acl: add vfs_remove_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_remove_acl() api that can be used to set
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

@@ -1479,3 +1479,68 @@ struct posix_acl *vfs_get_acl(struct user_namespace *mnt_userns,
 	return acl;
 }
 EXPORT_SYMBOL_GPL(vfs_get_acl);
+
+/**
+ * vfs_remove_acl - remove 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 removes posix acls.
+ *
+ * Return: On success 0, on error negative errno.
+ */
+int vfs_remove_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
+		   const char *acl_name)
+{
+	int acl_type;
+	int error;
+	struct inode *inode = d_inode(dentry);
+	struct inode *delegated_inode = NULL;
+
+	acl_type = posix_acl_type(acl_name);
+	if (acl_type < 0)
+		return -EINVAL;
+
+retry_deleg:
+	inode_lock(inode);
+
+	/*
+	 * We only care about restrictions the inode struct itself places upon
+	 * us otherwise POSIX ACLs aren't subject to any VFS restrictions.
+	 */
+	error = may_write_xattr(mnt_userns, inode);
+	if (error)
+		goto out_inode_unlock;
+
+	error = security_inode_remove_acl(mnt_userns, dentry, acl_name);
+	if (error)
+		goto out_inode_unlock;
+
+	error = try_break_deleg(inode, &delegated_inode);
+	if (error)
+		goto out_inode_unlock;
+
+	if (inode->i_opflags & IOP_XATTR)
+		error = set_posix_acl(mnt_userns, dentry, acl_type, NULL);
+	else if (unlikely(is_bad_inode(inode)))
+		error = -EIO;
+	else
+		error = -EOPNOTSUPP;
+	if (!error) {
+		fsnotify_xattr(dentry);
+		evm_inode_post_remove_acl(mnt_userns, dentry, acl_name);
+	}
+
+out_inode_unlock:
+	inode_unlock(inode);
+
+	if (delegated_inode) {
+		error = break_deleg_wait(&delegated_inode);
+		if (!error)
+			goto retry_deleg;
+	}
+
+	return error;
+}
+EXPORT_SYMBOL_GPL(vfs_remove_acl);

include/linux/evm.h

@@ -35,6 +35,12 @@ extern int evm_inode_removexattr(struct user_namespace *mnt_userns,
 				 struct dentry *dentry, const char *xattr_name);
 extern void evm_inode_post_removexattr(struct dentry *dentry,
 				       const char *xattr_name);
+static inline void evm_inode_post_remove_acl(struct user_namespace *mnt_userns,
+					     struct dentry *dentry,
+					     const char *acl_name)
+{
+	evm_inode_post_removexattr(dentry, acl_name);
+}
 extern int evm_inode_set_acl(struct user_namespace *mnt_userns,
 			     struct dentry *dentry, const char *acl_name,
 			     struct posix_acl *kacl);
@@ -123,6 +129,13 @@ static inline void evm_inode_post_removexattr(struct dentry *dentry,
 	return;
 }
 
+static inline void evm_inode_post_remove_acl(struct user_namespace *mnt_userns,
+					     struct dentry *dentry,
+					     const char *acl_name)
+{
+	return;
+}
+
 static inline int evm_inode_set_acl(struct user_namespace *mnt_userns,
 				    struct dentry *dentry, const char *acl_name,
 				    struct posix_acl *kacl)

include/linux/posix_acl.h

@@ -104,6 +104,8 @@ 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);
+int vfs_remove_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)
@@ -145,6 +147,12 @@ static inline struct posix_acl *vfs_get_acl(struct user_namespace *mnt_userns,
 {
 	return ERR_PTR(-EOPNOTSUPP);
 }
+
+static inline int vfs_remove_acl(struct user_namespace *mnt_userns,
+				 struct dentry *dentry, const char *acl_name)
+{
+	return -EOPNOTSUPP;
+}
 #endif /* CONFIG_FS_POSIX_ACL */
 
 struct posix_acl *get_inode_acl(struct inode *inode, int type);