netdev - Re: [PATCH v2 bpf 3/3] libbpf: ignore return values of setsockopt for XDP rings.

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Date:   Mon, 29 Mar 2021 08:28:57 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     "Loftus, Ciara" <ciara.loftus@...el.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "bjorn@...nel.org" <bjorn@...nel.org>,
        "magnus.karlsson@...il.com" <magnus.karlsson@...il.com>
Subject: Re: [PATCH v2 bpf 3/3] libbpf: ignore return values of setsockopt for
 XDP rings.

On Mon, Mar 29, 2021 at 1:41 AM Loftus, Ciara <ciara.loftus@...el.com> wrote:
>
> >
> > 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 <magnus.karlsson@...el.com>
> > > Signed-off-by: Ciara Loftus <ciara.loftus@...el.com>
> > > ---
> > >  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.