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Libbpf eBPF macro bpf_ntohl

v0.0.6

The bpf_ntohl macro is used to convert a 32-bit number from network byte order to host byte order.

Definition

#define ___bpf_mvb(x, b, n, m) ((__u##b)(x) << (b-(n+1)*8) >> (b-8) << (m*8))

#define ___bpf_swab32(x) ((__u32)(          \
              ___bpf_mvb(x, 32, 0, 3) | \
              ___bpf_mvb(x, 32, 1, 2) | \
              ___bpf_mvb(x, 32, 2, 1) | \
              ___bpf_mvb(x, 32, 3, 0)))

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
# define __bpf_ntohl(x)         __builtin_bswap32(x)
# define __bpf_constant_ntohl(x)    ___bpf_swab32(x)
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
# define __bpf_ntohl(x)         (x)
# define __bpf_constant_ntohl(x)    (x)
#else
# error "Fix your compiler's __BYTE_ORDER__?!"
#endif

#define bpf_ntohl(x)                \
    (__builtin_constant_p(x) ?      \
     __bpf_constant_ntohl(x) : __bpf_ntohl(x))

Usage

This macro implements the analog of the ntonl function from the standard C library. ntonl being short for "network to host long", converts a 32-bit number (a long) from network byte order to host byte order.

The implementation checks the endianness of the host system and if the number is a compile time constant or not. If the endianness of the system we are compiling on is already in network order, the macro simply returns the number as is. Otherwise if conversion is needed, and the number is a compile time constant, the conversion is done at compile time. If the number is not a compile time constant, a compiler builtin is used to emit byte swap instructions.

Example

Only allow a socket to bind to 192.168.1.254 and port 4040.

#define SERV4_IP        0xc0a801feU /* 192.168.1.254 */
#define SERV4_PORT      4040

SEC("cgroup/bind4")
int bind_v4_prog(struct bpf_sock_addr *ctx)
{
    struct bpf_sock *sk;

    sk = ctx->sk;
    if (!sk)
        return 0;

    if (sk->family != AF_INET)
        return 0;

    if (bpf_ntohl(ctx->user_ip4) != SERV4_IP ||
        bpf_ntohs(ctx->user_port) != SERV4_PORT)
        return 0;

    return 1;
}