fs: add new get acl method

The current way of setting and getting posix acls through the generic
xattr interface is error prone and type unsafe. The vfs needs to
interpret and fixup posix acls before storing or reporting it to
userspace. Various hacks exist to make this work. The code is hard to
understand and difficult to maintain in it's current form. Instead of
making this work by hacking posix acls through xattr handlers we are
building a dedicated posix acl api around the get and set inode
operations. This removes a lot of hackiness and makes the codepaths
easier to maintain. A lot of background can be found in [1].

Since some filesystem rely on the dentry being available to them when
setting posix acls (e.g., 9p and cifs) they cannot rely on the old get
acl inode operation to retrieve posix acl and need to implement their
own custom handlers because of that.

In a previous patch we renamed the old get acl inode operation to
->get_inode_acl(). We decided to rename it and implement a new one since
->get_inode_acl() is called generic_permission() and inode_permission()
both of which can be called during an filesystem's ->permission()
handler. So simply passing a dentry argument to ->get_acl() would have
amounted to also having to pass a dentry argument to ->permission(). We
avoided that change.

This adds a new ->get_acl() inode operations which takes a dentry
argument which filesystems such as 9p, cifs, and overlayfs can implement
to get posix acls.

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

Documentation/filesystems/locking.rst

@@ -84,6 +84,7 @@ prototypes::
 	int (*fileattr_set)(struct user_namespace *mnt_userns,
 			    struct dentry *dentry, struct fileattr *fa);
 	int (*fileattr_get)(struct dentry *dentry, struct fileattr *fa);
+	struct posix_acl * (*get_acl)(struct user_namespace *, struct dentry *, int);
 
 locking rules:
 	all may block
@@ -105,6 +106,7 @@ get_link:	no
 setattr:	exclusive
 permission:	no (may not block if called in rcu-walk mode)
 get_inode_acl:	no
+get_acl:	no
 getattr:	no
 listxattr:	no
 fiemap:		no

Documentation/filesystems/vfs.rst

@@ -443,6 +443,7 @@ As of kernel 2.6.22, the following members are defined:
 		int (*atomic_open)(struct inode *, struct dentry *, struct file *,
 				   unsigned open_flag, umode_t create_mode);
 		int (*tmpfile) (struct user_namespace *, struct inode *, struct file *, umode_t);
+		struct posix_acl * (*get_acl)(struct user_namespace *, struct dentry *, int);
 	        int (*set_acl)(struct user_namespace *, struct dentry *, struct posix_acl *, int);
 		int (*fileattr_set)(struct user_namespace *mnt_userns,
 				    struct dentry *dentry, struct fileattr *fa);

include/linux/fs.h

@@ -2172,6 +2172,8 @@ struct inode_operations {
 			   umode_t create_mode);
 	int (*tmpfile) (struct user_namespace *, struct inode *,
 			struct file *, umode_t);
+	struct posix_acl *(*get_acl)(struct user_namespace *, struct dentry *,
+				     int);
 	int (*set_acl)(struct user_namespace *, struct dentry *,
 		       struct posix_acl *, int);
 	int (*fileattr_set)(struct user_namespace *mnt_userns,