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From: Josh Kupershmidt <[email protected]>
To: pgsql-hackers <[email protected]>
Subject: Adding event mask validation for ModifyWaitEvent
Date: Mon, 6 Apr 2026 10:57:56 -0400
Message-ID: <CAK3UJREEiWR4W_8j=5oGn8d6yvu-2xBKSSPcEHg2giWUHrnSeQ@mail.gmail.com> (raw)

Hi,

Please find attached a patch implementing an old FIXME comment [1]
about validating the event mask in ModifyWaitEvent(). To prevent callers
of ModifyWaitEvent() from passing invalid flags that can be silently
accepted, I propose adding two checks:

 1. Prevent setting socket wait flags on an event that has no socket,
mirroring line 600 of AddWaitEventToSet() [2].

 2. Prevent promoting a non-latch event to a latch event
through ModifyWaitEvent(). The setup for latch events is handled in
AddWaitEventToSet(), but not supported in ModifyWaitEvent(). For example,
AddWaitEventToSet() enforces "cannot wait on more than one latch" [3] per
set, registering the latch pointer [4], and handles platform-specific latch
behavior [5]. We do still allow the behavior documented in the comment for
ModifyWaitEvent() [6] about setting a latch to NULL to disable it, and
enabling again as a latch later.

[1] The commit adding this code dates back to 2016, added in
https://github.com/postgres/postgres/commit/98a64d0bd71#diff-6e542ba2eb1d83ef90e65cdc0912b51a2951847...
[2]
https://github.com/postgres/postgres/blob/master/src/backend/storage/ipc/waiteventset.c#L600
[3]
https://github.com/postgres/postgres/blob/master/src/backend/storage/ipc/waiteventset.c#L588-L589
[4]
https://github.com/postgres/postgres/blob/master/src/backend/storage/ipc/waiteventset.c#L614-L615
[5]
https://github.com/postgres/postgres/blob/master/src/backend/storage/ipc/waiteventset.c#L616-L619
[6]
https://github.com/postgres/postgres/blob/master/src/backend/storage/ipc/waiteventset.c#L650-L651


Attachments:

  [application/x-patch] event-mask-validation-for-ModifyWaitEvent.patch (810B, 3-event-mask-validation-for-ModifyWaitEvent.patch)
  download | inline diff:
diff --git a/src/backend/storage/ipc/waiteventset.c b/src/backend/storage/ipc/waiteventset.c
index 0f228e1e7b8..098cff06a5d 100644
--- a/src/backend/storage/ipc/waiteventset.c
+++ b/src/backend/storage/ipc/waiteventset.c
@@ -695,7 +695,12 @@ ModifyWaitEvent(WaitEventSet *set, int pos, uint32 events, Latch *latch)
 	if (event->events & WL_LATCH_SET && events != event->events)
 		elog(ERROR, "cannot modify latch event");
 
-	/* FIXME: validate event mask */
+	/* Validate event mask */
+	if ((events & WL_SOCKET_MASK) && event->fd == PGINVALID_SOCKET)
+		elog(ERROR, "cannot wait on socket event without a socket");
+	if ((events & WL_LATCH_SET) && !(event->events & WL_LATCH_SET))
+		elog(ERROR, "cannot modify non-latch event to wait on latch");
+
 	event->events = events;
 
 	if (events == WL_LATCH_SET)

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  Subject: Re: Adding event mask validation for ModifyWaitEvent
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