On Tue, Jul 2, 2019 at 3:47 PM Maxim Mikityanskiy <maximmi@xxxxxxxxxxxx> wrote:
>
> On 2019-07-02 12:21, Magnus Karlsson wrote:
> >
> > +/* XDP_RING flags */
> > +#define XDP_RING_NEED_WAKEUP (1 << 0)
> > +
> > struct xdp_ring_offset {
> > __u64 producer;
> > __u64 consumer;
> > __u64 desc;
> > + __u64 flags;
> > };
> >
> > struct xdp_mmap_offsets {
>
> <snip>
>
> > @@ -621,9 +692,12 @@ static int xsk_getsockopt(struct socket *sock, int level, int optname,
> > case XDP_MMAP_OFFSETS:
> > {
> > struct xdp_mmap_offsets off;
> > + bool flags_supported = true;
> >
> > - if (len < sizeof(off))
> > + if (len < sizeof(off) - sizeof(off.rx.flags))
> > return -EINVAL;
> > + else if (len < sizeof(off))
> > + flags_supported = false;
> >
> > off.rx.producer = offsetof(struct xdp_rxtx_ring, ptrs.producer);
> > off.rx.consumer = offsetof(struct xdp_rxtx_ring, ptrs.consumer);
> > @@ -638,6 +712,16 @@ static int xsk_getsockopt(struct socket *sock, int level, int optname,
> > off.cr.producer = offsetof(struct xdp_umem_ring, ptrs.producer);
> > off.cr.consumer = offsetof(struct xdp_umem_ring, ptrs.consumer);
> > off.cr.desc = offsetof(struct xdp_umem_ring, desc);
> > + if (flags_supported) {
> > + off.rx.flags = offsetof(struct xdp_rxtx_ring,
> > + ptrs.flags);
> > + off.tx.flags = offsetof(struct xdp_rxtx_ring,
> > + ptrs.flags);
> > + off.fr.flags = offsetof(struct xdp_umem_ring,
> > + ptrs.flags);
> > + off.cr.flags = offsetof(struct xdp_umem_ring,
> > + ptrs.flags);
> > + }
>
> As far as I understood (correct me if I'm wrong), you are trying to
> preserve backward compatibility, so that if userspace doesn't support
> the flags field, you will determine that by looking at len and fall back
> to the old format.
That was the intention yes.
> However, two things are broken here:
>
> 1. The check `len < sizeof(off) - sizeof(off.rx.flags)` should be `len <
> sizeof(off) - 4 * sizeof(flags)`, because struct xdp_mmap_offsets
> consists of 4 structs xdp_ring_offset.
>
> 2. The old and new formats are not binary compatible, as flags are
> inserted in the middle of struct xdp_mmap_offsets.
You are correct. Since there are four copies of the xdp_ring_offset
this simple scheme will not work. I will instead create an internal
version 1 of the struct that I fill in and pass to user space if I
detect that user space is asking for the v1 size.
Thanks for catching Maxim. Keep'em coming.
/Magnus
