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

The following bug has been logged on the website:

Bug reference:      19458
Logged by:          Andrey Rachitskiy
Email address:      pl0h0yp1@gmail.com
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

> On 20 Apr 2026, at 18:38, Andrey Rachitskiy <pl0h0yp1@gmail.com> wrote:
>
> I propose a targeted backpatch for REL_14/15/16 in jsonpath_exec.c to align missing variable handling with newer
branchesand 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.

> On 29 Apr 2026, at 01:19, Nikita Malakhov <hukutoc@gmail.com> wrote:
>
> 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 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?


Best regards, Andrey Borodin.