Subject: Avoid core dump in pgstat_read_statsfile()

From: Bertrand Drouvot <[email-masked]>
Date: Wed, 23 Apr 2025 08:01:40 +0000

Hi hackers,

please find attached a patch to $SUBJECT.

Indeed one could generate a core dump similar to:

Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x0000000000a7a7a7 in pgstat_init_entry (kind=129, shhashent=0x7a55845cfd98) at pgstat_shmem.c:314
314             chunk = dsa_allocate0(pgStatLocal.dsa, pgstat_get_kind_info(kind)->shared_size);
(gdb) bt
#0  0x0000000000a7a7a7 in pgstat_init_entry (kind=129, shhashent=0x7a55845cfd98) at pgstat_shmem.c:314
#1  0x0000000000a72935 in pgstat_read_statsfile () at pgstat.c:1982
#2  0x0000000000a700a8 in pgstat_restore_stats () at pgstat.c:507

by:

1. creating a custom stat kind (non fixed_amount and write_to_file enabled)
2. generate stats
3. stop the engine
4. change the custom PGSTAT_KIND id linked to 1., compile and install
5. re-start the engine

I think that a check on pgstat_get_kind_info() is missing for this scenario, the
patch adds it. Such a check already exists for PGSTAT_FILE_ENTRY_FIXED and
for stats entry identified by name on disk, but not for PGSTAT_FILE_ENTRY_HASH.

The v18 existing checks mentioned above as well as the new check were missing
in pre-18 but I don't think it's worth a back-patch as the issue is unlikely to
occur without custom stats. Adding a v18 open item then.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
>From b80bcbcf05e44d31cc90c1de2e3200899db1294f Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <[email-masked]>
Date: Wed, 23 Apr 2025 07:19:44 +0000
Subject: [PATCH v1] Avoid core dump in pgstat_read_statsfile()

Adding a check that pgstat_get_kind_info() is not NULL when loading the
stats file. Without this extra check a non fixed_amount custom stats kind that
has write_to_file enabled would produce a core dump should its PGSTAT_KIND ID
change before a re-start.
---
 src/backend/utils/activity/pgstat.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index 21bdff106a9..d735a2c1a58 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -1908,6 +1908,13 @@ pgstat_read_statsfile(void)
 								 key.objid, t);
 							goto error;
 						}
+
+						if (!pgstat_get_kind_info(key.kind))
+						{
+							elog(WARNING, "could not find information of kind %u for entry of type %c",
+								 key.kind, t);
+							goto error;
+						}
 					}
 					else
 					{
-- 
2.34.1