Re: [PATCH v2 16/30] acl: add vfs_get_acl()

[Christian Brauner <brauner@kernel.org>]

On Tue, Sep 27, 2022 at 06:55:25PM -0400, Paul Moore wrote:
> On Mon, Sep 26, 2022 at 11:24 AM Christian Brauner <brauner@kernel.org> wrote:
> >
> > 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>
> > ---
> >
> > Notes:
> >     /* v2 */
> >     unchanged
> >
> >  fs/posix_acl.c                  | 131 ++++++++++++++++++++++++++++++--
> >  include/linux/posix_acl.h       |   9 +++
> >  include/linux/posix_acl_xattr.h |  10 +++
> >  3 files changed, 142 insertions(+), 8 deletions(-)
> 
> ...
> 
> > diff --git a/fs/posix_acl.c b/fs/posix_acl.c
> > index ef0908a4bc46..18873be583a9 100644
> > --- a/fs/posix_acl.c
> > +++ b/fs/posix_acl.c
> > @@ -1369,3 +1439,48 @@ int vfs_set_acl(struct user_namespace *mnt_userns, struct dentry *dentry,
> >         return error;
> >  }
> >  EXPORT_SYMBOL(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_getxattr(dentry, acl_name);
> > +       if (error)
> > +               return ERR_PTR(error);
> 
> I understand the desire to reuse the security_inode_getxattr() hook
> here, it makes perfect sense, but given that this patchset introduces
> an ACL specific setter hook I think it makes sense to have a matching
> getter hook.  It's arguably a little silly given the current crop of
> LSMs and their approach to ACLs, but if we are going to differentiate
> on the write side I think we might as well be consistent and
> differentiate on the read side as well.

Sure, I don't mind doing that. I'll add the infrastructure and then the
individual LSMs can add their own hooks.