Linux: Implement FS_IOC_GETVERSION

Provide access to file generation number on Linux.

Add test coverage.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #12856

tests/zfs-tests/cmd/Makefile.am

@@ -32,6 +32,7 @@ SUBDIRS = \
 
 if BUILD_LINUX
 SUBDIRS += \
+	getversion \
 	randfree_file \
 	user_ns_exec \
 	xattrtest

tests/zfs-tests/cmd/getversion/.gitignore

@@ -0,0 +1 @@
+/getversion

tests/zfs-tests/cmd/getversion/Makefile.am

@@ -0,0 +1,6 @@
+include $(top_srcdir)/config/Rules.am
+
+pkgexecdir = $(datadir)/@PACKAGE@/zfs-tests/bin
+
+pkgexec_PROGRAMS = getversion
+getversion_SOURCES = getversion.c

tests/zfs-tests/cmd/getversion/getversion.c

@@ -0,0 +1,48 @@
+/*
+ * This file and its contents are supplied under the terms of the
+ * Common Development and Distribution License ("CDDL"), version 1.0.
+ * You may only use this file in accordance with the terms of version
+ * 1.0 of the CDDL.
+ *
+ * A full copy of the text of the CDDL should have accompanied this
+ * source.  A copy of the CDDL is also available via the Internet at
+ * http://www.illumos.org/license/CDDL.
+ */
+
+/*
+ * Copyright 2021 iXsystems, Inc.
+ */
+
+/*
+ * FreeBSD and macOS expose file generation number through stat(2) and stat(1).
+ * Linux exposes it instead through an ioctl.
+ */
+
+#include <sys/ioctl.h>
+#include <sys/fcntl.h>
+#include <linux/fs.h>
+#include <err.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+
+int
+main(int argc, const char * const argv[])
+{
+	if (argc != 2)
+		errx(EXIT_FAILURE, "usage: %s filename", argv[0]);
+
+	int fd = open(argv[1], O_RDONLY);
+	if (fd == -1)
+		err(EXIT_FAILURE, "failed to open %s", argv[1]);
+
+	int gen = 0;
+	if (ioctl(fd, FS_IOC_GETVERSION, &gen) == -1)
+		err(EXIT_FAILURE, "FS_IOC_GETVERSION failed");
+
+	(void) close(fd);
+
+	(void) printf("%d\n", gen);
+
+	return (EXIT_SUCCESS);
+}