Fix has_column_privilege function corner case

According to the comments, when an invalid or dropped column oid is passed
to has_column_privilege(), the intention has always been to return NULL.
However, when the caller had table level privilege the invalid/missing
column was never discovered, because table permissions were checked first.

Fix that by introducing extended versions of pg_attribute_acl(check|mask)
and pg_class_acl(check|mask) which take a new argument, is_missing. When
is_missing is NULL, the old behavior is preserved. But when is_missing is
passed by the caller, no ERROR is thrown for dropped or missing
columns/relations, and is_missing is flipped to true. This in turn allows
has_column_privilege to check for column privileges first, providing the
desired semantics.

Not backpatched since it is a user visible behavioral change with no previous
complaints, and the fix is a bit on the invasive side.

Author: Joe Conway
Reviewed-By: Tom Lane
Reported by: Ian Barwick
Discussion: https://postgr.es/m/flat/9b5f4311-157b-4164-7fe7-077b4fe8ed84%40joeconway.com

src/backend/catalog/aclchk.c

@@ -3705,6 +3705,20 @@ pg_aclmask(ObjectType objtype, Oid table_oid, AttrNumber attnum, Oid roleid,
 AclMode
 pg_attribute_aclmask(Oid table_oid, AttrNumber attnum, Oid roleid,
 					 AclMode mask, AclMaskHow how)
+{
+	return pg_attribute_aclmask_ext(table_oid, attnum, roleid,
+									mask, how, NULL);
+}
+
+/*
+ * Exported routine for examining a user's privileges for a column
+ *
+ * Does the bulk of the work for pg_attribute_aclmask(), and allows other
+ * callers to avoid the missing attribute ERROR when is_missing is non-NULL.
+ */
+AclMode
+pg_attribute_aclmask_ext(Oid table_oid, AttrNumber attnum, Oid roleid,
+						 AclMode mask, AclMaskHow how, bool *is_missing)
 {
 	AclMode		result;
 	HeapTuple	classTuple;
@@ -3723,18 +3737,38 @@ pg_attribute_aclmask(Oid table_oid, AttrNumber attnum, Oid roleid,
 							   ObjectIdGetDatum(table_oid),
 							   Int16GetDatum(attnum));
 	if (!HeapTupleIsValid(attTuple))
+	{
+		if (is_missing != NULL)
+		{
+			/* return "no privileges" instead of throwing an error */
+			*is_missing = true;
+			return 0;
+		}
+		else
 			ereport(ERROR,
 					(errcode(ERRCODE_UNDEFINED_COLUMN),
 					 errmsg("attribute %d of relation with OID %u does not exist",
 							attnum, table_oid)));
+	}
+
 	attributeForm = (Form_pg_attribute) GETSTRUCT(attTuple);
 
-	/* Throw error on dropped columns, too */
+	/* Check dropped columns, too */
 	if (attributeForm->attisdropped)
+	{
+		if (is_missing != NULL)
+		{
+			/* return "no privileges" instead of throwing an error */
+			*is_missing = true;
+			ReleaseSysCache(attTuple);
+			return 0;
+		}
+		else
 			ereport(ERROR,
 					(errcode(ERRCODE_UNDEFINED_COLUMN),
 					 errmsg("attribute %d of relation with OID %u does not exist",
 							attnum, table_oid)));
+	}
 
 	aclDatum = SysCacheGetAttr(ATTNUM, attTuple, Anum_pg_attribute_attacl,
 							   &isNull);
@@ -3790,6 +3824,19 @@ pg_attribute_aclmask(Oid table_oid, AttrNumber attnum, Oid roleid,
 AclMode
 pg_class_aclmask(Oid table_oid, Oid roleid,
 				 AclMode mask, AclMaskHow how)
+{
+	return pg_class_aclmask_ext(table_oid, roleid, mask, how, NULL);
+}
+
+/*
+ * Exported routine for examining a user's privileges for a table
+ *
+ * Does the bulk of the work for pg_class_aclmask(), and allows other
+ * callers to avoid the missing relation ERROR when is_missing is non-NULL.
+ */
+AclMode
+pg_class_aclmask_ext(Oid table_oid, Oid roleid, AclMode mask,
+					 AclMaskHow how, bool *is_missing)
 {
 	AclMode		result;
 	HeapTuple	tuple;
@@ -3804,10 +3851,20 @@ pg_class_aclmask(Oid table_oid, Oid roleid,
 	 */
 	tuple = SearchSysCache1(RELOID, ObjectIdGetDatum(table_oid));
 	if (!HeapTupleIsValid(tuple))
+	{
+		if (is_missing != NULL)
+		{
+			/* return "no privileges" instead of throwing an error */
+			*is_missing = true;
+			return 0;
+		}
+		else
 			ereport(ERROR,
 					(errcode(ERRCODE_UNDEFINED_TABLE),
 					 errmsg("relation with OID %u does not exist",
 							table_oid)));
+	}
+
 	classForm = (Form_pg_class) GETSTRUCT(tuple);
 
