Subject: RE: [PATCH v2 bpf 3/3] libbpf: ignore return values of setsockopt for XDP rings.

From: "Loftus, Ciara" <[email-masked]>
Date: Tue, 30 Mar 2021 12:04:46 +0000

> >
> > >
> > > On Fri, Mar 26, 2021 at 02:29:46PM +0000, Ciara Loftus wrote:
> > > > During xsk_socket__create the XDP_RX_RING and XDP_TX_RING
> > > setsockopts
> > > > are called to create the rx and tx rings for the AF_XDP socket. If the ring
> > > > has already been set up, the setsockopt will return an error. However,
> > > > in the event of a failure during xsk_socket__create(_shared) after the
> > > > rings have been set up, the user may wish to retry the socket creation
> > > > using these pre-existing rings. In this case we can ignore the error
> > > > returned by the setsockopts. If there is a true error, the subsequent
> > > > call to mmap() will catch it.
> > > >
> > > > Fixes: 1cad07884239 ("libbpf: add support for using AF_XDP sockets")
> > > >
> > > > Acked-by: Magnus Karlsson <[email-masked]>
> > > > Signed-off-by: Ciara Loftus <[email-masked]>
> > > > ---
> > > >  tools/lib/bpf/xsk.c | 34 ++++++++++++++++------------------
> > > >  1 file changed, 16 insertions(+), 18 deletions(-)
> > > >
> > > > diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> > > > index d4991ddff05a..cfc4abf505c3 100644
> > > > --- a/tools/lib/bpf/xsk.c
> > > > +++ b/tools/lib/bpf/xsk.c
> > > > @@ -900,24 +900,22 @@ int xsk_socket__create_shared(struct
> xsk_socket
> > > **xsk_ptr,
> > > >     }
> > > >     xsk->ctx = ctx;
> > > >
> > > > -   if (rx) {
> > > > -           err = setsockopt(xsk->fd, SOL_XDP, XDP_RX_RING,
> > > > -                            &xsk->config.rx_size,
> > > > -                            sizeof(xsk->config.rx_size));
> > > > -           if (err) {
> > > > -                   err = -errno;
> > > > -                   goto out_put_ctx;
> > > > -           }
> > > > -   }
> > > > -   if (tx) {
> > > > -           err = setsockopt(xsk->fd, SOL_XDP, XDP_TX_RING,
> > > > -                            &xsk->config.tx_size,
> > > > -                            sizeof(xsk->config.tx_size));
> > > > -           if (err) {
> > > > -                   err = -errno;
> > > > -                   goto out_put_ctx;
> > > > -           }
> > > > -   }
> > > > +   /* The return values of these setsockopt calls are intentionally not
> > > checked.
> > > > +    * If the ring has already been set up setsockopt will return an error.
> > > However,
> > > > +    * this scenario is acceptable as the user may be retrying the socket
> > > creation
> > > > +    * with rings which were set up in a previous but ultimately
> > > unsuccessful call
> > > > +    * to xsk_socket__create(_shared). The call later to mmap() will fail if
> > > there
> > > > +    * is a real issue and we handle that return value appropriately there.
> > > > +    */
> > > > +   if (rx)
> > > > +           setsockopt(xsk->fd, SOL_XDP, XDP_RX_RING,
> > > > +                      &xsk->config.rx_size,
> > > > +                      sizeof(xsk->config.rx_size));
> > > > +
> > > > +   if (tx)
> > > > +           setsockopt(xsk->fd, SOL_XDP, XDP_TX_RING,
> > > > +                      &xsk->config.tx_size,
> > > > +                      sizeof(xsk->config.tx_size));
> > >
> > > Instead of ignoring the error can you remember that setsockopt was
> done
> > > in struct xsk_socket and don't do it the second time?
> >
> > Ideally we don't have to ignore the error. However in the event of failure
> struct xsk_socket is freed at the end of xsk_socket__create so we can't use it
> to remember state between subsequent calls to __create().
> 
> but umem is not, right? and fd is taken from there.

Yes, got it. We can add a new field to struct xsk_umem. It's much better than ignoring the return values.
I'll add this in the v3. Thanks for your suggestion!

Ciara