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

[Kumar Kartikeya Dwivedi]

On Tue, 9 Aug 2022 at 02:37, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Mon, Aug 8, 2022 at 6:33 AM Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> wrote:
> >
> > 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.
>
> But names should be also semantically meaningful, so KF_DESTRUCTIVE
> explains that kfunc can do destructive operations, which is better
> than just declaring that kfunc needs CAP_SYS_BOOT, as the latter is
> current implementation detail which has no bearing on kfunc definition
> itself.
>
> Unless we anticipate we'll have another "destructive" kfunc not using
> KF_DESTRUCTIVE and instead we'll add some other
> KF_CAP_SYS_WHATEVERELSE?
>

I just found it a bit odd that KF_DESTRUCTIVE would require
CAP_SYS_BOOT. When thinking about what one would write in the docs:
just that KF_DESTRUCTIVE kfuncs can do destructive operations? That
doesn't really capture what the flag ends up doing to the kfunc (it
limits use to those who have a certain cap on program load). There can
be several destructive operations (e.g. a frequently mentioned socket
kill helper that may be considered equally destructive for some
workload) but would probably require CAP_NET_ADMIN instead.

But anyway, I didn't really want to bikeshed over this :), we can give
it a better name next time something like this is added, and just go
with KF_DESTRUCTIVE for now.

> > 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
> > >