Linux 6.8 compat: handle mnt_idmap user_namespace change

struct mnt_idmap no longer has a struct user_namespace within it. The
only place where that matters is when mapping through the initial no-op
mapping. Detect this case and handle it by mapping the uid directly.

Signed-off-by: Rob Norris <[email protected]>
Sponsored-by: https://despairlabs.com/sponsor/

Author: robn

config/kernel-idmap_mnt_api.m4

@@ -23,3 +23,28 @@ AC_DEFUN([ZFS_AC_KERNEL_IDMAP_MNT_API], [
         ])
 ])
 
+dnl #
+dnl # 6.8 decouples mnt_idmap from user_namespace. This is all internal
+dnl # to mnt_idmap so we can't detect it directly, but we detect a related
+dnl # change as use that as a signal.
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_SRC_IDMAP_NO_USERNS], [
+	ZFS_LINUX_TEST_SRC([idmap_no_userns], [
+		#include <linux/uidgid.h>
+	], [
+		struct uid_gid_map *map = NULL;
+		map_id_down(map, 0);
+	])
+])
+
+
+AC_DEFUN([ZFS_AC_KERNEL_IDMAP_NO_USERNS], [
+	AC_MSG_CHECKING([whether idmapped mounts have a user namespace])
+	ZFS_LINUX_TEST_RESULT([idmap_no_userns], [
+		AC_MSG_RESULT([yes])
+		AC_DEFINE(HAVE_IDMAP_NO_USERNS, 1,
+			[mnt_idmap does not have user_namespace])
+	], [
+		AC_MSG_RESULT([no])
+	])
+])

config/kernel.m4

@@ -158,6 +158,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_SRC], [
 	ZFS_AC_KERNEL_SRC___COPY_FROM_USER_INATOMIC
 	ZFS_AC_KERNEL_SRC_USER_NS_COMMON_INUM
 	ZFS_AC_KERNEL_SRC_IDMAP_MNT_API
+	ZFS_AC_KERNEL_SRC_IDMAP_NO_USERNS
 	ZFS_AC_KERNEL_SRC_IATTR_VFSID
 	ZFS_AC_KERNEL_SRC_FILEMAP
 	ZFS_AC_KERNEL_SRC_WRITEPAGE_T
@@ -305,6 +306,7 @@ AC_DEFUN([ZFS_AC_KERNEL_TEST_RESULT], [
 	ZFS_AC_KERNEL___COPY_FROM_USER_INATOMIC
 	ZFS_AC_KERNEL_USER_NS_COMMON_INUM
 	ZFS_AC_KERNEL_IDMAP_MNT_API
+	ZFS_AC_KERNEL_IDMAP_NO_USERNS
 	ZFS_AC_KERNEL_IATTR_VFSID
 	ZFS_AC_KERNEL_FILEMAP
 	ZFS_AC_KERNEL_WRITEPAGE_T

include/os/linux/spl/sys/cred.h

@@ -73,7 +73,8 @@ static inline struct user_namespace *zfs_i_user_ns(struct inode *inode)
 static inline boolean_t zfs_no_idmapping(struct user_namespace *mnt_userns,
     struct user_namespace *fs_userns)
 {
-	return (zfs_is_init_userns(mnt_userns) || mnt_userns == fs_userns);
+	return (mnt_userns == NULL || zfs_is_init_userns(mnt_userns) ||
+	    mnt_userns == fs_userns);
 }
 
 static inline uid_t zfs_uid_to_vfsuid(zidmap_t *mnt_userns,

include/os/linux/spl/sys/types.h

@@ -57,12 +57,23 @@ typedef int			minor_t;
 struct user_namespace;
 #ifdef HAVE_IOPS_CREATE_IDMAP
 #include <linux/refcount.h>
+#ifdef HAVE_IDMAP_NO_USERNS
+#include <linux/user_namespace.h>
+struct mnt_idmap {
+	struct uid_gid_map uid_map;
+	struct uid_gid_map gid_map;
+	refcount_t count;
+};
+typedef struct mnt_idmap	zidmap_t;
+#define	idmap_owner(p)	(NULL)
+#else
 struct mnt_idmap {
 	struct user_namespace *owner;
 	refcount_t count;
 };
 typedef struct mnt_idmap	zidmap_t;
 #define	idmap_owner(p)	(((struct mnt_idmap *)p)->owner)
+#endif
 #else
 typedef struct user_namespace	zidmap_t;
 #define	idmap_owner(p)	((struct user_namespace *)p)