Re: BUG #19476: Segmentation fault in contrib/spi

Hi,

On Tue, 12 May 2026 at 21:14, PG Bug reporting form <noreply(at)postgresql(dot)org>
wrote:

>
> CREATE EXTENSION refint;
> CREATE TABLE c (id int4);
> CREATE UNIQUE INDEX ci ON c(id);
> CREATE TABLE b (refb int4);
> CREATE INDEX bi ON b(refb);
>
> CREATE TRIGGER at AFTER DELETE OR UPDATE ON c FOR EACH ROW
> EXECUTE FUNCTION check_foreign_key (1, 'cascade', 'id', 'b', 'refb');
>
> CREATE TRIGGER bt AFTER INSERT OR UPDATE ON b FOR EACH ROW
> EXECUTE FUNCTION check_primary_key ('refb', 'c', 'id');
>
> INSERT INTO c VALUES (10);
> INSERT INTO b VALUES (10);
> UPDATE c SET id = NULL WHERE id = 10;
>
> Backtrace:
> #0 __strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:76
> #1 0x0000563a92ca096e in quote_literal_cstr (rawstr=0x0) at quote.c:109
> #2 0x00007fa4a7cce5f8 in check_foreign_key (fcinfo=<optimized out>) at
> refint.c:489
> #3 0x0000563a929c1d52 in ExecCallTriggerFunc
> (trigdata=trigdata(at)entry=0x7fff8cae1100,
> tgindx=tgindx(at)entry=0, finfo=finfo(at)entry=0x563aa5ae4438,
> instr=instr(at)entry=0x0,
> per_tuple_context=per_tuple_context(at)entry=0x563aa5adb9c0) at
> trigger.c:2369
> #4 0x0000563a929c4110 in AfterTriggerExecute (estate=<optimized out>,
> event=0x563aa5aedbb0,
> relInfo=0x563aa5ae4068, src_relInfo=<optimized out>,
> dst_relInfo=<optimized out>,
> trigdesc=0x563aa5ae4278, finfo=0x563aa5ae4438, instr=0x0,
> per_tuple_context=<optimized out>, trig_tuple_slot1=0x0,
> trig_tuple_slot2=0x0)
> at trigger.c:4559
> #5 afterTriggerInvokeEvents (events=events(at)entry=0x563aa5a75050,
> firing_id=1,
> estate=estate(at)entry=0x563aa5ae3b20, delete_ok=delete_ok(at)entry=false)
> at
> trigger.c:4800
> #6 0x0000563a929c8e88 in AfterTriggerEndQuery
> (estate=estate(at)entry=0x563aa5ae3b20)
> at trigger.c:5182
> #7 0x0000563a929ed504 in standard_ExecutorFinish
> (queryDesc=0x563aa5996e20)
> at execMain.c:443
> #8 0x0000563a92bd96f8 in ProcessQuery (plan=0x563aa5af4168,
> sourceText=0x563aa59df7e0 "UPDATE c SET id = NULL WHERE id = 10;",
> params=0x0,
> queryEnv=0x0, dest=0x563aa5aeae28, qc=0x7fff8cae1460) at pquery.c:194
> #9 0x0000563a92bda41e in PortalRunMulti
> (portal=portal(at)entry=0x563aa5a60510,
> isTopLevel=isTopLevel(at)entry=true,
> setHoldSnapshot=setHoldSnapshot(at)entry=false,
> --Type <RET> for more, q to quit, c to continue without paging--c
> dest=dest(at)entry=0x563aa5aeae28, altdest=altdest(at)entry=0x563aa5aeae28,
> qc=qc(at)entry=0x7fff8cae1460) at pquery.c:1272
> #10 0x0000563a92bda7f7 in PortalRun (portal=portal(at)entry=0x563aa5a60510,
> count=count(at)entry=9223372036854775807, isTopLevel=isTopLevel(at)entry
> =true,
> dest=dest(at)entry=0x563aa5aeae28, altdest=altdest(at)entry=0x563aa5aeae28,
> qc=qc(at)entry=0x7fff8cae1460) at pquery.c:788
> #11 0x0000563a92bd63fa in exec_simple_query (
> query_string=0x563aa59df7e0 "UPDATE c SET id = NULL WHERE id = 10;") at
> postgres.c:1274
> #12 0x0000563a92bd7fc5 in PostgresMain (dbname=<optimized out>,
> username=<optimized out>)
> at postgres.c:4770
> #13 0x0000563a92bd23bd in BackendMain (startup_data=<optimized out>,
> startup_data_len=<optimized out>) at backend_startup.c:124
> #14 0x0000563a92b24932 in postmaster_child_launch (child_type=<optimized
> out>, child_slot=1,
> startup_data=startup_data(at)entry=0x7fff8cae1900,
> startup_data_len=startup_data_len(at)entry=24,
> client_sock=client_sock(at)entry=0x7fff8cae1920)
> at launch_backend.c:290
> #15 0x0000563a92b283d2 in BackendStartup (client_sock=0x7fff8cae1920) at
> postmaster.c:3569
> #16 ServerLoop () at postmaster.c:1703
> #17 0x0000563a92b29ea6 in PostmasterMain (argc=argc(at)entry=3,
> argv=argv(at)entry=0x563aa5985bf0)
> at postmaster.c:1401
> #18 0x0000563a92800a5a in main (argc=3, argv=0x563aa5985bf0) at main.c:227
>

