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

[Carlos Llamas]

On Fri, Jul 9, 2021 at 2:56 AM Christian Brauner
<christian.brauner@xxxxxxxxxx> 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
>
> Hey Carlos,
>
> The model will be one file per feature?

Yes. I dropped a previous single bitmask file idea per Greg's suggestion.
The file per feature improves on a number of areas such as feature count
limit, readability and it's easier to manage (add/remove features).

>
> Instead of calling the directory "caps" should this maybe be called
> "features"? I'm not fuzzed about it and if you want to keep "caps"
> that's fine. The term is just a bit overused and makes me think of other
> things than this.

I have no problems switching over to "features".

>
> > in the first place. Note that older capabilities are assumed
> > to be supported and only new ones will be added.
>
> What if you ever want to deprecate one? :)

If the file for a feature doesn't exist then such feature is not supported.
So we can avoid creating such file if a feature were to be deprecated.

>
> >
> > Signed-off-by: Carlos Llamas <cmllamas@xxxxxxxxxx>
> > ---
> >  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,
>
> I know this is the oneway spam _detection_ feature but this file makes
> it sound like the binder driver has the capability to generate one-way
> spam. :) Maybe name at least name the file "oneway_spam_detection".

That's true. I'll rename it as suggested.

>
> > +};
> > +
> >  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)
>
> You can drop the goto here and just always return directly.

I also noticed this and I decided to keep it consistent with init_binder_logs()
structure. But I don't have a strong preference so I'll switch to
early returns.

>
> > +{
> > +     struct dentry *dentry, *root;
>
> Please name this "dir" instead of "root". "root" is conventionally used
> for sb->s_root and especially here in this file I only ever used it to
> indicate s_root.

ok, sounds good.

>
> > +     int ret = 0;
> > +
> > +     root = binderfs_create_dir(sb->s_root, "caps");
> > +     if (IS_ERR(root)) {
> > +             ret = PTR_ERR(root);
>
>         return 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);
>
>         return PTR_ERR(root);
>
> > +             goto out;
> > +     }
> > +
> > +out:
> > +     return ret;
> > +}
> > +
> >  static int init_binder_logs(struct super_block *sb)
> >  {
> >       struct dentry *binder_logs_root_dir, *dentry, *proc_log_dir;
> > @@ -723,6 +764,10 @@ static int binderfs_fill_super(struct super_block *sb, struct fs_context *fc)
> >                       name++;
> >       }
> >
> > +     ret = init_binder_caps(sb);
> > +     if (ret)
> > +             return ret;
> > +
> >       if (info->mount_opts.stats_mode == binderfs_stats_mode_global)
> >               return init_binder_logs(sb);
> >
> > --
> > 2.32.0.93.g670b81a890-goog
> >

thanks,
carlos llamas