NetBSD Problem Report #60358

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Date: Tue, 23 Jun 2026 17:34:05 +0000 (UTC)
From: campbell+netbsd@mumble.net
Reply-To: campbell+netbsd@mumble.net
To: gnats-bugs@NetBSD.org
Subject: kevent EVFILT_PROC races with process exit
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X-From4GNATS: "campbell+netbsd@mumble.net via gnats" <gnats-admin@NetBSD.org>

>Number:         60358
>Category:       kern
>Synopsis:       kevent EVFILT_PROC races with process exit
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    thorpej
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Jun 23 17:35:00 +0000 2026
>Last-Modified:  Tue Jun 23 18:00:02 +0000 2026
>Originator:     Taylor R Campbell
>Release:        current, 11/10/9/...(?)
>Organization:
The WaitBSD Foundevent, Inc.
>Environment:
>Description:

	kevent EVFILT_PROC (with NOTE_EXIT) fails if the process has
	already exited, even if it is guaranteed to exist and to be
	usable with wait() because it is a zombie child of the current
	process.

	This is the root cause of mysterious diff3(1) test failures:

	https://gnats.NetBSD.org/60357
	bin/60357: diff3(1): tests are failing since BSD diff import

>How-To-Repeat:
#include <sys/event.h>
#include <sys/wait.h>

#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(void)
{
	struct kevent ev;
	int kq;
	pid_t child;
	int nev;

	if ((kq = kqueue()) == -1)
		err(EXIT_FAILURE, "kqueue");

	if ((child = fork()) == -1)
		err(EXIT_FAILURE, "fork");
	if (child == 0)
		_exit(1);
	sleep(1);
	EV_SET(&ev, child, EVFILT_PROC, EV_ADD, NOTE_EXIT, 0, NULL);
	if (kevent(kq, &ev, 1, NULL, 0, NULL) == -1)
		err(EXIT_FAILURE, "kevent add");
	nev = kevent(kq, NULL, 0, &ev, 1, NULL);
	if (nev == -1)
		err(EXIT_FAILURE, "kevent wait");
	if (nev == 0)
		errx(EXIT_FAILURE, "kevent failed to wait");
}

>Fix:

	Allow EVFILT_PROC to be registered on zombie children of the
	current process, in which case NOTE_EXIT should fire
	immediately.

>Release-Note:

>Audit-Trail:

Responsible-Changed-From-To: kern-bug-people->thorpej
Responsible-Changed-By: riastradh@NetBSD.org
Responsible-Changed-When: Tue, 23 Jun 2026 17:38:15 +0000
Responsible-Changed-Why:
You made the latest change to kern_event.c replacing proc_find_raw
(which can return zombies) by proc_find, can you take a look?

https://mail-index.netbsd.org/source-changes/2021/10/10/msg132905.html


From: "Taylor R Campbell" <riastradh@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/60358 CVS commit: src/usr.bin
Date: Tue, 23 Jun 2026 17:57:14 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Tue Jun 23 17:57:14 UTC 2026

 Modified Files:
 	src/usr.bin/diff: pr.c pr.h
 	src/usr.bin/diff3: diff3.c

 Log Message:
 diff3(1): Just use waitpid(2), no need for kevent(2).

 Sidesteps

 PR kern/60358: kevent EVFILT_PROC races with process exit

 in order to fix

 PR bin/60357: diff3(1): tests are failing since BSD diff import

 The FreeBSD version used kevent(2) until February of this year:
 https://cgit.freebsd.org/src/commit/?id=404753664a5e145d98f1749d1c7bc046c8aa32c3
 So this reduces diff from FreeBSD somewhat, except FreeBSD uses
 capsicum and process fds rather than pids.


 To generate a diff of this commit:
 cvs rdiff -u -r1.1 -r1.2 src/usr.bin/diff/pr.c src/usr.bin/diff/pr.h
 cvs rdiff -u -r1.1 -r1.2 src/usr.bin/diff3/diff3.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

>Unformatted:

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