acl: add vfs_get_acl()

In previous patches we implemented get and set inode operations for all
non-stacking filesystems that support posix acls but didn't yet
implement get and/or set acl inode operations. This specifically
affected cifs and 9p.

Now we can build a posix acl api based solely on get and set inode
operations. We add a new vfs_get_acl() api that can be used to get posix
acls. This finally removes all type unsafety and type conversion issues
explained in detail in [1] that we aim to get rid of.

After we finished building the vfs api we can switch stacking
filesystems to rely on the new posix api and then finally switch the
xattr system calls themselves to rely on the posix acl api.

Link: https://lore.kernel.org/all/20220801145520.1532837-1-brauner@kernel.org [1]
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/include/linux/posix_acl_xattr.h b/include/linux/posix_acl_xattr.h
index b86b7f170d43..bf30296389d7 100644
--- a/include/linux/posix_acl_xattr.h
+++ b/include/linux/posix_acl_xattr.h
@@ -38,6 +38,9 @@ void posix_acl_fix_xattr_to_user(void *value, size_t size);
 void posix_acl_getxattr_idmapped_mnt(struct user_namespace *mnt_userns,
 				     const struct inode *inode,
 				     void *value, size_t size);
+ssize_t vfs_posix_acl_to_xattr(struct user_namespace *mnt_userns,
+			       struct inode *inode, const struct posix_acl *acl,
+			       void *buffer, size_t size);
 #else
 static inline void posix_acl_fix_xattr_from_user(void *value, size_t size)
 {
@@ -51,6 +54,13 @@ posix_acl_getxattr_idmapped_mnt(struct user_namespace *mnt_userns,
 				size_t size)
 {
 }
+static inline ssize_t vfs_posix_acl_to_xattr(struct user_namespace *mnt_userns,
+					     struct inode *inode,
+					     const struct posix_acl *acl,
+					     void *buffer, size_t size)
+{
+	return 0;
+}
 #endif
 
 struct posix_acl *posix_acl_from_xattr(struct user_namespace *user_ns,