Hi hackers,
While working on a rebase for [0], I noticed some weird behavior on the stats.
The issue is that [0], in conjonction with b14e9ce7d5, does introduce padding
in
the PgStat_HashKey:
(gdb) ptype /o struct PgStat_HashKey
/* offset | size */ type = struct PgStat_HashKey {
/* 0 | 4 */ uint32 kind;
/* 4 | 4 */ Oid dboid;
/* 8 | 8 */ uint64 objid;
/* 16 | 4 */ RelFileNumber relfile;
/* XXX 4-byte padding */
/* total size (bytes): 24 */
}
But, the keys are initialized that way:
"
PgStat_HashKey key = {.kind = kind,.dboid = dboid,.objid = objid,.relfile =
refile};
"
which could lead to random data in the padding bytes.
We are using sizeof(PgStat_HashKey) in pgstat_cmp_hash_key() and we compute the
hash hash key in pgstat_hash_hash_key() using the PgStat_HashKey struct size as
input: this lead to unexpected results if the keys contain random data in the
padding bytes.
Even if currently there is no issues, as without [0] there is no padding:
(gdb) ptype /o struct PgStat_HashKey
/* offset | size */ type = struct PgStat_HashKey {
/* 0 | 4 */ uint32 kind;
/* 4 | 4 */ Oid dboid;
/* 8 | 8 */ uint64 objid;
/* total size (bytes): 16 */
}
I think that we should ensure to $SUBJECT (to prevent unexpected results should
padding be introduced in the future).
For example we currently ensure the same for LOCALLOCKTAG localtag in
LockHeldByMe()
while there is no padding:
gdb) ptype /o struct LOCALLOCKTAG
/* offset | size */ type = struct LOCALLOCKTAG {
/* 0 | 16 */ LOCKTAG lock;
/* 16 | 4 */ LOCKMODE mode;
/* total size (bytes): 20 */
}
So, please find attached a patch to $SUBJECT.
[0]:
https://www.postgresql.org/message-id/flat/ZlGYokUIlERemvpB%40ip-10-97-1-34.eu-west-3.compute.internal
Looking forward to your feedback,
Regards,
--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
>From 0a590aad0ff014dff60e5ee73bef2c5d7a659804 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <[email protected]>
Date: Sat, 2 Nov 2024 14:21:18 +0000
Subject: [PATCH v1] Clear padding in PgStat_HashKey keys
PgStat_HashKey keys are currently initialized in a way that could result in random
data in the padding bytes (if there was padding in PgStat_HashKey which is not
the case currently).
We are using sizeof(PgStat_HashKey) in pgstat_cmp_hash_key() and we compute the
hash hash key in pgstat_hash_hash_key() using the PgStat_HashKey struct size as
input. So, we have to ensure that no random data can be stored in the padding
bytes (if any) of a PgStat_HashKey key.
---
src/backend/utils/activity/pgstat.c | 3 +++
src/backend/utils/activity/pgstat_shmem.c | 18 ++++++++++++++++--
2 files changed, 19 insertions(+), 2 deletions(-)
100.0% src/backend/utils/activity/
diff --git a/src/backend/utils/activity/pgstat.c b/src/backend/utils/activity/pgstat.c
index be48432cc3..ea8c5691e8 100644
--- a/src/backend/utils/activity/pgstat.c
+++ b/src/backend/utils/activity/pgstat.c
@@ -938,6 +938,9 @@ pgstat_fetch_entry(PgStat_Kind kind, Oid dboid, uint64 objid)
pgstat_prep_snapshot();
+ /* clear padding */
+ memset(&key, 0, sizeof(struct PgStat_HashKey));
+
key.kind = kind;
key.dboid = dboid;
key.objid = objid;
diff --git a/src/backend/utils/activity/pgstat_shmem.c b/src/backend/utils/activity/pgstat_shmem.c
index a09c6fee05..c1b7ff76b1 100644
--- a/src/backend/utils/activity/pgstat_shmem.c
+++ b/src/backend/utils/activity/pgstat_shmem.c
@@ -432,11 +432,18 @@ PgStat_EntryRef *
pgstat_get_entry_ref(PgStat_Kind kind, Oid dboid, uint64 objid, bool create,
bool *created_entry)
{
- PgStat_HashKey key = {.kind = kind,.dboid = dboid,.objid = objid};
+ PgStat_HashKey key;
PgStatShared_HashEntry *shhashent;
PgStatShared_Common *shheader = NULL;
PgStat_EntryRef *entry_ref;
+ /* clear padding */
+ memset(&key, 0, sizeof(struct PgStat_HashKey));
+
+ key.kind = kind;
+ key.dboid = dboid;
+ key.objid = objid;
+
/*
* passing in created_entry only makes sense if we possibly could create
* entry.
@@ -908,10 +915,17 @@ pgstat_drop_database_and_contents(Oid dboid)
bool
pgstat_drop_entry(PgStat_Kind kind, Oid dboid, uint64 objid)
{
- PgStat_HashKey key = {.kind = kind,.dboid = dboid,.objid = objid};
+ PgStat_HashKey key;
PgStatShared_HashEntry *shent;
bool freed = true;
+ /* clear padding */
+ memset(&key, 0, sizeof(struct PgStat_HashKey));
+
+ key.kind = kind;
+ key.dboid = dboid;
+ key.objid = objid;
+
/* delete local reference */
if (pgStatEntryRefHash)
{
--
2.34.1
