Re: Fix segfault while accessing half-initialized hash table in pgstat_shmem.c

Em qua., 3 de set. de 2025 às 03:34, Mikhail Kot <mikhail.kot@databricks.com>
escreveu:

> Hi,
>
> I've encountered the following segmentation fault lately. It happens when
> Postgres is experiencing high memory pressure. There are multiple OOM
> errors in
> the log as well.
>
> Core was generated by `postgres: neondb_owner neondb ::1(46658) BIND
>           '.
> Program terminated with signal SIGSEGV, Segmentation fault.
> #0  pg_atomic_read_u32_impl (ptr=0x8) at
> ../../../../src/include/port/atomics/generic.h:48
> #1  pg_atomic_read_u32 (ptr=0x8) at
> ../../../../src/include/port/atomics.h:239
> #2  LWLockAttemptLock (lock=lock@entry=0x4,
> mode=mode@entry=LW_EXCLUSIVE) at lwlock.c:821
> #3  0x000056446bce129f in LWLockConditionalAcquire (lock=0x4,
> mode=mode@entry=LW_EXCLUSIVE) at lwlock.c:1386
> #4  0x000056446bd0bacf in pgstat_lock_entry
> (entry_ref=entry_ref@entry=0x56446d9f4340, nowait=nowait@entry=true)
> at pgstat_shmem.c:625
> #5  0x000056446bd0a3c9 in pgstat_relation_flush_cb
> (entry_ref=0x56446d9f4340, nowait=<optimized out>) at
> pgstat_relation.c:794
> #6  0x000056446bd069f5 in pgstat_flush_pending_entries
> (nowait=<optimized out>) at pgstat.c:1217
> #7  pgstat_report_stat (force=<optimized out>, force@entry=false) at
> pgstat.c:658
> #8  0x000056446bcf16c1 in PostgresMain (dbname=<optimized out>,
> username=<optimized out>) at postgres.c:4623
> #9  0x000056446bc716b3 in BackendRun (port=<optimized out>,
> port=<optimized out>) at postmaster.c:4465
> #10 BackendStartup (port=<optimized out>) at postmaster.c:4193
> #11 ServerLoop () at postmaster.c:1782
> #12 0x000056446bc726ea in PostmasterMain (argc=argc@entry=3,
> argv=argv@entry=0x56446cd803b0) at postmaster.c:1466
> #13 0x000056446b9d5a00 in main (argc=3, argv=0x56446cd803b0) at main.c:238
>
> The error originates from pgstat_shmem.c file where shhashent is left in
> half-initialized state if pgstat_init_entry(), calling dsa_allocate0(),
> errors
> out with OOM. Then shhashent causes a segmentation fault on access. I
> propose a
> patch which solves this issue. The patch is for main branch, but the code
> is
> nearly identical in Postgres 13-17 so I suggest backporting it to other
> supported versions.
>
> The patch changes pgstat_init_entry()'s behaviour, returning NULL if memory
> allocation failed.

I'm wondering if it wouldn't be better to raise elog(ERROR), and avoid
many checks for this NULL.

best regards,
Ranier Vilela