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

diff --git a/src/backend/catalog/storage.c b/src/backend/catalog/storage.c
index 11b3ea40200836779044b16a547f1a5bc2b3cffe..a3e554e372e2ed9916c5de26ab0db0c9dd1d028b 100644 (file)
--- 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)