Add __pledge_open regress.

regress/sys/kern/pledge/open/Makefile

@@ -0,0 +1,51 @@
+#	$OpenBSD: Makefile,v 1.1 2026/03/27 05:06:33 dgl Exp $
+
+PROG=	open
+LDFLAGS=	-static
+
+run-no-pledge: ${PROG}
+	./${PROG} "[NO PLEDGE]" /etc/myname
+REGRESS_TARGETS+=	run-no-pledge
+
+run-unknown-file: ${PROG}
+	if ./${PROG} "stdio" /etc/myname; \
+	    then false; else [[ $$? == 134 ]]; fi
+REGRESS_TARGETS+=	run-unknown-file
+
+run-hosts: ${PROG}
+	./${PROG} "stdio dns fattr" /etc/hosts
+REGRESS_TARGETS+=	run-hosts
+
+run-hosts-no-dns: ${PROG}
+	if ./${PROG} "stdio" /etc/hosts; \
+	    then false; else [[ $$? == 134 ]]; fi
+REGRESS_TARGETS+=	run-hosts-no-dns
+
+run-hosts-no-dns-error: ${PROG}
+	if ./${PROG} "stdio error" /etc/hosts; \
+	    then false; else [[ $$? == 2 ]]; fi
+REGRESS_TARGETS+=	run-hosts-no-dns-error
+
+run-hosts-no-dns-but-rpath: ${PROG}
+	./${PROG} "stdio rpath fattr" /etc/hosts
+REGRESS_TARGETS+=	run-hosts-no-dns-but-rpath
+
+run-devnull: ${PROG}
+	./${PROG} "stdio" /dev/null
+REGRESS_TARGETS+=	run-devnull
+
+run-empty-promises: ${PROG}
+	if ./${PROG} "" /dev/null; \
+	    then false; else [[ $$? == 134 ]]; fi
+REGRESS_TARGETS+=	run-empty-promises
+
+run-zoneinfo: ${PROG}
+	./${PROG} "stdio fattr" /usr/share/zoneinfo/Asia/Ulaanbaatar
+REGRESS_TARGETS+=	run-zoneinfo
+
+run-zoneinfo-dotdot: ${PROG}
+	if ./${PROG} "stdio" /usr/share/zoneinfo/Asia/..; \
+	    then false; else [[ $$? == 134 ]]; fi
+REGRESS_TARGETS+=	run-zoneinfo-dotdot
+
+.include <bsd.regress.mk>

regress/sys/kern/pledge/open/open.c

@@ -0,0 +1,60 @@
+/*	$OpenBSD: open.c,v 1.1 2026/03/27 05:06:33 dgl Exp $	*/
+/*
+ * Copyright (c) 2026 David Leadbeater <dgl@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 <sys/stat.h>
+
+#include <err.h>
+#include <fcntl.h>
+#include <string.h>
+#include <unistd.h>
+
+/*
+ * __pledge_open(2) is libc internal. This is for testing kernel semantics
+ * only, do not use this outside of regress.
+ */
+int _libc___pledge_open(char *path, int flags, ...);
+
+#define NO_PLEDGE "[NO PLEDGE]"
+
+int
+main(int argc, char **argv)
+{
+	int fd;
+	char *promise, *path;
+	struct stat sb;
+
+	if (argc != 3)
+		errx(1, "argc: %d", argc);
+
+	promise = argv[1];
+	path = argv[2];
+
+	if (strcmp(promise, NO_PLEDGE) != 0 && pledge(promise, NULL) == -1)
+		err(1, "pledge %s", promise);
+
+	fd = _libc___pledge_open(path, O_RDONLY);
+	if (fd == -1)
+		err(2, "open %s", path);
+
+	if (fstat(fd, &sb) == -1)
+		err(3, "fstat %s", path);
+
+	/* __pledge_open marks fds so certain operations are not allowed. */
+	if (strcmp(promise, NO_PLEDGE) != 0 && S_ISREG(sb.st_mode))
+		if (fchmod(fd, 0) != -1)
+			errx(4, "fchmod succeeded");
+}