git.postgresql.org Git - postgresql.git/commitdiff (original) (raw)

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](/gitweb/?p=postgresql.git;a=object;h=66ec2d80)\-3b-[68487680](/gitweb/?p=postgresql.git;a=object;h=68487680)@[27595217](/gitweb/?p=postgresql.git;a=object;h=27595217)
Backpatch-through: 13

{

ForkNumber fork;

BlockNumber nblocks[MAX_FORKNUM + 1];

- BlockNumber total_blocks = 0;

+ uint64 total_blocks = 0;

SMgrRelation srel;

srel = smgropen(pendingsync->rlocator, INVALID_PROC_NUMBER);

* 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)