From 28d8cf0bbfe2d8e27749a1453bd2771c8e41c31b Mon Sep 17 00:00:00 2001
From: guenther <>
Date: Mon, 19 Jan 2026 23:01:00 +0000
Subject: While almost all the libc locks are taken and released in the same
 libc call, flockfile() and ftrylockfile() can be called when single-threaded
 and then--while 'holding' the lock--the process can create another thread,
 resulting in a broken state.  Have the f{lock,trylock,unlock}file() APIs
 *always* do real locking so the exposed state is always consistent.

ok dlg@
---
 src/regress/lib/libc/stdio_threading/Makefile      |  2 +-
 .../lib/libc/stdio_threading/flockfile/Makefile    |  6 ++
 .../stdio_threading/flockfile/flockfile_test.c     | 78 ++++++++++++++++++++++
 3 files changed, 85 insertions(+), 1 deletion(-)
 create mode 100644 src/regress/lib/libc/stdio_threading/flockfile/Makefile
 create mode 100755 src/regress/lib/libc/stdio_threading/flockfile/flockfile_test.c

(limited to 'src/regress/lib/libc')

diff --git a/src/regress/lib/libc/stdio_threading/Makefile b/src/regress/lib/libc/stdio_threading/Makefile
index e42481afc2..5823b6e284 100644
--- a/src/regress/lib/libc/stdio_threading/Makefile
+++ b/src/regress/lib/libc/stdio_threading/Makefile
@@ -1,3 +1,3 @@
-SUBDIR += fopen fread fwrite fgetln fgets fputs
+SUBDIR += fopen fread fwrite fgetln fgets fputs flockfile
 
 .include <bsd.subdir.mk>
diff --git a/src/regress/lib/libc/stdio_threading/flockfile/Makefile b/src/regress/lib/libc/stdio_threading/flockfile/Makefile
new file mode 100644
index 0000000000..171ccc7ac7
--- /dev/null
+++ b/src/regress/lib/libc/stdio_threading/flockfile/Makefile
@@ -0,0 +1,6 @@
+TOPDIR=${.CURDIR}
+PROG=flockfile_test
+CFLAGS+=-I${TOPDIR}/../include/
+LDFLAGS+=-lpthread
+
+.include <bsd.regress.mk>
diff --git a/src/regress/lib/libc/stdio_threading/flockfile/flockfile_test.c b/src/regress/lib/libc/stdio_threading/flockfile/flockfile_test.c
new file mode 100755
index 0000000000..034a753cd9
--- /dev/null
+++ b/src/regress/lib/libc/stdio_threading/flockfile/flockfile_test.c
@@ -0,0 +1,78 @@
+/*	$OpenBSD: flockfile_test.c,v 1.1 2026/01/19 23:01:00 guenther Exp $	*/
+/*
+ * Copyright (c) 2026 Philip Guenther <guenther@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <stdio.h>
+#include <pthread.h>
+#include "local.h"
+
+pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
+pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
+int did_test1 = 0;
+
+static void *
+test1(void *v)
+{
+	int r;
+
+	if (ftrylockfile(stdin) == 0)
+		errx(1, "Lock on stdin never held");
+
+	if ((r = pthread_mutex_lock(&lock)))
+		errc(1, r, "could not lock lock");
+	did_test1 = 1;
+	if ((r = pthread_cond_signal(&cond)))
+		errc(1, r, "could not signal cond");
+	if ((r = pthread_mutex_unlock(&lock)))
+		errc(1, r, "could not unlock lock");
+
+	flockfile(stdin);
+	funlockfile(stdin);
+	return NULL;
+}
+
+int
+main(void)
+{
+	int fd, r;
+	pthread_t thr;
+
+	/*
+	 * Make sure flockfile() is handled correctly before the first
+	 * thread is created.
+	 */
+	flockfile(stdin);
+	flockfile(stdin);
+
+	if ((r = pthread_create(&thr, NULL, test1, NULL)))
+		warnc(r, "could not create thread");
+
+	if ((r = pthread_mutex_lock(&lock)))
+		errc(1, r, "could not lock lock");
+	while (did_test1 == 0)
+		if ((r = pthread_cond_wait(&cond, &lock)))
+			errc(1, r, "could not wait on cond");
+	if ((r = pthread_mutex_unlock(&lock)))
+		errc(1, r, "could not unlock lock");
+
+	funlockfile(stdin);
+	funlockfile(stdin);
+
+	if ((r = pthread_join(thr, NULL)))
+		warnc(r, "could not join thread");
+
+	exit(0);
+}
-- 
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