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>

fs/posix_acl.c

@@ -111,7 +111,9 @@ void forget_all_cached_acls(struct inode *inode)
 }
 EXPORT_SYMBOL(forget_all_cached_acls);
 
-struct posix_acl *get_inode_acl(struct inode *inode, int type)
+static struct posix_acl *__get_acl(struct user_namespace *mnt_userns,
+				   struct dentry *dentry, struct inode *inode,
+				   int type)
 {
 	void *sentinel;
 	struct posix_acl **p;
@@ -144,19 +146,21 @@ struct posix_acl *get_inode_acl(struct inode *inode, int type)
 	cmpxchg(p, ACL_NOT_CACHED, sentinel);
 
 	/*
-	 * Normally, the ACL returned by ->get_inode_acl will be cached.
+	 * Normally, the ACL returned by ->get{_inode}_acl will be cached.
 	 * A filesystem can prevent that by calling
-	 * forget_cached_acl(inode, type) in ->get_inode_acl.
+	 * forget_cached_acl(inode, type) in ->get{_inode}_acl.
 	 *
-	 * If the filesystem doesn't have a get_inode_acl() function at all,
+	 * If the filesystem doesn't have a get{_inode}_ acl() function at all,
 	 * we'll just create the negative cache entry.
 	 */
-	if (!inode->i_op->get_inode_acl) {
+	if (dentry && inode->i_op->get_acl) {
+		acl = inode->i_op->get_acl(mnt_userns, dentry, type);
+	} else if (inode->i_op->get_inode_acl) {
+		acl = inode->i_op->get_inode_acl(inode, type, false);
+	} else {
 		set_cached_acl(inode, type, NULL);
 		return NULL;
 	}
-	acl = inode->i_op->get_inode_acl(inode, type, false);
-
 	if (IS_ERR(acl)) {
 		/*
 		 * Remove our sentinel so that we don't block future attempts
@@ -174,6 +178,11 @@ struct posix_acl *get_inode_acl(struct inode *inode, int type)
 		posix_acl_release(acl);
 	return acl;
 }
+
+struct posix_acl *get_inode_acl(struct inode *inode, int type)
+{
+	return __get_acl(&init_user_ns, NULL, inode, type);
+}
 EXPORT_SYMBOL(get_inode_acl);
 
 /*
@@ -1120,6 +1129,67 @@ posix_acl_to_xattr(struct user_namespace *user_ns, const struct posix_acl *acl,
 }
 EXPORT_SYMBOL (posix_acl_to_xattr);
 
