How to use kernel libcrc32c (or same functions) in userspace programmes?

Question

I want to do some CRC check in my own userspace programme. And I find that the kernel crypto lib is already in the system, and come with SSE4.2 support.

I tried to directly #include <linux/crc32c.h> and run gcc with -I/usr/src/linux/include/. However, it doesnot work.

Any way to use some kind of libcrc32c ?

Answer 1

You can use kernel crypto CRC32c (and other hash/cipher functions) from user-space via socket family AF_ALG on Linux:

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <sys/socket.h>
#include <linux/if_alg.h>
#include <sys/param.h>
#include <string.h>
#include <strings.h>

int
main (int argc, char **argv) {

    int sds[2] = { -1, -1 };

    struct sockaddr_alg sa = {
        .salg_family = AF_ALG,
        .salg_type   = "hash",
        .salg_name   = "crc32c"
    };

    if ((sds[0] = socket(AF_ALG, SOCK_SEQPACKET, 0)) == -1 )
        return -1;

    if( bind(sds[0], (struct sockaddr *) &sa, sizeof(sa)) != 0 )
        return -1;

    if( (sds[1] = accept(sds[0], NULL, 0)) == -1 )
        return -1;

    char *s = "hello";
    size_t n = strlen(s);
    if (send(sds[1], s, n, MSG_MORE) != n)
        return -1;

    int crc32c = 0x00000000;
    if(read(sds[1], &crc32c, 4) != 4)
        return -1;

    printf("%08X\n", crc32c);
    return 0;
}

If you're hashing files or socket data you can speed it up using zero-copy approach to avoid kernel -> user-space buffer copy with sendfile and/or splice.

Happy coding.