bufmgr: Fix undefined behaviour with, unrealistically, large temp_buffers

anarazel · anarazel · commit 558cf8038768 · 2023-03-30T10:26:10.000-07:00
Quoting Melanie: > Since if buffer is INT_MAX, then the -(buffer + 1) version invokes > undefined behavior while the -buffer - 1 version doesn't. All other places were already using the correct version. I (Andres), copied the code into more places in a patch. Melanie caught it in review, but to prevent more people from copying the bad code, fix it. Even if it is a theoretical issue. We really ought to wrap these accesses in a helper function... As this is a theoretical issue, don't backpatch. Reported-by: Melanie Plageman <melanieplageman@gmail.com> Discussion: https://postgr.es/m/CAAKRu_aW2SX_LWtwHgfnqYpBrunMLfE9PD6-ioPpkh92XH0qpg@mail.gmail.com
diff --git a/src/backend/storage/buffer/localbuf.c b/src/backend/storage/buffer/localbuf.c
@@ -305,7 +305,7 @@ MarkLocalBufferDirty(Buffer buffer)
 	fprintf(stderr, "LB DIRTY %d\n", buffer);
 #endif
 
-	bufid = -(buffer + 1);
+	bufid = -buffer - 1;
 
 	Assert(LocalRefCount[bufid] > 0);
 
