tmpfs: test link accounting with O_TMPFILE

Mount tmpfs with "nr_inodes=3" for easy check.

Link: http://lkml.kernel.org/r/20190219215016.GA20084@avx2
Signed-off-by: Alexey Dobriyan <[e-mail]>
Reviewed-by: Andrew Morton <[e-mail]>
Cc: Darrick J. Wong <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Matej Kupljen <[e-mail]>
Cc: Al Viro <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

tools/testing/selftests/Makefile

···
 ifneq (1, $(quicktest))
 TARGETS += timers
 endif
+TARGETS += tmpfs
 TARGETS += user
 TARGETS += vm
 TARGETS += x86

tools/testing/selftests/tmpfs/.gitignore

···
+/bug-link-o-tmpfile

tools/testing/selftests/tmpfs/Makefile

···
+CFLAGS += -Wall -O2
+CFLAGS += -D_GNU_SOURCE
+
+TEST_GEN_PROGS :=
+TEST_GEN_PROGS += bug-link-o-tmpfile
+
+include ../lib.mk

tools/testing/selftests/tmpfs/bug-link-o-tmpfile.c

···
+/*
+ * Copyright (c) 2019 Alexey Dobriyan <[e-mail]>
+ *
+ * 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.
+ */
+/* Test that open(O_TMPFILE), linkat() doesn't screw accounting. */
+#include <errno.h>
+#include <sched.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <sys/mount.h>
+#include <unistd.h>
+
+int main(void)
+{
+	int fd;
+
+	if (unshare(CLONE_NEWNS) == -1) {
+		if (errno == ENOSYS || errno == EPERM) {
+			fprintf(stderr, "error: unshare, errno %d\n", errno);
+			return 4;
+		}
+		fprintf(stderr, "error: unshare, errno %d\n", errno);
+		return 1;
+	}
+	if (mount(NULL, "/", NULL, MS_PRIVATE|MS_REC, NULL) == -1) {
+		fprintf(stderr, "error: mount '/', errno %d\n", errno);
+		return 1;
+	}
+
+	/* Our heroes: 1 root inode, 1 O_TMPFILE inode, 1 permanent inode. */
+	if (mount(NULL, "/tmp", "tmpfs", 0, "nr_inodes=3") == -1) {
+		fprintf(stderr, "error: mount tmpfs, errno %d\n", errno);
+		return 1;
+	}
+
+	fd = openat(AT_FDCWD, "/tmp", O_WRONLY|O_TMPFILE, 0600);
+	if (fd == -1) {
+		fprintf(stderr, "error: open 1, errno %d\n", errno);
+		return 1;
+	}
+	if (linkat(fd, "", AT_FDCWD, "/tmp/1", AT_EMPTY_PATH) == -1) {
+		fprintf(stderr, "error: linkat, errno %d\n", errno);
+		return 1;
+	}
+	close(fd);
+
+	fd = openat(AT_FDCWD, "/tmp", O_WRONLY|O_TMPFILE, 0600);
+	if (fd == -1) {
+		fprintf(stderr, "error: open 2, errno %d\n", errno);
+		return 1;
+	}
+
+	return 0;
+}