[PATCH] xfs: show build options in sysfs

[Eric Sandeen <sandeen@redhat.com>]

This adds the "build options" string to a sysfs entry:

# cat /sys/fs/xfs/features/build_opts 
ACLs, security attributes, realtime, scrub, no debug

because right now we only get it in dmesg and scraping dmesg
is not a great option.

This is really /build options/, not features as in "on-disk,
superblock features" like XFS_SB_FEAT_* - putting this under a
features/ dir will leave the window open open to do supported
superblock features ala ext4 & f2fs in the future if desired.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---

No sure if it would make sense to have i.e.

/sys/fs/xfs/features/build_options
/sys/fs/xfs/features/finobt
/sys/fs/xfs/features/rmapbt
...

all in the same features/ dir ?

Also I didn't test module unload/teardown as I'm testing on xfs root.

diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
index a14d11d78bd8..bc0e7fd63567 100644
--- a/fs/xfs/xfs_super.c
+++ b/fs/xfs/xfs_super.c
@@ -55,9 +55,10 @@
 static const struct super_operations xfs_super_operations;
 struct bio_set xfs_ioend_bioset;
 
-static struct kset *xfs_kset;		/* top-level xfs sysfs dir */
+static struct kset *xfs_kset;			/* top-level xfs sysfs dir */
+static struct xfs_kobj xfs_features_kobj;	/* global features */
 #ifdef DEBUG
-static struct xfs_kobj xfs_dbg_kobj;	/* global debug sysfs attrs */
+static struct xfs_kobj xfs_dbg_kobj;		/* global debug sysfs attrs */
 #endif
 
 /*
@@ -2134,11 +2135,16 @@ init_xfs_fs(void)
 	if (error)
 		goto out_free_stats;
 
+	xfs_features_kobj.kobject.kset = xfs_kset;
+	error = xfs_sysfs_init(&xfs_features_kobj, &xfs_features_ktype,
+				NULL, "features");
+	if (error)
+		goto out_remove_stats_kobj;
 #ifdef DEBUG
 	xfs_dbg_kobj.kobject.kset = xfs_kset;
 	error = xfs_sysfs_init(&xfs_dbg_kobj, &xfs_dbg_ktype, NULL, "debug");
 	if (error)
-		goto out_remove_stats_kobj;
+		goto out_remove_features_kobj;
 #endif
 
 	error = xfs_qm_init();
@@ -2155,8 +2161,10 @@ init_xfs_fs(void)
  out_remove_dbg_kobj:
 #ifdef DEBUG
 	xfs_sysfs_del(&xfs_dbg_kobj);
- out_remove_stats_kobj:
+ out_remove_features_kobj:
 #endif
+	xfs_sysfs_del(&xfs_features_kobj);
+ out_remove_stats_kobj:
 	xfs_sysfs_del(&xfsstats.xs_kobj);
  out_free_stats:
 	free_percpu(xfsstats.xs_stats);
@@ -2186,6 +2194,7 @@ exit_xfs_fs(void)
 #ifdef DEBUG
 	xfs_sysfs_del(&xfs_dbg_kobj);
 #endif
+	xfs_sysfs_del(&xfs_features_kobj);
 	xfs_sysfs_del(&xfsstats.xs_kobj);
 	free_percpu(xfsstats.xs_stats);
 	kset_unregister(xfs_kset);
diff --git a/fs/xfs/xfs_sysfs.c b/fs/xfs/xfs_sysfs.c
index cabda13f3c64..98f36ad16237 100644
--- a/fs/xfs/xfs_sysfs.c
+++ b/fs/xfs/xfs_sysfs.c
@@ -222,6 +222,28 @@ struct kobj_type xfs_dbg_ktype = {
 
 #endif /* DEBUG */
 
+/* features */
+
+STATIC ssize_t
+build_opts_show(
+	struct kobject	*kobject,
+	char		*buf)
+{
+	return snprintf(buf, PAGE_SIZE, "%s\n", XFS_BUILD_OPTIONS);
+}
+XFS_SYSFS_ATTR_RO(build_opts);
+
+static struct attribute *xfs_features_attrs[] = {
+	ATTR_LIST(build_opts),
+	NULL,
+};
+
+struct kobj_type xfs_features_ktype = {
+	.release = xfs_sysfs_release,
+	.sysfs_ops = &xfs_sysfs_ops,
+	.default_attrs = xfs_features_attrs,
+};
+
 /* stats */
 
 static inline struct xstats *
diff --git a/fs/xfs/xfs_sysfs.h b/fs/xfs/xfs_sysfs.h
index e9f810fc6731..e475f6b7eb91 100644
--- a/fs/xfs/xfs_sysfs.h
+++ b/fs/xfs/xfs_sysfs.h
@@ -11,6 +11,7 @@ extern struct kobj_type xfs_mp_ktype;	/* xfs_mount */
 extern struct kobj_type xfs_dbg_ktype;	/* debug */
 extern struct kobj_type xfs_log_ktype;	/* xlog */
 extern struct kobj_type xfs_stats_ktype;	/* stats */
+extern struct kobj_type xfs_features_ktype;	/* features*/
 
 static inline struct xfs_kobj *
 to_kobj(struct kobject *kobject)