Re: [PATCH] ANDROID: binderfs: add capabilities support

[Carlos Llamas <cmllamas@google.com>]

On Wed, Jul 7, 2021 at 10:59 AM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Wed, Jul 07, 2021 at 04:24:19PM +0000, Carlos Llamas wrote:
> > Provide userspace with a mechanism to discover binder driver
> > capabilities to refrain from using these unsupported features
> > in the first place. Note that older capabilities are assumed
> > to be supported and only new ones will be added.
>
> What defines "new" vs. "old"?  Where was the line drawn?

Any feature after "one way spam" may be added to the capabilities.
I'll amend the commit message to indicate this.

>
> > Signed-off-by: Carlos Llamas <cmllamas@google.com>
> > ---
> >  drivers/android/binderfs.c | 45 ++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 45 insertions(+)
> >
> > diff --git a/drivers/android/binderfs.c b/drivers/android/binderfs.c
> > index e80ba93c62a9..f793887f6dc8 100644
> > --- a/drivers/android/binderfs.c
> > +++ b/drivers/android/binderfs.c
> > @@ -58,6 +58,10 @@ enum binderfs_stats_mode {
> >       binderfs_stats_mode_global,
> >  };
> >
> > +struct binder_capabilities {
> > +     bool oneway_spam;
> > +};
> > +
> >  static const struct constant_table binderfs_param_stats[] = {
> >       { "global", binderfs_stats_mode_global },
> >       {}
> > @@ -69,6 +73,10 @@ static const struct fs_parameter_spec binderfs_fs_parameters[] = {
> >       {}
> >  };
> >
> > +static struct binder_capabilities binder_caps = {
> > +     .oneway_spam = true,
> > +};
> > +
> >  static inline struct binderfs_info *BINDERFS_SB(const struct super_block *sb)
> >  {
> >       return sb->s_fs_info;
> > @@ -583,6 +591,39 @@ static struct dentry *binderfs_create_dir(struct dentry *parent,
> >       return dentry;
> >  }
> >
> > +static int binder_caps_show(struct seq_file *m, void *unused)
> > +{
> > +     bool *cap = m->private;
> > +
> > +     seq_printf(m, "%d\n", *cap);
> > +
> > +     return 0;
> > +}
> > +DEFINE_SHOW_ATTRIBUTE(binder_caps);
> > +
> > +static int init_binder_caps(struct super_block *sb)
> > +{
> > +     struct dentry *dentry, *root;
> > +     int ret = 0;
> > +
> > +     root = binderfs_create_dir(sb->s_root, "caps");
> > +     if (IS_ERR(root)) {
> > +             ret = PTR_ERR(root);
> > +             goto out;
> > +     }
> > +
> > +     dentry = binderfs_create_file(root, "oneway_spam",
> > +                                   &binder_caps_fops,
> > +                                   &binder_caps.oneway_spam);
> > +     if (IS_ERR(dentry)) {
> > +             ret = PTR_ERR(dentry);
> > +             goto out;
>
> If this fails, you still report that an error happened, yet you do not
> remove the directory?  Is that intended?

Propagating an error in this case will kill the super block along with the
capabilities entries. At a user level the binderfs will fail to be mounted,
so anything created up until this point is removed.

>
> And where is this new file documented?  Where are the existing binderfs
> files documented?

oh! we do have "Documentation/admin-guide/binderfs.rst". I'll add a
brief explanation of these new capabilities files there.

>
> thanks,
>
> greg k-h

thanks,
carlos llamas