Libbpf eBPF macro bpf_ksym_exists

v0.2

The bpf_ksym_exists macro is used to check if a __weak symbol has been defined.

Definition

#define bpf_ksym_exists(sym) ({ \
    _Static_assert(!__builtin_constant_p(!!sym), #sym " should be marked as __weak"); \
    !!sym; \
})

Usage

This macro is used to check if a weak symbol has been defined. If the symbol is defined, it is better practice to use this macro to check instead of doing a == 0 check. This is because this macro includes a static assertion to see if the symbol is marked as weak, this can avoid unintended bugs.

Example

extern int bpf_dynptr_from_xdp(struct xdp_md *x, u64 flags, struct bpf_dynptr *ptr__uninit) __weak __ksym;

SEC("xdp.frags")
int example_prog(struct xdp_md *ctx)
{
    if (bpf_ksym_exists(bpf_dynptr_from_xdp)) {
        struct bpf_dynptr ptr;
        if (bpf_dynptr_from_xdp(ctx, 0, &ptr) < 0)
            return XDP_DROP;

        __u8 buf[sizeof(struct ethhdr)];
        struct ethhdr *eth = bpf_dynptr_slice(&ptr, buf, sizeof(buf));
        if (!eth)
            return XDP_DROP;

        if (eth->h_proto == htons(ETH_P_IP))
            return XDP_PASS;
    } else {
        void *data_end = (void *)(long)ctx->data_end;
        void *data = (void *)(long)ctx->data;

        if (data + sizeof(struct ethhdr) > data_end)
            return XDP_DROP;

        struct ethhdr *eth = data;
        if (eth->h_proto == htons(ETH_P_IP))
            return XDP_PASS;
    }
}