+/**
+ * vfs_posix_acl_to_xattr - convert from kernel to userspace representation
+ * @mnt_userns: user namespace of the mount
+ * @inode: inode the posix acls are set on
+ * @acl: the posix acls as represented by the vfs
+ * @buffer: the buffer into which to convert @acl
+ * @size: size of @buffer
+ *
+ * This converts @acl from the VFS representation in the filesystem idmapping
+ * to the uapi form reportable to userspace. And mount and caller idmappings
+ * are handled appropriately.
+ *
+ * Return: On success, the size of the stored uapi posix acls, on error a
+ * negative errno.
+ */
+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)
+
+{
+	struct posix_acl_xattr_header *ext_acl = buffer;
+	struct posix_acl_xattr_entry *ext_entry;
+	struct user_namespace *fs_userns, *caller_userns;
+	ssize_t real_size, n;
+	vfsuid_t vfsuid;
+	vfsgid_t vfsgid;
+
+	real_size = posix_acl_xattr_size(acl->a_count);
+	if (!buffer)
+		return real_size;
+	if (real_size > size)
+		return -ERANGE;
+
+	ext_entry = (void *)(ext_acl + 1);
+	ext_acl->a_version = cpu_to_le32(POSIX_ACL_XATTR_VERSION);
+
+	fs_userns = i_user_ns(inode);
+	caller_userns = current_user_ns();
+	for (n=0; n < acl->a_count; n++, ext_entry++) {
+		const struct posix_acl_entry *acl_e = &acl->a_entries[n];
+		ext_entry->e_tag  = cpu_to_le16(acl_e->e_tag);
+		ext_entry->e_perm = cpu_to_le16(acl_e->e_perm);
+		switch(acl_e->e_tag) {
+		case ACL_USER:
+			vfsuid = make_vfsuid(mnt_userns, fs_userns, acl_e->e_uid);
+			ext_entry->e_id = cpu_to_le32(from_kuid(
+				caller_userns, vfsuid_into_kuid(vfsuid)));
+			break;
+		case ACL_GROUP:
+			vfsgid = make_vfsgid(mnt_userns, fs_userns, acl_e->e_gid);
+			ext_entry->e_id = cpu_to_le32(from_kgid(
+				caller_userns, vfsgid_into_kgid(vfsgid)));
+			break;
+		default:
+			ext_entry->e_id = cpu_to_le32(ACL_UNDEFINED_ID);
+			break;
+		}
+	}
+	return real_size;
+}
+
 static int
 posix_acl_xattr_get(const struct xattr_handler *handler,
 		    struct dentry *unused, struct inode *inode,
@@ -1364,3 +1434,48 @@ int vfs_set_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
 	return error;
 }
 EXPORT_SYMBOL_GPL(vfs_set_acl);
+
+/**
+ * vfs_get_acl - get posix acls
+ * @mnt_userns: user namespace of the mount
+ * @dentry: the dentry based on which to retrieve the posix acls
+ * @acl_name: the name of the posix acl
+ *
+ * This function retrieves @kacl from the filesystem. The caller must all
+ * posix_acl_release() on @kacl.
+ *
+ * Return: On success POSIX ACLs in VFS format, on error negative errno.
+ */
+struct posix_acl *vfs_get_acl(struct user_namespace *mnt_userns,
+			      struct dentry *dentry, const char *acl_name)
+{
+	struct inode *inode = d_inode(dentry);
+	struct posix_acl *acl;
+	int acl_type, error;
+
+	acl_type = posix_acl_type(acl_name);
+	if (acl_type < 0)
+		return ERR_PTR(-EINVAL);
+
+	/*
+	 * The VFS has no restrictions on reading POSIX ACLs so calling
+	 * something like xattr_permission() isn't needed. Only LSMs get a say.
+	 */
+	error = security_inode_get_acl(mnt_userns, dentry, acl_name);
+	if (error)
+		return ERR_PTR(error);
+
+	if (!IS_POSIXACL(inode))
+		return ERR_PTR(-EOPNOTSUPP);
+	if (S_ISLNK(inode->i_mode))
+		return ERR_PTR(-EOPNOTSUPP);
+
+	acl = __get_acl(mnt_userns, dentry, inode, acl_type);
+	if (IS_ERR(acl))
+		return acl;
+	if (!acl)
+		return ERR_PTR(-ENODATA);
+
+	return acl;
+}
+EXPORT_SYMBOL_GPL(vfs_get_acl);

include/linux/posix_acl.h

@@ -102,6 +102,8 @@ static inline void cache_no_acl(struct inode *inode)
 
 int vfs_set_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
 		const char *acl_name, struct posix_acl *kacl);
+struct posix_acl *vfs_get_acl(struct user_namespace *mnt_userns,
+			      struct dentry *dentry, const char *acl_name);
 #else
 static inline int posix_acl_chmod(struct user_namespace *mnt_userns,
 				  struct dentry *dentry, umode_t mode)
@@ -136,6 +138,13 @@ static inline int vfs_set_acl(struct user_namespace *mnt_userns,
 {
 	return -EOPNOTSUPP;
 }
+
+static inline struct posix_acl *vfs_get_acl(struct user_namespace *mnt_userns,
+					    struct dentry *dentry,
+					    const char *acl_name)
+{
+	return ERR_PTR(-EOPNOTSUPP);
+}
 #endif /* CONFIG_FS_POSIX_ACL */
 
 struct posix_acl *get_inode_acl(struct inode *inode, int type);

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,