Fix theoretical bug in tuplesort

david-rowley · david-rowley · commit 6f1c7a2d0fe0 · 2021-07-13T12:44:36.000+12:00
This fixes a theoretical bug in tuplesort.c which, if a bounded sort was used in combination with a byval Datum sort (tuplesort_begin_datum), when switching the sort to a bounded heap in make_bounded_heap(), we'd call free_sort_tuple(). The problem was that when sorting Datums of a byval type, the tuple is NULL and free_sort_tuple() would free the memory for it regardless of that. This would result in a crash. Here we fix that simply by adding a check to see if the tuple is NULL before trying to disassociate and free any memory belonging to it. The reason this bug is only theoretical is that nowhere in the current code base do we do tuplesort_set_bound() when performing a Datum sort. However, let's backpatch a fix for this as if any extension uses the code in this way then it's likely to cause problems. Author: Ronan Dunklau Discussion: https://postgr.es/m/CAApHDvpdoqNC5FjDb3KUTSMs5dg6f+XxH4Bg_dVcLi8UYAG3EQ@mail.gmail.com Backpatch-through: 9.6, oldest supported version
diff --git a/src/backend/utils/sort/tuplesort.c b/src/backend/utils/sort/tuplesort.c
@@ -4585,6 +4585,9 @@ leader_takeover_tapes(Tuplesortstate *state)
 static void
 free_sort_tuple(Tuplesortstate *state, SortTuple *stup)
 {
-	FREEMEM(state, GetMemoryChunkSpace(stup->tuple));
-	pfree(stup->tuple);
+	if (stup->tuple)
+	{
+		FREEMEM(state, GetMemoryChunkSpace(stup->tuple));
+		pfree(stup->tuple);
+	}
 }

Original file line number	Diff line number	Diff line change
`@@ -4585,6 +4585,9 @@ leader_takeover_tapes(Tuplesortstate *state)`
`4585`	`4585`	`static void`
`4586`	`4586`	`free_sort_tuple(Tuplesortstate state, SortTuple stup)`
`4587`	`4587`	`{`
`4588`		`- FREEMEM(state, GetMemoryChunkSpace(stup->tuple));`
`4589`		`- pfree(stup->tuple);`
	`4588`	`+ if (stup->tuple)`
	`4589`	`+ {`
	`4590`	`+ FREEMEM(state, GetMemoryChunkSpace(stup->tuple));`
	`4591`	`+ pfree(stup->tuple);`
	`4592`	`+ }`
`4590`	`4593`	`}`