!superuser())
-		ereport(ERROR,
-				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-				 (errmsg("must be superuser to get directory listings"))));
-
 	if (SRF_IS_FIRSTCALL())
 	{
 		bool		missing_ok = false;