fs: pass dentry to set 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 set acl inode
operation. But since ->set_acl() is required in order to use the generic
posix acl xattr handlers filesystems that do not implement this inode
operation cannot use the handler and need to implement their own
dedicated posix acl handlers.

Update the ->set_acl() inode method to take a dentry argument. This
allows all filesystems to rely on ->set_acl().

As far as I can tell all codepaths can be switched to rely on the dentry
instead of just the inode. Note that the original motivation for passing
the dentry separate from the inode instead of just the dentry in the
xattr handlers was because of security modules that call
security_d_instantiate(). This hook is called during
d_instantiate_new(), d_add(), __d_instantiate_anon(), and
d_splice_alias() to initialize the inode's security context and possibly
to set security.* xattrs. Since this only affects security.* xattrs this
is completely irrelevant for posix acls.

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

1 files changed, 13 insertions, 9 deletions

diff --git a/fs/posix_acl.c b/fs/posix_acl.c
index 74dc0f571dc9..c4bc58a1160e 100644
--- a/fs/posix_acl.c
+++ b/fs/posix_acl.c
@@ -578,19 +578,20 @@ EXPORT_SYMBOL(__posix_acl_chmod);
  * posix_acl_chmod - chmod a posix acl
  *
  * @mnt_userns:	user namespace of the mount @inode was found from
- * @inode:	inode to check permissions on
+ * @dentry:	dentry to check permissions on
  * @mode:	the new mode of @inode
  *
- * If the inode has been found through an idmapped mount the user namespace of
+ * If the dentry has been found through an idmapped mount the user namespace of
  * the vfsmount must be passed through @mnt_userns. This function will then
  * take care to map the inode according to @mnt_userns before checking
  * permissions. On non-idmapped mounts or if permission checking is to be
  * performed on the raw inode simply passs init_user_ns.
  */
 int
- posix_acl_chmod(struct user_namespace *mnt_userns, struct inode *inode,
+ posix_acl_chmod(struct user_namespace *mnt_userns, struct dentry *dentry,
 		    umode_t mode)
 {
+	struct inode *inode = d_inode(dentry);
 	struct posix_acl *acl;
 	int ret = 0;
 
@@ -609,7 +610,7 @@ int
 	ret = __posix_acl_chmod(&acl, GFP_KERNEL, mode);
 	if (ret)
 		return ret;
-	ret = inode->i_op->set_acl(mnt_userns, inode, acl, ACL_TYPE_ACCESS);
+	ret = inode->i_op->set_acl(mnt_userns, dentry, acl, ACL_TYPE_ACCESS);
 	posix_acl_release(acl);
 	return ret;
 }
@@ -1139,9 +1140,11 @@ posix_acl_xattr_get(const struct xattr_handler *handler,
 }
 
 int
-set_posix_acl(struct user_namespace *mnt_userns, struct inode *inode,
+set_posix_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
 	      int type, struct posix_acl *acl)
 {
+	struct inode *inode = d_inode(dentry);
+
 	if (!IS_POSIXACL(inode))
 		return -EOPNOTSUPP;
 	if (!inode->i_op->set_acl)
@@ -1157,14 +1160,14 @@ set_posix_acl(struct user_namespace *mnt_userns, struct inode *inode,
 		if (ret)
 			return ret;
 	}
-	return inode->i_op->set_acl(mnt_userns, inode, acl, type);
+	return inode->i_op->set_acl(mnt_userns, dentry, acl, type);
 }
 EXPORT_SYMBOL(set_posix_acl);
 
 static int
 posix_acl_xattr_set(const struct xattr_handler *handler,
 			   struct user_namespace *mnt_userns,
-			   struct dentry *unused, struct inode *inode,
+			   struct dentry *dentry, struct inode *inode,
 			   const char *name, const void *value, size_t size,
 			   int flags)
 {
@@ -1186,7 +1189,7 @@ posix_acl_xattr_set(const struct xattr_handler *handler,
 		if (IS_ERR(acl))
 			return PTR_ERR(acl);
 	}
-	ret = set_posix_acl(mnt_userns, inode, handler->flags, acl);
+	ret = set_posix_acl(mnt_userns, dentry, handler->flags, acl);
 	posix_acl_release(acl);
 	return ret;
 }
@@ -1215,10 +1218,11 @@ const struct xattr_handler posix_acl_default_xattr_handler = {
 };
 EXPORT_SYMBOL_GPL(posix_acl_default_xattr_handler);
 
-int simple_set_acl(struct user_namespace *mnt_userns, struct inode *inode,
+int simple_set_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
 		   struct posix_acl *acl, int type)
 {
 	int error;
+	struct inode *inode = d_inode(dentry);
 
 	if (type == ACL_TYPE_ACCESS) {
 		error = posix_acl_update_mode(mnt_userns, inode,