Add o_tmpfile_linkat test binary

Add a test binary that uses o_tmpfile and linkat to create a file in a
given dir.  We have something similar, but it's weirdly specific to a
given test.  This is a simpler building block that could be used by more
tests.

Signed-off-by: Zach Brown <zab@versity.com>

tests/.gitignore

@@ -9,3 +9,4 @@ src/find_xattrs
 src/stage_tmpfile
 src/create_xattr_loop
 src/o_tmpfile_umask
+src/o_tmpfile_linkat

tests/Makefile

@@ -12,7 +12,8 @@ BIN := src/createmany			\
 	src/find_xattrs			\
 	src/create_xattr_loop		\
 	src/fragmented_data_extents	\
-	src/o_tmpfile_umask
+	src/o_tmpfile_umask		\
+	src/o_tmpfile_linkat
 
 DEPS := $(wildcard src/*.d)
 

tests/src/o_tmpfile_linkat.c

@@ -0,0 +1,71 @@
+/*
+ * Copyright (C) 2023 Versity Software, Inc.  All rights reserved.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License v2 as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ */
+
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <sys/stat.h>
+#include <assert.h>
+#include <limits.h>
+
+static void linkat_tmpfile(char *dir, char *lpath)
+{
+	char proc_self[PATH_MAX];
+	int ret;
+	int fd;
+
+	fd = open(dir, O_RDWR | O_TMPFILE, 0777);
+	if (fd < 0) {
+		perror("open(O_TMPFILE)");
+		exit(1);
+	}
+
+	snprintf(proc_self, sizeof(proc_self), "/proc/self/fd/%d", fd);
+
+	ret = linkat(AT_FDCWD, proc_self, AT_FDCWD, lpath, AT_SYMLINK_FOLLOW);
+	if (ret < 0) {
+		perror("linkat");
+		exit(1);
+	}
+
+	close(fd);
+}
+
+/*
+ * Use O_TMPFILE and linkat to create a new visible file, used to test
+ * the O_TMPFILE creation path by inspecting the created file.
+ */
+int main(int argc, char **argv)
+{
+	char *lpath;
+	char *dir;
+
+	if (argc < 3) {
+		printf("%s <open_dir> <linkat_path>\n", argv[0]);
+		return 1;
+	}
+
+	dir = argv[1];
+	lpath = argv[2];
+
+	linkat_tmpfile(dir, lpath);
+
+	return 0;
+}