[bug report] lwt: Fix return values of BPF xmit ops

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

Hello Yan Zhai,

The patch 29b22badb7a8: "lwt: Fix return values of BPF xmit ops" from
Aug 17, 2023 (linux-next), leads to the following Smatch static
checker warning:

	net/core/lwt_bpf.c:131 bpf_input()
	error: double free of 'skb'

net/core/lwt_bpf.c
    38  static int run_lwt_bpf(struct sk_buff *skb, struct bpf_lwt_prog *lwt,
    39                         struct dst_entry *dst, bool can_redirect)
    40  {
    41          int ret;
    42  
    43          /* Migration disable and BH disable are needed to protect per-cpu
    44           * redirect_info between BPF prog and skb_do_redirect().
    45           */
    46          migrate_disable();
    47          local_bh_disable();
    48          bpf_compute_data_pointers(skb);
    49          ret = bpf_prog_run_save_cb(lwt->prog, skb);
    50  
    51          switch (ret) {
    52          case BPF_OK:
    53          case BPF_LWT_REROUTE:
    54                  break;
    55  
    56          case BPF_REDIRECT:
    57                  if (unlikely(!can_redirect)) {
    58                          pr_warn_once("Illegal redirect return code in prog %s\n",
    59                                       lwt->name ? : "<unknown>");
    60                          ret = BPF_OK;
    61                  } else {
    62                          skb_reset_mac_header(skb);
    63                          skb_do_redirect(skb);
    64                          ret = BPF_REDIRECT;

If skb_do_redirect() returns -EINVAL it means the skb has been freed.
Originally we preserved error code but now we just return BPF_REDIRECT.

    65                  }
    66                  break;
    67  
    68          case BPF_DROP:
    69                  kfree_skb(skb);
    70                  ret = -EPERM;
    71                  break;

regards,
dan carpenter