Re: BUG #19634: Hash partition with large MODULUS causes "invalid memory alloc request size"

Hello,

I propose following patch that adds the check to new_partition_bound
routine:

postgres=# CREATE TABLE t (id int) PARTITION BY HASH (id);
CREATE TABLE
postgres=# CREATE TABLE t_p0 PARTITION OF t FOR VALUES WITH (MODULUS
268435457,
REMAINDER 0);
ERROR:  hash partitions bounds are too large
DETAIL:  Creating hash partitions for modulus 268435457 would require
too much memory.
HINT:  Reduce the number of partitions.
postgres=#

Regards,

Pierre Forstmann

Le 21/08/2026 à 05:44, PG Bug reporting form a écrit :
> The following bug has been logged on the website:
>
> Bug reference: 19634
> Logged by: Zheng Hacker
> Email address: hackerzheng666(at)gmail(dot)com
> PostgreSQL version: 19beta3
> Operating system: Linux x86_64
> Description:
>
> Creating a hash partition with MODULUS >= 268435457 causes an internal error
> "invalid memory alloc request size" on any subsequent query against the
> partitioned table. The table becomes permanently unusable — SELECT,
> INSERT,
> and all other operations fail with the same error. Only DROP TABLE works.
>
> Minimal reproducer (tested on PG 20devel commit 609f969, 2026-08-21):
>
> CREATE TABLE t (id int) PARTITION BY HASH (id);
> CREATE TABLE t_p0 PARTITION OF t FOR VALUES WITH (MODULUS 268435457,
> REMAINDER 0);
> SELECT * FROM t; -- ERROR: invalid memory alloc request size 1073741828
>
> Root cause:
> In src/backend/partitioning/partbounds.c, create_hash_bounds() (line 390)
> allocates an array indexed by greatest_modulus:
>
> boundinfo->nindexes = greatest_modulus;
> boundinfo->indexes = palloc_array(int, greatest_modulus);
>
> When greatest_modulus >= 268435457, this requests 268435457 * 4 =
> 1073741828
> bytes, exceeding MaxAllocSize (1073741823 = 1GB - 1). No bounds check
> exists
> on the modulus value before this allocation.
>
> The validation in check_new_partition_bound() (line ~2927) runs AFTER
> create_hash_bounds() is called during partition descriptor loading, so it
> never gets a chance to reject the invalid modulus.
>
> Impact:
> - Affects all versions since hash partitioning was introduced (PG 11+)
> - The partition is created successfully (CREATE TABLE succeeds)
> - But any access to the parent table fails permanently
> - Only DROP TABLE recovers the table
> - Any unprivileged user with CREATE TABLE permission can trigger this
>
> Suggested fix:
> Add a bounds check in create_hash_bounds() before the allocation, or
> validate modulus against MaxAllocSize in check_new_partition_bound()
> before partition descriptor loading.
>
> PostgreSQL version: 20devel (commit 609f969)
> OS: Ubuntu 22.04 x86_64
>
>
>
>