Thread: BUG #18669: pg_get_shmem_allocations show wrong memory for 11 structures

The following bug has been logged on the website:

Bug reference:      18669
Logged by:          Oleg Ivanov
Email address:      o15611@gmail.com
PostgreSQL version: 17.0
Operating system:   Linux
Description:

pg_shmem_allocations hide real sizes for 11 structures, mainly hash
tables.

select * from (select *, lead(off) over(order by off) - off as diff from
pg_shmem_allocations) as a where a.diff<>a.allocated_size order by 5 desc;

                      name                      |    off    | size |
allocated_size |  diff   
------------------------------------------------+-----------+------+----------------+---------
 PREDICATELOCK hash                             | 144359936 | 2896 |
  2944 | 1260416
 Shared Buffer Lookup Table                     | 141591296 | 2896 |
  2944 |  928640
 LOCK hash                                      | 142520064 | 2896 |
  2944 |  695168
 PROCLOCK hash                                  | 143215232 | 2896 |
  2944 |  695168
 PREDICATELOCKTARGET hash                       | 143914624 | 2896 |
  2944 |  445312
 RWConflictPool                                 | 145883904 |   24 |
   128 |  292992
 PredXactList                                   | 145620352 |   88 |
   128 |  205184
 Proc Header                                    | 146444672 |  136 |
   256 |  114944
 SERIALIZABLEXID hash                           | 145825536 | 2896 |
  2944 |   58368
 WaitEventCustom hash by wait event information | 147903488 | 2896 |
  2944 |    6400
 WaitEventCustom hash by name                   | 147909888 | 2896 |
  2944 |    6400
(11 rows)

diff is the real size. "Shared Buffer Lookup Table" (Buffer Mapping Hash)
can not be 2944 byte for 128

Robert Haas shown similar idea to check the correct sizes, but used wrong
query, so hiddnig the real sizes was not obviouse.
https://www.postgresql.org/message-id/CA%2BTgmoazhJnk25jKPXtRPxNddM8ZxDdJ7-OP5wiWSBsXJ2AWBg%40mail.gmail.com

select *, off - lag(off + allocated_size, 1) over () as hole_size from
(select * from pg_shmem_allocations order by 2) x order by 5 desc limit
14;
                name                 |    off    |  size   | allocated_size
| hole_size 
-------------------------------------+-----------+---------+----------------+-----------
 DSM Registry Data                   |     54656 |      16 |            128
|          
 <anonymous>                         |           | 4739584 |        4739584
|          
 PredXactList                        | 145620352 |      88 |            128
|   1257472
 Buffer Strategy Status              | 142519936 |      28 |            128
|    925696
 Fast Path Strong Relation Lock Data | 143910400 |    4100 |           4224
|    692224
 PROCLOCK hash                       | 143215232 |    2896 |           2944
|    692224
 PREDICATELOCK hash                  | 144359936 |    2896 |           2944
|    442368
 FinishedSerializableTransactions    | 146176896 |      16 |            128
|    292864
 SERIALIZABLEXID hash                | 145825536 |    2896 |           2944
|    205056
 Proc Array                          | 146559616 |     524 |            640
|    114688
 RWConflictPool                      | 145883904 |      24 |            128
|     55424
 WaitEventCustom hash by name        | 147909888 |    2896 |           2944
|      3456
                                     | 147916288 | 1939968 |        1939968
|      3456
 Buffer Descriptors                  |   5731072 | 1048576 |        1048576
|         0
(14 rows)

For checking:

select * from pg_shmem_allocations order by off; 
                      name                      |    off    |   size    |
allocated_size 
------------------------------------------------+-----------+-----------+----------------
...
 Checkpoint BufferIds                           | 141263616 |    327680 |
     327680
 Shared Buffer Lookup Table                     | 141591296 |      2896 |
       2944
 Buffer Strategy Status                         | 142519936 |        28 |
        128
...

142519936-141591296=928640 - lie: not 2944
141591296-141263616=327680 - true.

Hidding memory can be a security hole wich may be used to inject a delayed
multi-step vulnerability.
I suggest to calculate and show the real sizes.