[bug report] bpf: Add bpf_dynptr_slice and bpf_dynptr_slice_rdwr

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

Hello Joanne Koong,

The patch 66e3a13e7c2c: "bpf: Add bpf_dynptr_slice and
bpf_dynptr_slice_rdwr" from Mar 1, 2023 (linux-next), leads to the
following Smatch static checker warning:

	tools/testing/selftests/bpf/progs/test_cls_redirect_dynptr.c:403 forward_with_gre()
	error: 'encap_gre' dereferencing possible ERR_PTR()

tools/testing/selftests/bpf/progs/test_cls_redirect_dynptr.c
    396 
    397         encap_gre = bpf_dynptr_slice_rdwr(dynptr, 0, encap_buffer, sizeof(encap_buffer));
    398         if (!encap_gre) {
    399                 metrics->errors_total_encap_buffer_too_small++;
    400                 return TC_ACT_SHOT;
    401         }
    402 
--> 403         encap_gre->ip.protocol = IPPROTO_GRE;
                ^^^^^^^^^^^

The bpf_dynptr_slice() function accidentally propagates error pointers
from bpf_xdp_pointer() so it would crash here.


    404         encap_gre->ip.daddr = next_hop->s_addr;
    405         encap_gre->ip.saddr = ENCAPSULATION_IP;
    406         encap_gre->ip.tot_len =
    407                 bpf_htons(bpf_ntohs(encap_gre->ip.tot_len) + delta);
    408         encap_gre->gre.flags = 0;
    409         encap_gre->gre.protocol = bpf_htons(proto);
    410         pkt_ipv4_checksum((void *)&encap_gre->ip);
    411 
    412         if (encap_gre == encap_buffer)
    413                 bpf_dynptr_write(dynptr, 0, encap_buffer, sizeof(encap_buffer), 0);
    414 
    415         return bpf_redirect(skb->ifindex, 0);
    416 }

regards,
dan carpenter