Add unshare03 test

Add a test case based on kernel self-test unshare_test.c to check that
the kernel handles the EMFILE error when a parent process changes file
descriptor limits and the child process tries to unshare (CLONE_FILES).

Reviewed-by: Andrea Cervesato <andrea.cervesato@suse.com>
Signed-off-by: Wei Gao <wegao@suse.com>

Author: Wei Gao via ltp

runtest/syscalls

@@ -1718,6 +1718,7 @@ unlinkat01 unlinkat01
 
 unshare01 unshare01
 unshare02 unshare02
+unshare03 unshare03
 
 #
 # These tests require an unmounted block device

testcases/kernel/syscalls/unshare/.gitignore

@@ -1,2 +1,3 @@
 /unshare01
 /unshare02
+/unshare03

testcases/kernel/syscalls/unshare/unshare03.c

@@ -0,0 +1,78 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2024 Al Viro <viro@zeniv.linux.org.uk>
+ * Copyright (C) 2024 Wei Gao <wegao@suse.com>
+ */
+
+/*\
+ * This test case based on kernel self-test unshare_test.c to check that
+ * the kernel handles the EMFILE error when a parent process changes file
+ * descriptor limits and the child process tries to unshare (CLONE_FILES).
+ */
+
+#define _GNU_SOURCE
+
+#include "tst_test.h"
+#include "config.h"
+#include "lapi/sched.h"
+
+#define FS_NR_OPEN "/proc/sys/fs/nr_open"
+#define NR_OPEN_LIMIT 1024
+#define NR_OPEN_DUP 64
+
+#ifdef HAVE_UNSHARE
+
+static void run(void)
+{
+	int nr_open;
+	int nr_limit;
+	struct rlimit rlimit;
+	struct tst_clone_args args = {
+		.flags = CLONE_FILES,
+		.exit_signal = SIGCHLD,
+	};
+
+	SAFE_FILE_SCANF(FS_NR_OPEN, "%d", &nr_open);
+	tst_res(TDEBUG, "Maximum number of file descriptors: %d", nr_open);
+
+	nr_limit = nr_open + NR_OPEN_LIMIT;
+	SAFE_FILE_PRINTF(FS_NR_OPEN, "%d", nr_limit);
+
+	SAFE_GETRLIMIT(RLIMIT_NOFILE, &rlimit);
+
+	rlimit.rlim_cur = nr_limit;
+	rlimit.rlim_max = nr_limit;
+
+	SAFE_SETRLIMIT(RLIMIT_NOFILE, &rlimit);
+	tst_res(TDEBUG, "Set new maximum number of file descriptors to : %d",
+		nr_limit);
+
+	SAFE_DUP2(2, nr_open + NR_OPEN_DUP);
+
+	if (!SAFE_CLONE(&args)) {
+		SAFE_FILE_PRINTF(FS_NR_OPEN, "%d", nr_open);
+		TST_EXP_FAIL(unshare(CLONE_FILES), EMFILE);
+		exit(0);
+	}
+
+}
+
+static void setup(void)
+{
+	clone3_supported_by_kernel();
+}
+
+static struct tst_test test = {
+	.forks_child = 1,
+	.needs_root = 1,
+	.test_all = run,
+	.setup = setup,
+	.save_restore = (const struct tst_path_val[]) {
+		{FS_NR_OPEN, NULL, TST_SR_TCONF},
+		{}
+	},
+};
+
+#else
+TST_TEST_TCONF("unshare syscall is undefined.");
+#endif