Thanks for the report and the clear backtrace.

I had a look at this and I think the immediate crash comes from
check_foreign_key()'s cascade-update branch in contrib/spi/refint.c,
which calls SPI_getvalue() for the new key value and then passes the
result straight into snprintf("%s", ...):

nv = SPI_getvalue(newtuple, tupdesc, fn);
...
snprintf(sql + strlen(sql), sizeof(sql) - strlen(sql),
" %s = %s%s%s %s ",
args2[k], (is_char_type > 0) ? "'" : "",
nv, (is_char_type > 0) ? "'" : "", (k < nkeys) ? ", " : "");

For an UPDATE that sets the referenced key to NULL, SPI_getvalue()
returns NULL, so nv is NULL. On stock glibc/Postgres builds snprintf
substitutes "(null)" and SQL parses that as the NULL keyword, so the
cascade silently does the right thing. On builds with _FORTIFY_SOURCE
glibc's strlen check dereferences the NULL pointer instead, which
matches the backtrace you posted.

I am attaching a two-patch series that addresses this.

0001 is the minimal fix: emit the SQL NULL keyword when SPI_getvalue()
returns NULL, instead of passing a NULL pointer to snprintf. This is
small and targeted, and is sufficient on its own to fix the reported
crash. I did not add a regression test for 0001 because the bug only
manifests on hardened libc builds; on stock builds the bad snprintf
happens to produce a result that the SQL parser accepts.

0002 is a follow-up that I think makes sense but is not strictly
required for this bug. While looking at the cascade-update path I
noticed two pre-existing issues in the same code:

- The new key values were embedded into the prepared cascade UPDATE
query text as literals. Combined with plan caching (the cache key
only gained the operation type in 8cfbdf8f4df), the first set of
new values gets reused for every subsequent cascade UPDATE.

- Char-like new values were wrapped in single quotes without escaping,
so a value containing a single quote produced malformed SQL.

0002 switches that branch to use SPI parameters for the new key values
instead of embedding them into the SQL text. That naturally handles
NULLs via the SPI nulls array, lets a single cached plan handle later
UPDATEs with different keys, and avoids the quoting issue entirely.
It also adds a refint regression test that runs two cascade UPDATEs in
a row (one to NULL, one to a non-NULL value) which catches the cached-
plan reuse.

I'm happy with just 0001 if reviewers prefer to keep the fix minimal,
given that refint is intended mainly as a documentation example
(8cfbdf8f4df explicitly mentioned not back-patching changes here?).

Regards,
Ayush