netdev - Re: [PATCH bpf 1/2] xsk: fix to reject invalid flags in xsk

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Date:   Fri, 8 Mar 2019 12:06:23 +0100
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Maciej Fijalkowski <maciejromanfijalkowski@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Netdev <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf 1/2] xsk: fix to reject invalid flags in xsk_bind

On Fri, 8 Mar 2019 at 11:46, Maciej Fijalkowski
<maciejromanfijalkowski@...il.com> wrote:
>
[...]
> > > > +     flags = sxdp->sxdp_flags;
> > > > +     if (flags & ~(XDP_SHARED_UMEM | XDP_COPY | XDP_ZEROCOPY))
> > > > +             return -EINVAL;
> > > > +
> > >
> > > What about setting more than one flag at a time? Is it allowed/make any sense?
> > > After a quick look it seems that they exclude each other, e.g. you can't force
> > > a zero copy and copy mode at the same time. And for XDP_SHARED_UMEM two
> > > remaining flags can't be set.
> > >
> > > So maybe check here also that only one particular flag is set by doing:
> > >
> > > if (hweight32(flags & (XDP_SHARED_UMEM | XDP_COPY | XDP_ZEROCOPY)) > 1)
> > >         return -EINVAL;
> > >
> > > just like we do it for IFLA_XDP_FLAGS in net/core/rtnetlink.c?
> > >
> >
> > We have flag semantic checks further down, and my rational was to
> > *only* check unknown flags first. IMO the current patch is easier to
> > understand, than your suggested one.
> >
>
> Hmm thought that bailing out earlier would be better and we could drop the
> actual copy flags checks for shared umem. For xdp_umem_assign_dev() instead of
> passing flags you could just pass a boolean whether you're doing zero copy or
> not. And that brings up the question whether we really need a XDP_COPY flag?
>

I'd prefer doing that as a follow-up patch.

XDP_COPY is needed to explicitly enable copy-mode. No flags is "select
the best option", and COPY/ZEROCOPY is to explicitly select a mode.