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From: PG Bug reporting form <[email protected]>
To: [email protected]
Cc: [email protected]
Subject: BUG #19528: Assert failure in generate_normalized_query() via Squashed Array Literals
Date: Fri, 19 Jun 2026 04:40:56 +0000
Message-ID: <[email protected]> (raw)

The following bug has been logged on the website:

Bug reference:      19528
Logged by:          Yuelin Wang
Email address:      [email protected]
PostgreSQL version: 19beta1
Operating system:   Linux (Ubuntu 24.04, x86_64)
Description:        

The buffer is allocated at line 2841 with a per-location budget of 10 bytes:

```c
norm_query_buflen = query_len + jstate->clocations_count * 10;
```

The comment explains the budget: a `$n` placeholder is at most 11 bytes and
the original constant is at least 1 byte, so net expansion is at most 10
bytes per location. This holds for non-squashed constants.

For squashed array elements, the sprintf at line 2883 writes `"$N /*, ...
*/"` instead of just `"$N"`:

```c
n_quer_loc += sprintf(norm_query + n_quer_loc, "$%d%s",
                      num_constants_replaced + 1 +
jstate->highest_extern_param_id,
                      locs[i].squashed ? " /*, ... */" : "");
```

`" /*, ... */"` is 11 bytes, so a squashed entry writes `"$N"` (2 bytes)
plus `" /*, ... */"` (11 bytes) = 13 bytes to replace a 1-byte constant, a
net expansion of 12 bytes. The budget of 10 bytes is exceeded by 2 bytes per
squashed location. With N squashed array elements the buffer overflows by 2N
bytes.

Any `ARRAY[a,b]` literal records its second element as a squashed
`clocations` entry. Ten such arrays produce a 20-byte overflow, which is
sufficient to trigger the Assert at line 2908:

```
TRAP: failed Assert("n_quer_loc <= norm_query_buflen"),
File: "pg_stat_statements.c", Line: 2908
```

In a Release build (assertions disabled) the overflow is a true heap write
past the `palloc` allocation, corrupting adjacent allocator metadata.

### Reproduction

`pg_stat_statements` must be listed in `shared_preload_libraries`. The
triggering role holds no special privileges beyond `LOGIN`.

```sql
CREATE EXTENSION IF NOT EXISTS pg_stat_statements;

CREATE ROLE vuln_004_lowpriv LOGIN PASSWORD 'vuln004';
GRANT pg_read_all_stats TO vuln_004_lowpriv;
SET ROLE vuln_004_lowpriv;

SELECT ARRAY[1,2], ARRAY[1,2], ARRAY[1,2], ARRAY[1,2], ARRAY[1,2],
       ARRAY[1,2], ARRAY[1,2], ARRAY[1,2], ARRAY[1,2], ARRAY[1,2];
```

### Observed Output

psql output:

```
SET
server closed the connection unexpectedly
        This probably means the server terminated abnormally
        before or while processing the request.
connection to server was lost
```

Server log (PostgreSQL 19beta1, cassert build, 2026-06-19):

```
TRAP: failed Assert("n_quer_loc <= norm_query_buflen"), File:
"pg_stat_statements.c", Line: 2908, PID: 2671635
postgres: yuelinwang postgres [local]
SELECT(ExceptionalCondition+0x103)[0x5602a92f55b8]
/data/.../pg_stat_statements.so(+0x655b)[0x7d147957e55b]
/data/.../pg_stat_statements.so(+0x9ef7)[0x7d1479581ef7]
/data/.../pg_stat_statements.so(+0xb3ba)[0x7d14795833ba]
postgres: yuelinwang postgres [local]
SELECT(parse_analyze_fixedparams+0x120)[...]
postgres: yuelinwang postgres [local] SELECT(PostgresMain+0x1142)[...]

LOG:  client backend (PID 2671635) was terminated by signal 6: Aborted
DETAIL:  Failed process was running: SELECT ARRAY[1,2], ARRAY[1,2],
ARRAY[1,2], ARRAY[1,2], ARRAY[1,2],
         ARRAY[1,2], ARRAY[1,2], ARRAY[1,2], ARRAY[1,2], ARRAY[1,2];
LOG:  terminating any other active server processes
LOG:  all server processes terminated; reinitializing
LOG:  database system was interrupted; last known up at 2026-06-19 12:34:57
+08
LOG:  database system was not properly shut down; automatic recovery in
progress
LOG:  database system is ready to accept connections
```

### Expected vs Actual

| Step | Expected | Actual |
|---|---|---|
| SELECT with 10 ARRAY[1,2] literals | returns one row of 10 arrays |
backend crashes (SIGABRT signal 6) |
| Server log | nothing | `TRAP: failed Assert("n_quer_loc <=
norm_query_buflen"), File: "pg_stat_statements.c", Line: 2908` |
| Server state | unaffected | crash recovery triggered, postmaster
reinitializes |
| Connection | stays open | `server closed the connection unexpectedly` |

### Fix

Account for the extra 11 bytes appended when a squashed suffix is written.
One correct approach:

```c
norm_query_buflen = query_len + jstate->clocations_count *
    (10 + (jstate->has_squashed_lists ? 11 : 0));
```

A simpler conservative fix that covers the worst case (`"$2147483648 /*, ...
*/"` = 22 bytes replacing a 1-byte constant, net 21 bytes):

```c
norm_query_buflen = query_len + jstate->clocations_count * 22;
```

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