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>

3 files changed, 4 insertions, 3 deletions

diff --git a/fs/ext2/acl.c b/fs/ext2/acl.c
index bf298967c5b8..440d5f1e9d47 100644
--- a/fs/ext2/acl.c
+++ b/fs/ext2/acl.c
@@ -219,11 +219,12 @@ __ext2_set_acl(struct inode *inode, struct posix_acl *acl, int type)
  * inode->i_mutex: down
  */
 int
-ext2_set_acl(struct user_namespace *mnt_userns, struct inode *inode,
+ext2_set_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
 	     struct posix_acl *acl, int type)
 {
 	int error;
 	int update_mode = 0;
+	struct inode *inode = d_inode(dentry);
 	umode_t mode = inode->i_mode;
 
 	if (type == ACL_TYPE_ACCESS && acl) {
diff --git a/fs/ext2/acl.h b/fs/ext2/acl.h
index 925ab6287d35..3841becb94ff 100644
--- a/fs/ext2/acl.h
+++ b/fs/ext2/acl.h
@@ -56,7 +56,7 @@ static inline int ext2_acl_count(size_t size)
 
 /* acl.c */
 extern struct posix_acl *ext2_get_acl(struct inode *inode, int type, bool rcu);
-extern int ext2_set_acl(struct user_namespace *mnt_userns, struct inode *inode,
+extern int ext2_set_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
 			struct posix_acl *acl, int type);
 extern int ext2_init_acl (struct inode *, struct inode *);
 
diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
index 918ab2f9e4c0..e97e77be64f3 100644
--- a/fs/ext2/inode.c
+++ b/fs/ext2/inode.c
@@ -1652,7 +1652,7 @@ int ext2_setattr(struct user_namespace *mnt_userns, struct dentry *dentry,
 	}
 	setattr_copy(&init_user_ns, inode, iattr);
 	if (iattr->ia_valid & ATTR_MODE)
-		error = posix_acl_chmod(&init_user_ns, inode, inode->i_mode);
+		error = posix_acl_chmod(&init_user_ns, dentry, inode->i_mode);
 	mark_inode_dirty(inode);
 
 	return error;