Add zfs module feature and property compatibility

This change is required to ease the transition when upgrading
from 0.7.x to 0.8.x.  It allows 0.8.x user space utilities to
remain compatible with 0.7.x and older kernel modules.

Reviewed-by: Don Brady <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #8231 

1 files changed, 30 insertions, 6 deletions

diff --git a/module/zcommon/zfeature_common.c b/module/zcommon/zfeature_common.c
index 40ce01b94..80f4a0562 100644
--- a/module/zcommon/zfeature_common.c
+++ b/module/zcommon/zfeature_common.c
@@ -163,25 +163,49 @@ deps_contains_feature(const spa_feature_t *deps, const spa_feature_t feature)
 static boolean_t
 zfs_mod_supported_impl(const char *scope, const char *name, const char *sysfs)
 {
-	struct stat64 statbuf;
-	char *path;
 	boolean_t supported = B_FALSE;
-	int len;
-
-	len = asprintf(&path, "%s/%s/%s", sysfs, scope, name);
+	char *path;
 
+	int len = asprintf(&path, "%s%s%s%s%s", sysfs,
+	    scope == NULL ? "" : "/", scope == NULL ? "" : scope,
+	    name == NULL ? "" : "/", name == NULL ? "" : name);
 	if (len > 0) {
+		struct stat64 statbuf;
 		supported = !!(stat64(path, &statbuf) == 0);
 		free(path);
 	}
+
 	return (supported);
 }
 
 boolean_t
 zfs_mod_supported(const char *scope, const char *name)
 {
-	return (zfs_mod_supported_impl(scope, name, ZFS_SYSFS_DIR) ||
+	boolean_t supported;
+
+	/*
+	 * Check both the primary and alternate sysfs locations to determine
+	 * if the required functionality is supported.
+	 */
+	supported = (zfs_mod_supported_impl(scope, name, ZFS_SYSFS_DIR) ||
 	    zfs_mod_supported_impl(scope, name, ZFS_SYSFS_ALT_DIR));
+
+	/*
+	 * For backwards compatibility with kernel modules that predate
+	 * supported feature/property checking.  Report the feature/property
+	 * as supported if the kernel module is loaded but the requested
+	 * scope directory does not exist.
+	 */
+	if (supported == B_FALSE) {
+		struct stat64 statbuf;
+		if ((stat64(ZFS_SYSFS_DIR, &statbuf) == 0) &&
+		    !zfs_mod_supported_impl(scope, NULL, ZFS_SYSFS_DIR) &&
+		    !zfs_mod_supported_impl(scope, NULL, ZFS_SYSFS_ALT_DIR)) {
+			supported = B_TRUE;
+		}
+	}
+
+	return (supported);
 }
 #endif