Subject: BUG #19458: OOM killer in jsonb_path_exists_opr (@?) with malformed JSONPath containing non-existent variables

From: PG Bug reporting form <[email-masked]>
Date: Fri, 17 Apr 2026 11:20:34 +0000

The following bug has been logged on the website:

Bug reference:      19458
Logged by:          Andrey Rachitskiy
Email address:      [email-masked]
PostgreSQL version: 14.22
Operating system:   Debian GNU/Linux 12 (bookworm)
Description:        

Description:
During fuzzing of the jsonb_path_exists_opr (operator jsonb @? jsonpath, a
two-argument version of jsonb_path_exists()), a pathological query was
discovered that causes uncontrolled memory consumption, leading to OOM
Killer on PostgreSQL versions REL_14/15/16_STABLE.
On versions 17 and 18, the same query returns a proper error instead of
crashing the server.
This bug was found using AFL++ as a fuzzer and LibBlobStamper as a tool for
creating syntactically correct arguments.

Reproduction:
Execute the following query:
```sql
select '[3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
13558284848669739, 3472328296227668016, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328295419228208, 3472328296227680304,
3528904766546522246, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296328343600,
3472328296227680304, 3472328296227680299, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3470920921344127024, 3906362710315511856,
3472328296228075062, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472334893297446960, 3472328090069248816,
13511005849006128, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
3472328296227680304, 3472328296227680304, 3472328296227680304,
13563782407139376, 4337019423877509168]'::jsonb @? '(-$?(0 <
($"〰〭〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰" - $?(0 < $"〰〰〰〰〰〰〰〰〰〰〰〰〰〰" - $?(0 + $ <
$"㘰〰㘶〰")."〰〰〰〰")."〰〰〰〰〰〰〰〰") - 0?(+$ < $"〰
〰〰〰")."ほ〰〰㘰")."〰〰〰〶〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰")'::jsonpath;
```

Expected result:
The query should return an error, as happens on versions 17 and 18:
ERROR:  could not find jsonpath variable "〰〭〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰〰"

Actual result (14, 15, 16):
  - Memory consumption grows until the kernel kills the postgres process via
OOM Killer
  - Client loses connection:
       server closed the connection unexpectedly
       This probably means the server terminated abnormally
       before or while processing the request.

Kernel log:
516294.487767] Out of memory: Killed process 1135405 (postgres)
total-vm:13521932kB, anon-rss:9170792kB, file-rss:92kB, shmem-rss:1848kB,
UID:1002 pgtables:26176kB oom_score_adj:0

--
Regards,
Andrey Rachitskiy
Postgres Professional

---

From: Andrey Rachitskiy <[email-masked]>
Date: Mon, 20 Apr 2026 18:38:38 +0500

I propose a targeted backpatch for REL_14/15/16 in jsonpath_exec.c to align
missing variable handling with newer branches and prevent pathological
memory growth on malformed/hostile jsonpath expressions.

Why this is not a full backport
This is intentionally not a full backport of the REL_17 jsonpath executor
refactoring (callbacks for variable access, broader executor integration,
etc.).

Why this still fixes the reported issue
On REL_14/15/16, with current behavior (missing vars -> null), some crafted
jsonpath expressions continue deep evaluation and can consume very large
memory, leading to OOM.
With this patch, execution fails early on undefined variable, matching
REL_17 behavior for this case and avoiding runaway memory use.

—
Regards,
Andrey Rachitskiy
Postgres Professional

---

From: Andrey Borodin <[email-masked]>
Date: Thu, 23 Apr 2026 23:55:56 +0500

> On 20 Apr 2026, at 18:38, Andrey Rachitskiy <[email-masked]> wrote:
> 
> I propose a targeted backpatch for REL_14/15/16 in jsonpath_exec.c to align missing variable handling with newer branches and prevent pathological memory growth on malformed/hostile jsonpath expressions.

Hi! Thank you for the report and proposed fix. I've took a look into the
patch.

So we can use vars like this:

# SELECT jsonb_path_exists(
'{"x": 42}'::jsonb,
'$ ? ($"threshold" < 50)'::jsonpath,
'{"threshold": 10}'::jsonb -- HERE go vars
);

Operator @? is doing the same, but without supplied vars. And this thread
essentially points to buggy handling of vars:

# SELECT j @? '$"no_such_var"'
FROM (VALUES
  ('{"important": "data"}'::jsonb),
  ('42'::jsonb),
  ('null'::jsonb),
  ('false'::jsonb)
) AS t(j);
 ?column?  
----------
 t
 t
 t
 t
(4 rows)

It basically says that path with value of var "no_such_var" exists everywhere.

I think it's a bug, but we would need a JSON Path expert here.

17+ throws an error, which seems suspicious to me too. @? is expected to
operate in silent mode. Perhaps, we should just return NULL instead of t.
By using RETURN_ERROR macro. But it might sound overly invasive for back
branches.

Even if we are going to throw an error, we can give mode details. I'd suggest
instead of "could not find jsonpath variable \"%s\"" throwing something like
"no variables supplied to reference by variable \"%s\"" or something along
those lines.

Besides this, the direction of the fix looks good to me. Thank you!

