Avoid integer overflow while testing wal_skip_threshold condition.

tglsfdc · tglsfdc · commit 49a38238e2b6 · 2025-01-30T15:36:07.000-05:00
smgrDoPendingSyncs had two distinct risks of integer overflow while deciding which way to ensure durability of a newly-created relation. First, it accumulated the total size of all forks in a variable of type BlockNumber (uint32). While we restrict an individual fork's size to fit in that, I don't believe there's such a restriction on all of them added together. Second, it proceeded to multiply the sum by BLCKSZ, which most certainly could overflow a uint32. (The exact expression is total_blocks * BLCKSZ / 1024. The compiler might choose to optimize that to total_blocks * 8, which is not at quite as much risk of overflow as a literal reading would be, but it's still wrong.) If an overflow did occur it could lead to a poor choice to shove a very large relation into WAL instead of fsync'ing it. This wouldn't be fatal, but it could be inefficient. Change total_blocks to uint64 which should be plenty, and rearrange the comparison calculation to be overflow-safe. I noticed this while looking for ramifications of the proposed change in MAX_KILOBYTES. It's not entirely clear to me why wal_skip_threshold is limited to MAX_KILOBYTES in the first place, but in any case this code is unsafe regardless of the range of wal_skip_threshold. Oversight in c6b9204 which introduced wal_skip_threshold, so back-patch to v13. Discussion: https://postgr.es/m/1a01f0-66ec2d80-3b-68487680@27595217 Backpatch-through: 13
diff --git a/src/backend/catalog/storage.c b/src/backend/catalog/storage.c
@@ -774,7 +774,7 @@ smgrDoPendingSyncs(bool isCommit, bool isParallelWorker)
 	{
 		ForkNumber	fork;
 		BlockNumber nblocks[MAX_FORKNUM + 1];
-		BlockNumber total_blocks = 0;
+		uint64		total_blocks = 0;
 		SMgrRelation srel;
 
 		srel = smgropen(pendingsync->rnode, InvalidBackendId);
@@ -818,7 +818,7 @@ smgrDoPendingSyncs(bool isCommit, bool isParallelWorker)
 		 * main fork is longer than ever but FSM fork gets shorter.
 		 */
 		if (pendingsync->is_truncated ||
-			total_blocks * BLCKSZ / 1024 >= wal_skip_threshold)
+			total_blocks >= wal_skip_threshold * (uint64) 1024 / BLCKSZ)
 		{
 			/* allocate the initial array, or extend it, if needed */
 			if (maxrels == 0)
