netdev - Re: Ask for help about bpf map

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Date:   Tue, 13 Jul 2021 09:16:57 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     "luwei (O)" <luwei32@...wei.com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        David Ahern <dahern@...italocean.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: Ask for help about bpf map

On Mon, Jul 12, 2021 at 11:35 PM luwei (O) <luwei32@...wei.com> wrote:
>
> Hi, List:
>
>        I am a beginner about bpf and working on XDP now. I meet a
> problem and feel difficult to figure it out.
>
>        In my following codes, I use two ways to define my_map: in SEC
> maps and SEC .maps respectively. When I load the xdp_kern.o file,
>
> It has different results. The way I load is: ip link set dev ens3 xdp
> obj xdp1_kern.o sec xdp1.
>
>        when I define my_map using SEC maps, it loads successfully but
> fails to load using SEC .maps, it reports:
>
> "
>
> [12] TYPEDEF __u32 type_id=13
> [13] INT unsigned int size=4 bits_offset=0 nr_bits=32 encoding=(none)
> [14] FUNC_PROTO (anon) return=2 args=(10 ctx)
> [15] FUNC xdp_prog1 type_id=14
> [16] INT char size=1 bits_offset=0 nr_bits=8 encoding=SIGNED
> [17] ARRAY (anon) type_id=16 index_type_id=4 nr_elems=4
> [18] VAR _license type_id=17 linkage=1
> [19] DATASEC .maps size=0 vlen=1 size == 0
>
>
> Prog section 'xdp1' rejected: Permission denied (13)!
>   - Type:         6
>   - Instructions: 9 (0 over limit)
>   - License:      GPL
>
> Verifier analysis:
>
> 0: (b7) r1 = 0
> 1: (63) *(u32 *)(r10 -4) = r1
> last_idx 1 first_idx 0
> regs=2 stack=0 before 0: (b7) r1 = 0
> 2: (bf) r2 = r10
> 3: (07) r2 += -4
> 4: (18) r1 = 0x0

this shouldn't be 0x0.

I suspect you have an old iproute2 which doesn't yet use libbpf to
load BPF programs, so .maps definition is not yet supported. cc'ing
netdev@...r, David and Toke

> 6: (85) call bpf_map_lookup_elem#1
> R1 type=inv expected=map_ptr
> processed 6 insns (limit 1000000) max_states_per_insn 0 total_states 0
> peak_states 0 mark_read 0
>
> Error fetching program/map!
>
> "
>
> I tried to search google, and only found the following page:
>
> https://stackoverflow.com/questions/67553794/what-is-variable-attribute-sec-means
>
> Does anyone meet the same problem or help to explain this or provide me
> some suggestions ? Thanks !
>
>
> === xdp1_kern.c ===
>
> #define KBUILD_MODNAME "foo"
> #include <uapi/linux/bpf.h>
> #include <linux/time.h>
> #include <linux/in.h>
> #include <linux/if_ether.h>
> #include <linux/if_packet.h>
> #include <linux/if_vlan.h>
> #include <linux/ip.h>
> #include <linux/ipv6.h>
> #include <bpf/bpf_helpers.h>
>
> struct {
>      __uint(type, BPF_MAP_TYPE_HASH);
>      __uint(max_entries, 1024);
>      __type(key, int);
>      __type(value, int);
> } my_map SEC(".maps");
>
> #if 0
> #define PIN_GLOBAL_NS           2
> struct bpf_elf_map {
>          __u32 type;
>          __u32 size_key;
>          __u32 size_value;
>          __u32 max_elem;
>          __u32 flags;
>          __u32 id;
>          __u32 pinning;
> };
>
> struct bpf_elf_map SEC("maps") my_map = {
>          .type = BPF_MAP_TYPE_HASH,
>          .size_key = sizeof(int),
>          .size_value = sizeof(int),
>          .pinning        = PIN_GLOBAL_NS,
>          .max_elem = 65535,
> };
> #endif
>
> SEC("xdp1")
> int xdp_prog1(struct xdp_md *ctx)
> {
>      int key = 0;
>      struct map_elem *val;
>
>      val = bpf_map_lookup_elem(&my_map, &key);
>      if (val) {
>          return XDP_PASS;
>      }
>
>      return XDP_PASS;
> }
>
> char _license[] SEC("license") = "GPL";
>
> --
> Best Regards,
> Lu Wei
>