netdev - Re: [QUESTION] bpf, iproute2/tc: verifier fails because of a map

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Message-ID: <CAADnVQL87R07UM3prVPwVmz_e2+uuO67QmXJxXqECgjt3S=54w@mail.gmail.com>
Date:   Tue, 7 Feb 2023 10:31:37 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Justin Iurman <justin.iurman@...ege.be>
Cc:     bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [QUESTION] bpf, iproute2/tc: verifier fails because of a map

On Tue, Feb 7, 2023 at 3:09 AM Justin Iurman <justin.iurman@...ege.be> wrote:
>
> Hello,
>
> CC'ing netdev as well, since I initially suspected an issue in iproute2.
> However, after having recompiled iproute2 with libbpf, I'm still stuck
> facing the same problem.
>
> Environment:
>   - OS: Ubuntu 20.04.5 LTS
>   - kernel: 5.4.0-137-generic x86_64 (CONFIG_DEBUG_INFO_BTF=y)
>   - clang version 10.0.0-4ubuntu1
>   - iproute2-6.1.0, libbpf 1.2.0 (iproute2-ss200127 installed by default
> without libbpf)
>
> Note: same result with kernel 6.2.0-rc6+ (net-next), as a test to be
> aligned with latest iproute2 version (just in case).
>
> Long story short: I can't for the life of me make the ebpf program load
> correctly with tc. What's the cause? Well, a map, and the verifier
> doesn't like it. I must definitely be doing something wrong, but can't
> find what. Here is a reproducible and minimal example:
>
> #include "vmlinux.h"
> #include <bpf/bpf_helpers.h>
>
> #define TC_ACT_OK 0
> #define MAX_BYTES 2048
>
> char _license[] SEC("license") = "GPL";
>
> struct mystruct_t {
>         __u8 bytes[MAX_BYTES];
> };
>
> struct {
>         __uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
>         __uint(key_size, sizeof(__u8));
>         __uint(value_size, sizeof(struct mystruct_t));
>         __uint(max_entries, 1);
> } percpu_map SEC(".maps");
>
> SEC("egress")
> int xxx(struct __sk_buff *skb)
> {
>         __u8 idx = 0;
>         struct mystruct_t *x = bpf_map_lookup_elem(&percpu_map, &idx);
>         return TC_ACT_OK;
> }
>
>
> Here is how I compile the whole thing:
>
> git clone --recursive --depth 1 https://github.com/libbpf/libbpf
> ./deps/libbpf
> make -j -C deps/libbpf/src/ BUILD_STATIC_ONLY=y DESTDIR="build"
> INCLUDEDIR= LIBDIR= UAPIDIR= install
>
> git clone --recursive --depth 1 https://github.com/libbpf/bpftool
> ./deps/bpftool
> make -j -C deps/bpftool/src/
> deps/bpftool/src/bpftool btf dump file /sys/kernel/btf/vmlinux format c
>  > build/vmlinux.h
>
> clang -g -O2 -Wall -Wextra -target bpf -D__TARGET_ARCH_x86_64 -I build/
> -c program.c -o build/program.o
>
>
> I noticed that "clang-bpf-co-re" is OFF when compiling bpftool, don't
> know if it's part of the problem or not. Here is what the "build"
> directory looks like after that:
>
> $ ls -al build
> [...]
> drwxr-xr-x 2 justin justin    4096 fév  6 18:20 bpf
> -rw-rw-r-- 1 justin justin 3936504 fév  6 18:20 libbpf.a
> drwxr-xr-x 2 justin justin    4096 fév  6 18:20 pkgconfig
> -rw-rw-r-- 1 justin justin   10592 fév  7 10:42 program.o
> -rw-rw-r-- 1 justin justin 2467774 fév  7 10:42 vmlinux.h
>
>
> And here is the verifier error I got when loading it with tc (qdisc
> clsact already attached):
>
> $ sudo ../deps/iproute2/tc/tc filter add dev eno2 egress bpf da obj
> program.o sec egress
>
> libbpf: map 'percpu_map': failed to create: Invalid argument(-22)
> libbpf: failed to load object 'program.o'
> Unable to load program

It's likely due to
__uint(key_size, sizeof(__u8));

https://docs.kernel.org/bpf/map_array.html
"The key type is an unsigned 32-bit integer (4 bytes) and the map is
of constant size."