Best regards, Andrey Borodin.

---

From: Nikita Malakhov <[email-masked]>
Date: Tue, 28 Apr 2026 23:19:15 +0300

Hi!

According to the Jsonpath standard, malformed expression should return an
error,
but not all cases of malformation are thoroughly described.

When this functionality was developed (Jsonpath and SQL/JSON) the absence
of the variable was considered as malformation and was decided to throw an
error
in threads long time ago. In case this behavior to be a subject for change
it surely
should not be backported, but the error-throwing code has to.
I agree that sometimes Json (-path) functionality provides very little info
on errors,
and it could be extended.

Changing this behavior is subject for Hackers and should be approved by Tom
Lane,
Adres Freund and other people responsible for including Jsonpath and
SQL/JSON
into Postgres.

On Tue, Apr 28, 2026 at 7:51 PM Andrey Borodin <[email-masked]> wrote:

> > I propose a targeted backpatch for REL_14/15/16 in jsonpath_exec.c to
> align missing variable handling with newer branches and prevent
> pathological memory growth on malformed/hostile jsonpath expressions.
>

-- 
Regards,
Nikita Malakhov
Postgres Professional
The Russian Postgres Company
https://postgrespro.ru/

---

From: Andrey Borodin <[email-masked]>
Date: Thu, 30 Apr 2026 17:03:29 +0500

> On 29 Apr 2026, at 01:19, Nikita Malakhov <[email-masked]> wrote:
> 
> According to the Jsonpath standard, malformed expression should return an error,
> of the variable was considered as malformation and was decided to throw an error
> in threads long time ago. In case this behavior to be a subject for change it surely

I think you just explained very well why we throw an error. Your arguments against
silent mode are valid and we don't need to consider RETURN_ERROR any further.

Current master behavior throws an error, in this thread author propose to backport it.
This might be behavior change for some users. But it seems to me we have to backport,
because

SELECT '42'::jsonb @? '$"no_such_var"';

should not return true. What do you think?

---

From: Nikita Malakhov <[email-masked]>
Date: Tue, 26 May 2026 14:17:45 +0300

Hi!

Sorry for the late answer -  I missed this email.
No objections against backporting error throwing code, sorry if my
explanation
was not clear enough, of course OOM is very bad behavior and should be
corrected,
even if this would result in changes for some users (hopefully not).
My point in the previous message was only against silent mode.

Thanks!

---

From: Andrey Borodin <[email-masked]>
Date: Sun, 31 May 2026 18:33:06 +0500

> On 26 May 2026, at 16:17, Nikita Malakhov <[email-masked]> wrote:
> 
> of course OOM is very bad behavior and should be corrected

I think the biggest problem is not OOM, but incorrect results for query
SELECT '42'::jsonb @? '$"no_such_var"';

---

From: Amit Langote <[email-masked]>
Date: Fri, 5 Jun 2026 17:58:14 +0900

Hi,

Before I dig into the patch properly after the weekend, one question
on the report itself: has anyone traced why the old path runs away on
memory? We've characterized it as missing-var, then null, then
evaluation continues, then OOM, but I don't think the actual growing
allocation has been pinned down. Mostly want to understand whether the
same runaway is reachable without a missing variable, since raising
the error early wouldn't catch those cases.

- Thanks, Amit

---

From: Andrey Rachitskiy <[email-masked]>
Date: Fri, 5 Jun 2026 15:03:11 +0500

The growing allocation is leaked temporary JsonValueLists in
executePredicate() (local lseq/rseq, ~1482–1547) and the arithmetic helpers
executeBinaryArithmExpr() / executeUnaryArithmExpr() (~1561–1684). Each
nested comparison or arithmetic subexpression materializes operands via
executeItemOptUnwrapResult[NoThrow]() → executeNextItem() →
JsonValueListAppend() (~1165, ~2451), but the interim lists are never freed
before return. For @? specifically, executeJsonPath() also leaks a local
vals list in strict exists mode (~579–586).

Missing vars make the AFL case worse by returning null instead of error, so
evaluation continues deep into nested $?()/comparisons instead of stopping
at the first $"…" reference. The same leak mechanism is reachable without
missing vars — Tom Lane demonstrated this on master (5a2043bf713) with $[*]
? (@ < $) on a large array.

Our missing-variable patch fixes the reported OOM and the @? semantics bug
by aborting early. Whether REL_14/15/16 also need a broader fix for interim
JsonValueList cleanup is beyond what I can confidently propose; I've tried
to pin down where the growth happens for that discussion.

---

From: Nikita Malakhov <[email-masked]>
Date: Fri, 5 Jun 2026 13:57:27 +0300

Hi!

Thank you very much for this investigation! I'd take a look into the patch
after the weekend.

On Fri, Jun 5, 2026 at 1:03 PM Andrey Rachitskiy <[email-masked]> wrote:

> Missing vars make the AFL case worse by returning null instead of error,
> so evaluation continues deep into nested $?()/comparisons instead of
> stopping at the first $"…" reference. The same leak mechanism is reachable
> without missing vars — Tom Lane demonstrated this on master (5a2043bf713)
> with $[*] ? (@ < $) on a large array.
>