[PATCH v4 04/30] fs: add new get acl method

[Christian Brauner]

On Fri, Sep 30, 2022 at 10:53:05AM +0200, Miklos Szeredi wrote:
> On Thu, 29 Sept 2022 at 17:31, Christian Brauner <brauner at kernel.org> wrote:
> 
> > 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.
> 
> This is confusing.   For example overlayfs ends up with two functions
> that are similar, but not quite the same:
> 
>  ovl_get_acl -> ovl_get_acl_path -> vfs_get_acl -> __get_acl(mnt_userns, ...)
> 
>  ovl_get_inode_acl -> get_inode_acl -> __get_acl(&init_user_ns, ...)
> 
> So what's the difference and why do we need both?  If one can retrive
> the acl without dentry, then why do we need the one with the dentry?

The ->get_inode_acl() method is called during generic_permission() and
inode_permission() both of which are called from various filesystems in
their ->permission inode operations. There's no dentry available during
the permission inode operation and there are filesystems like 9p and
cifs that need a dentry.

> If a filesystem cannot implement a get_acl() without a dentry, then
> what will happen to caller's that don't have a dentry?

This happens today for cifs where posix acls can be created and read but
they cannot be used for permission checking where no inode is available.
New filesystems shouldn't have this issue.