 	/*
@@ -4468,7 +4525,22 @@ AclResult
 pg_attribute_aclcheck(Oid table_oid, AttrNumber attnum,
 					  Oid roleid, AclMode mode)
 {
-	if (pg_attribute_aclmask(table_oid, attnum, roleid, mode, ACLMASK_ANY) != 0)
+	return pg_attribute_aclcheck_ext(table_oid, attnum, roleid, mode, NULL);
+}
+
+
+/*
+ * Exported routine for checking a user's access privileges to a column
+ *
+ * Does the bulk of the work for pg_attribute_aclcheck(), and allows other
+ * callers to avoid the missing attribute ERROR when is_missing is non-NULL.
+ */
+AclResult
+pg_attribute_aclcheck_ext(Oid table_oid, AttrNumber attnum,
+					  Oid roleid, AclMode mode, bool *is_missing)
+{
+	if (pg_attribute_aclmask_ext(table_oid, attnum, roleid, mode,
+								 ACLMASK_ANY, is_missing) != 0)
 		return ACLCHECK_OK;
 	else
 		return ACLCHECK_NO_PRIV;
@@ -4581,7 +4653,21 @@ pg_attribute_aclcheck_all(Oid table_oid, Oid roleid, AclMode mode,
 AclResult
 pg_class_aclcheck(Oid table_oid, Oid roleid, AclMode mode)
 {
-	if (pg_class_aclmask(table_oid, roleid, mode, ACLMASK_ANY) != 0)
+	return pg_class_aclcheck_ext(table_oid, roleid, mode, NULL);
+}
+
+/*
+ * Exported routine for checking a user's access privileges to a table
+ *
+ * Does the bulk of the work for pg_class_aclcheck(), and allows other
+ * callers to avoid the missing relation ERROR when is_missing is non-NULL.
+ */
+AclResult
+pg_class_aclcheck_ext(Oid table_oid, Oid roleid,
+					  AclMode mode, bool *is_missing)
+{
+	if (pg_class_aclmask_ext(table_oid, roleid, mode,
+							 ACLMASK_ANY, is_missing) != 0)
 		return ACLCHECK_OK;
 	else
 		return ACLCHECK_NO_PRIV;

src/backend/utils/adt/acl.c

@@ -2444,8 +2444,7 @@ column_privilege_check(Oid tableoid, AttrNumber attnum,
 					   Oid roleid, AclMode mode)
 {
 	AclResult	aclresult;
-	HeapTuple	attTuple;
-	Form_pg_attribute attributeForm;
+	bool		is_missing = false;
 
 	/*
 	 * If convert_column_name failed, we can just return -1 immediately.
@@ -2454,42 +2453,25 @@ column_privilege_check(Oid tableoid, AttrNumber attnum,
 		return -1;
 
 	/*
-	 * First check if we have the privilege at the table level.  We check
-	 * existence of the pg_class row before risking calling pg_class_aclcheck.
-	 * Note: it might seem there's a race condition against concurrent DROP,
-	 * but really it's safe because there will be no syscache flush between
-	 * here and there.  So if we see the row in the syscache, so will
-	 * pg_class_aclcheck.
+	 * Check for column-level privileges first. This serves in
+	 * part as a check on whether the column even exists, so we
+	 * need to do it before checking table-level privilege.
 	 */
-	if (!SearchSysCacheExists1(RELOID, ObjectIdGetDatum(tableoid)))
+	aclresult = pg_attribute_aclcheck_ext(tableoid, attnum, roleid,
+										  mode, &is_missing);
+	if (aclresult == ACLCHECK_OK)
+		return 1;
+	else if (is_missing)
 		return -1;
 
