From 1e25cdb214543d8b661cf01bbdb6f8e2b1a0381e Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Thu, 30 Jan 2025 15:36:07 -0500 Subject: [PATCH] Avoid integer overflow while testing wal_skip_threshold condition. 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 c6b92041d which introduced wal_skip_threshold, so back-patch to v13. Discussion: https://postgr.es/m/1a01f0-66ec2d80-3b-68487680@27595217 Backpatch-through: 13 --- src/backend/catalog/storage.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/src/backend/catalog/storage.c b/src/backend/catalog/storage.c index 11b3ea40200..a3e554e372e 100644 --- a/src/backend/catalog/storage.c +++ b/src/backend/catalog/storage.c @@ -763,7 +763,7 @@ 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->rlocator, INVALID_PROC_NUMBER); @@ -807,7 +807,7 @@ 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)