MIME-Version: 1.0
From: Josh Kupershmidt <schmiddy@gmail.com>
Date: Mon, 6 Apr 2026 10:57:56 -0400
Message-ID: 
 <CAK3UJREEiWR4W_8j=5oGn8d6yvu-2xBKSSPcEHg2giWUHrnSeQ@mail.gmail.com>
Subject: Adding event mask validation for ModifyWaitEvent
To: pgsql-hackers <pgsql-hackers@postgresql.org>
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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-6e542ba2eb1d83ef90e65cdc0912b51a295184701c7e3bd236937c43c4cac4b9R704
[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

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<div dir=3D"ltr"><div dir=3D"ltr">Hi,<div><br></div><div>Please find attach=
ed a patch implementing an old FIXME comment [1] about=C2=A0validating the =
event mask in=C2=A0ModifyWaitEvent(). To prevent callers of=C2=A0ModifyWait=
Event() from passing invalid flags that can be silently accepted, I propose=
 adding two checks:</div><div><br></div><div>=C2=A01. Prevent setting socke=
t wait flags on an event that has no socket, mirroring line 600 of=C2=A0Add=
WaitEventToSet() [2].</div><div><br></div><div>=C2=A02. Prevent promoting a=
 non-latch event to a latch event through=C2=A0ModifyWaitEvent(). The setup=
 for latch events is handled in AddWaitEventToSet(), but not supported in=
=C2=A0ModifyWaitEvent(). For example, AddWaitEventToSet() enforces &quot;ca=
nnot wait on more than one latch&quot; [3] per set, registering the latch p=
ointer [4], and handles platform-specific latch behavior [5]. We do still a=
llow 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=
.</div><div><br></div><div>[1] The commit adding this code dates back to 20=
16, added in <a href=3D"https://github.com/postgres/postgres/commit/98a64d0=
bd71#diff-6e542ba2eb1d83ef90e65cdc0912b51a295184701c7e3bd236937c43c4cac4b9R=
704" target=3D"_blank">https://github.com/postgres/postgres/commit/98a64d0b=
d71#diff-6e542ba2eb1d83ef90e65cdc0912b51a295184701c7e3bd236937c43c4cac4b9R7=
04</a></div><div>[2]=C2=A0 <a href=3D"https://github.com/postgres/postgres/=
blob/master/src/backend/storage/ipc/waiteventset.c#L600" target=3D"_blank">=
https://github.com/postgres/postgres/blob/master/src/backend/storage/ipc/wa=
iteventset.c#L600</a></div><div>[3]=C2=A0<a href=3D"https://github.com/post=
gres/postgres/blob/master/src/backend/storage/ipc/waiteventset.c#L588-L589"=
 target=3D"_blank">https://github.com/postgres/postgres/blob/master/src/bac=
kend/storage/ipc/waiteventset.c#L588-L589</a></div><div>[4]=C2=A0<a href=3D=
"https://github.com/postgres/postgres/blob/master/src/backend/storage/ipc/w=
aiteventset.c#L614-L615" target=3D"_blank">https://github.com/postgres/post=
gres/blob/master/src/backend/storage/ipc/waiteventset.c#L614-L615</a></div>=
<div>[5]=C2=A0<a href=3D"https://github.com/postgres/postgres/blob/master/s=
rc/backend/storage/ipc/waiteventset.c#L616-L619" target=3D"_blank">https://=
github.com/postgres/postgres/blob/master/src/backend/storage/ipc/waitevents=
et.c#L616-L619</a></div><div>[6]=C2=A0<a href=3D"https://github.com/postgre=
s/postgres/blob/master/src/backend/storage/ipc/waiteventset.c#L650-L651" ta=
rget=3D"_blank">https://github.com/postgres/postgres/blob/master/src/backen=
d/storage/ipc/waiteventset.c#L650-L651</a></div><div><br></div></div>
</div>

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