Re: [PATCH bpf-next v3 1/3] bpf: add destructive kfunc flag

[Kumar Kartikeya Dwivedi]

On Mon, 8 Aug 2022 at 14:41, Artem Savkov <asavkov@xxxxxxxxxx> wrote:
>
> On Mon, Aug 08, 2022 at 02:14:33PM +0200, Kumar Kartikeya Dwivedi wrote:
> > On Mon, 8 Aug 2022 at 11:48, Artem Savkov <asavkov@xxxxxxxxxx> wrote:
> > >
> > > Add KF_DESTRUCTIVE flag for destructive functions. Functions with this
> > > flag set will require CAP_SYS_BOOT capabilities.
> > >
> > > Signed-off-by: Artem Savkov <asavkov@xxxxxxxxxx>
> > > ---
> > >  include/linux/btf.h   | 1 +
> > >  kernel/bpf/verifier.c | 5 +++++
> > >  2 files changed, 6 insertions(+)
> > >
> > > diff --git a/include/linux/btf.h b/include/linux/btf.h
> > > index cdb376d53238..51a0961c84e3 100644
> > > --- a/include/linux/btf.h
> > > +++ b/include/linux/btf.h
> > > @@ -49,6 +49,7 @@
> > >   * for this case.
> > >   */
> > >  #define KF_TRUSTED_ARGS (1 << 4) /* kfunc only takes trusted pointer arguments */
> > > +#define KF_DESTRUCTIVE  (1 << 5) /* kfunc performs destructive actions */
> > >
> >
> > Please also document this flag in Documentation/bpf/kfuncs.rst.
>
> Ok, will do.
>
> > And maybe instead of KF_DESTRUCTIVE, it might be more apt to call this
> > KF_CAP_SYS_BOOT. While it is true you had a destructive flag for
> > programs being loaded earlier, so there was a mapping between the two
> > UAPI and kfunc flags, what it has boiled down to is that this flag
> > just requires CAP_SYS_BOOT (in addition to other capabilities) during
> > load. So that name might express the intent a bit better. We might
> > soon have similar flags encoding requirements of other capabilities on
> > load.
> >
> > The flag rename is just a suggestion, up to you.
>
> This makes sense right now, but if going forward we'll add stricter
> signing requirements or other prerequisites we'll either have to rename
> the flag back, or add those as separate flags. I guess the decision here

IMO we should do that when the time comes, for now it should reflect
the current state.
To me this helper requiring cap_sys_boot is just like how some
existing stable helpers are gated behind bpf_capable or
perfmon_capable.
When it requires that the program calling it be signed, we can revisit this.

> depends on whether some of non-destructive bpf programs might ever require
> CAP_SYS_BOOT capabilities or not.

These are just internal kernel flags, so refactoring/renaming is not a
big deal when it is needed. E.g. we've changed just how kfuncs are
registered twice since the support was added not long ago :).

>
> --
>  Artem
>