On Wed, Mar 24, 2021 at 3:46 PM Ciara Loftus <ciara.loftus@xxxxxxxxx> 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")
>
> Signed-off-by: Ciara Loftus <ciara.loftus@xxxxxxxxx>
> ---
> 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 ec3c23299329..1f1c4c11c292 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -904,24 +904,22 @@ 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));
Thanks Ciara!
This is a pragmatic solution, but I do not see any better way around
it since these operations are irreversible. And it works without any
fix to the kernel which is good and you have a comment explaining
things clearly. With that said, it would be nice as a follow up to
bpf-next to actually return a unique error value (among the ones that
this function can return) when the rings have already been mapped.
This way the user can react to this in a more informed way in the
future.
Acked-by: Magnus Karlsson <magnus.karlsson@xxxxxxxxx>
>
> err = xsk_get_mmap_offsets(xsk->fd, &off);
> if (err) {
> --
> 2.17.1
>
