Check whether the kernel supports i_uid/gid_read/write helpers

Since the concept of a kuid and the need to translate from it to
ordinary integer type was added in kernel version 3.5 implement necessary
plumbing to be able to detect this condition during compile time. If
the kernel doesn't support the kuid then just fall back to directly
accessing the respective struct inode's members

config/kernel-kuid-helpers.m4

@@ -0,0 +1,22 @@
+dnl #
+dnl # 3.5 API change,
+dnl # Since usernamespaces were introduced in kernel version 3.5, it
+dnl # became necessary to go through one more level of indirection
+dnl # when dealing with uid/gid - namely the kuid type.
+dnl #
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_KUID_HELPERS], [
+	AC_MSG_CHECKING([whether i_(uid|gid)_(read|write) exist])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/fs.h>
+	],[
+		struct inode *ip = NULL;
+		(void) i_uid_read(ip);
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(HAVE_KUID_HELPERS, 1,
+		    [i_(uid|gid)_(read|write) exist])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -92,6 +92,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_MAKE_REQUEST_FN
 	ZFS_AC_KERNEL_GENERIC_IO_ACCT
 	ZFS_AC_KERNEL_FPU
+	ZFS_AC_KERNEL_KUID_HELPERS
 
 	AS_IF([test "$LINUX_OBJ" != "$LINUX"], [
 		KERNELMAKE_PARAMS="$KERNELMAKE_PARAMS O=$LINUX_OBJ"

include/linux/vfs_compat.h

@@ -28,6 +28,7 @@
 #define	_ZFS_VFS_H
 
 #include <sys/taskq.h>
+#include <sys/cred.h>
 #include <linux/backing-dev.h>
 
 /*
@@ -352,6 +353,58 @@ static inline struct inode *file_inode(const struct file *f)
 }
 #endif /* HAVE_FILE_INODE */
 
+#ifdef HAVE_KUID_HELPERS
+static inline uid_t zfs_uid_read_impl(struct inode *ip)
+{
+	return (from_kuid(kcred->user_ns, ip->i_uid));
+}
+
+static inline uid_t zfs_uid_read(struct inode *ip)
+{
+	return (zfs_uid_read_impl(ip));
+}
+
+static inline gid_t zfs_gid_read_impl(struct inode *ip)
+{
+	return (from_kgid(kcred->user_ns, ip->i_gid));
+}
+
+static inline gid_t zfs_gid_read(struct inode *ip)
+{
+	return (zfs_gid_read_impl(ip));
+}
+
+static inline void zfs_uid_write(struct inode *ip, uid_t uid)
+{
+	ip->i_uid = make_kuid(kcred->user_ns, uid);
+}
+
+static inline void zfs_gid_write(struct inode *ip, gid_t gid)
+{
+	ip->i_gid = make_kgid(kcred->user_ns, gid);
+}
+#else
+static inline uid_t zfs_uid_read(struct inode *ip)
+{
+	return (ip->i_uid);
+}
+
+static inline gid_t zfs_gid_read(struct inode *ip)
+{
+	return (ip->i_gid);
+}
+
+static inline void zfs_uid_write(struct inode *ip, uid_t uid)
+{
+	ip->i_uid = uid;
+}
+
+static inline void zfs_gid_write(struct inode *ip, gid_t gid)
+{
+	ip->i_gid = gid;
+}
+#endif
+
 /*
  * 2.6.38 API change
  */