-	aclresult = pg_class_aclcheck(tableoid, roleid, mode);
-
+	/* Next check if we have the privilege at the table level */
+	aclresult = pg_class_aclcheck_ext(tableoid, roleid, mode, &is_missing);
 	if (aclresult == ACLCHECK_OK)
-		return true;
-
-	/*
-	 * No table privilege, so try per-column privileges.  Again, we have to
-	 * check for dropped attribute first, and we rely on the syscache not to
-	 * notice a concurrent drop before pg_attribute_aclcheck fetches the row.
-	 */
-	attTuple = SearchSysCache2(ATTNUM,
-							   ObjectIdGetDatum(tableoid),
-							   Int16GetDatum(attnum));
-	if (!HeapTupleIsValid(attTuple))
-		return -1;
-	attributeForm = (Form_pg_attribute) GETSTRUCT(attTuple);
-	if (attributeForm->attisdropped)
-	{
-		ReleaseSysCache(attTuple);
+		return 1;
+	else if (is_missing)
 		return -1;
-	}
-	ReleaseSysCache(attTuple);
-
-	aclresult = pg_attribute_aclcheck(tableoid, attnum, roleid, mode);
-
-	return (aclresult == ACLCHECK_OK);
+	else
+		return 0;
 }
 
 /*

src/include/utils/acl.h

@@ -233,8 +233,14 @@ extern void RemoveRoleFromObjectACL(Oid roleid, Oid classid, Oid objid);
 
 extern AclMode pg_attribute_aclmask(Oid table_oid, AttrNumber attnum,
 									Oid roleid, AclMode mask, AclMaskHow how);
+extern AclMode pg_attribute_aclmask_ext(Oid table_oid, AttrNumber attnum,
+										Oid roleid, AclMode mask,
+										AclMaskHow how, bool *is_missing);
 extern AclMode pg_class_aclmask(Oid table_oid, Oid roleid,
 								AclMode mask, AclMaskHow how);
+extern AclMode pg_class_aclmask_ext(Oid table_oid, Oid roleid,
+									AclMode mask, AclMaskHow how,
+									bool *is_missing);
 extern AclMode pg_database_aclmask(Oid db_oid, Oid roleid,
 								   AclMode mask, AclMaskHow how);
 extern AclMode pg_proc_aclmask(Oid proc_oid, Oid roleid,
@@ -256,9 +262,14 @@ extern AclMode pg_type_aclmask(Oid type_oid, Oid roleid,
 
 extern AclResult pg_attribute_aclcheck(Oid table_oid, AttrNumber attnum,
 									   Oid roleid, AclMode mode);
+extern AclResult pg_attribute_aclcheck_ext(Oid table_oid, AttrNumber attnum,
+										   Oid roleid, AclMode mode,
+										   bool *is_missing);
 extern AclResult pg_attribute_aclcheck_all(Oid table_oid, Oid roleid,
 										   AclMode mode, AclMaskHow how);
 extern AclResult pg_class_aclcheck(Oid table_oid, Oid roleid, AclMode mode);
+extern AclResult pg_class_aclcheck_ext(Oid table_oid, Oid roleid,
+									   AclMode mode, bool *is_missing);
 extern AclResult pg_database_aclcheck(Oid db_oid, Oid roleid, AclMode mode);
 extern AclResult pg_proc_aclcheck(Oid proc_oid, Oid roleid, AclMode mode);
 extern AclResult pg_language_aclcheck(Oid lang_oid, Oid roleid, AclMode mode);

src/test/regress/expected/privileges.out

@@ -1362,7 +1362,13 @@ select has_column_privilege('mytable','........pg.dropped.2........','select');
 select has_column_privilege('mytable',2::int2,'select');
  has_column_privilege 
 ----------------------
- t
+ 
+(1 row)
+
+select has_column_privilege('mytable',99::int2,'select');
+ has_column_privilege 
+----------------------
+ 
 (1 row)
 
 revoke select on table mytable from regress_priv_user3;
@@ -1372,6 +1378,12 @@ select has_column_privilege('mytable',2::int2,'select');
  
 (1 row)
 
+select has_column_privilege('mytable',99::int2,'select');
+ has_column_privilege 
+----------------------
+ 
+(1 row)
+
 drop table mytable;
 -- Grant options
 SET SESSION AUTHORIZATION regress_priv_user1;

src/test/regress/sql/privileges.sql

@@ -836,8 +836,10 @@ alter table mytable drop column f2;
 select has_column_privilege('mytable','f2','select');
 select has_column_privilege('mytable','........pg.dropped.2........','select');
 select has_column_privilege('mytable',2::int2,'select');
+select has_column_privilege('mytable',99::int2,'select');
 revoke select on table mytable from regress_priv_user3;
 select has_column_privilege('mytable',2::int2,'select');
+select has_column_privilege('mytable',99::int2,'select');
 drop table mytable;
 
 -- Grant options