Cross-platform xattr user namespace compatibility

ZFS on Linux originally implemented xattr namespaces in a way that is
incompatible with other operating systems.  On illumos, xattrs do not
have namespaces.  Every xattr name is visible.  FreeBSD has two
universally defined namespaces: EXTATTR_NAMESPACE_USER and
EXTATTR_NAMESPACE_SYSTEM.  The system namespace is used for protected
FreeBSD-specific attributes such as MAC labels and pnfs state.  These
attributes have the namespace string "freebsd:system:" prefixed to the
name in the encoding scheme used by ZFS.  The user namespace is used
for general purpose user attributes and obeys normal access control
mechanisms.  These attributes have no namespace string prefixed, so
xattrs written on illumos are accessible in the user namespace on
FreeBSD, and xattrs written to the user namespace on FreeBSD are
accessible by the same name on illumos.

Linux has several xattr namespaces.  On Linux, ZFS encodes the
namespace in the xattr name for every namespace, including the user
namespace.  As a consequence, an xattr in the user namespace with the
name "foo" is stored by ZFS with the name "user.foo" and therefore
appears on FreeBSD and illumos to have the name "user.foo" rather than
"foo".  Conversely, none of the xattrs written on FreeBSD or illumos
are accessible on Linux unless the name happens to be prefixed with one
of the Linux xattr namespaces, in which case the namespace is stripped
from the name.  This makes xattrs entirely incompatible between Linux
and other platforms.

We want to make the encoding of user namespace xattrs compatible across
platforms.  A critical requirement of this compatibility is for xattrs
from existing pools from FreeBSD and illumos to be accessible by the
same names in the user namespace on Linux.  It is also necessary that
existing pools with xattrs written by Linux retain access to those
xattrs by the same names on Linux.  Making user namespace xattrs from
Linux accessible by the correct names on other platforms is important.
The handling of other namespaces is not required to be consistent.

Add a fallback mechanism for listing and getting xattrs to treat xattrs
as being in the user namespace if they do not match a known prefix.

Do not allow setting or getting xattrs with a name that is prefixed
with one of the namespace names used by ZFS on supported platforms.

Allow choosing between legacy illumos and FreeBSD compatability and
legacy Linux compatibility with a new tunable.  This facilitates
replication and migration of pools between hosts with different
compatibility needs.

The tunable controls whether or not to prefix the namespace to the
name.  If the xattr is already present with the alternate prefix,
remove it so only the new version persists.  By default the platform's
existing convention is used.

Signed-off-by: Ryan Moeller <ryan@iXsystems.com>

include/sys/fs/zfs.h

@@ -1709,7 +1709,6 @@ typedef enum {
 #define	ZFS_EV_HIST_DSID	"history_dsid"
 #define	ZFS_EV_RESILVER_TYPE	"resilver_type"
 
-
 /*
  * We currently support block sizes from 512 bytes to 16MB.
  * The benefits of larger blocks, and thus larger IO, need to be weighed
@@ -1731,7 +1730,6 @@ typedef enum {
 #define	SPA_OLD_MAXBLOCKSIZE	(1ULL << SPA_OLD_MAXBLOCKSHIFT)
 #define	SPA_MAXBLOCKSIZE	(1ULL << SPA_MAXBLOCKSHIFT)
 
-
 /* supported encryption algorithms */
 enum zio_encrypt {
 	ZIO_CRYPT_INHERIT = 0,
@@ -1749,6 +1747,34 @@ enum zio_encrypt {
 #define	ZIO_CRYPT_ON_VALUE	ZIO_CRYPT_AES_256_GCM
 #define	ZIO_CRYPT_DEFAULT	ZIO_CRYPT_OFF
 
+/*
+ * xattr namespace prefixes.  These are forbidden in xattr names.
+ *
+ * For cross-platform compatibility, xattrs in the user namespace should not be
+ * prefixed with the namespace name, but for backwards compatibility with older
+ * ZFS on Linux versions we do prefix the namespace.
+ */
+#define	ZFS_XA_NS_FREEBSD_PREFIX		"freebsd:"
+#define	ZFS_XA_NS_FREEBSD_PREFIX_LEN		strlen("freebsd:")
+#define	ZFS_XA_NS_LINUX_SECURITY_PREFIX		"security."
+#define	ZFS_XA_NS_LINUX_SECURITY_PREFIX_LEN	strlen("security.")
+#define	ZFS_XA_NS_LINUX_SYSTEM_PREFIX		"system."
+#define	ZFS_XA_NS_LINUX_SYSTEM_PREFIX_LEN	strlen("system.")
+#define	ZFS_XA_NS_LINUX_TRUSTED_PREFIX		"trusted."
+#define	ZFS_XA_NS_LINUX_TRUSTED_PREFIX_LEN	strlen("trusted.")
+#define	ZFS_XA_NS_LINUX_USER_PREFIX		"user."
+#define	ZFS_XA_NS_LINUX_USER_PREFIX_LEN		strlen("user.")
+
+#define	ZFS_XA_NS_PREFIX_MATCH(ns, name) \
+	(strncmp(name, ZFS_XA_NS_##ns##_PREFIX, \
+	ZFS_XA_NS_##ns##_PREFIX_LEN) == 0)
+
+#define	ZFS_XA_NS_PREFIX_FORBIDDEN(name) \
+	(ZFS_XA_NS_PREFIX_MATCH(FREEBSD, name) || \
+	    ZFS_XA_NS_PREFIX_MATCH(LINUX_SECURITY, name) || \
+	    ZFS_XA_NS_PREFIX_MATCH(LINUX_SYSTEM, name) || \
+	    ZFS_XA_NS_PREFIX_MATCH(LINUX_TRUSTED, name) || \
+	    ZFS_XA_NS_PREFIX_MATCH(LINUX_USER, name))
 
 #ifdef	__cplusplus
 }

man/man4/zfs.4

@@ -2129,6 +2129,30 @@ When enabled, the maximum number of pending allocations per top-level vdev
 is limited by
 .Sy zfs_vdev_queue_depth_pct .
 .
+.It Sy zfs_xattr_compat Ns = Ns 0 Ns | Ns 1 Pq int
+Control the naming scheme used when setting new xattrs in the user namespace.
+If
+.Sy 0
+.Pq the default on Linux ,
+user namespace xattr names are prefixed with the namespace, to be backwards
+compatible with previous versions of ZFS on Linux.
+If
+.Sy 1
+.Pq the default on Fx ,
+user namespace xattr names are not prefixed, to be backwards compatible with
+previous versions of ZFS on illumos and
+.Fx .
+.Pp
+Either naming scheme can be read on this and future versions of ZFS, regardless
+of this tunable, but legacy ZFS on illumos or
+.Fx
+are unable to read user namespace xattrs written in the Linux format, and
+legacy versions of ZFS on Linux are unable to read user namespace xattrs written
+in the legacy ZFS format.
+.Pp
+An existing xattr with the alternate naming scheme is removed when overwriting
+the xattr so as to not accumulate duplicates.
+.
 .It Sy zio_requeue_io_start_cut_in_line Ns = Ns Sy 0 Ns | Ns 1 Pq int
 Prioritize requeued I/O.
 .