create /dev/fakeDevice supporting read, write and ioctl

Question

I have a piece of software running on an embedded device (x86, recent linux). To ease development, use automated tests, etc., I want to run it on my host system. The code compiles just fine with some tweaks to the build-system. The next step would be creating "virtual devices".

The application do not use any kind of library but communicates with several devices directly by read, write and ioctl-calls. The devices represent custom hardware with a custom protocol. To create a virtual environement, I need to respond to this calls. One possible way would be:

• create device drivers for every needed device (/dev/deviceA, /dev/deviceB, /dev/deviceC, ...)
• create another device driver to communicate back to userspace (say, /dev/deviceSimulation)
• all virtual devices will forward every call to /dev/deviceSimulation
• another userspace application interacts with /dev/deviceSimulation and keeps track of the state of the simulation.

Is there an easier way to do this without the roundtrip trough the linux kernel?

Answer 1

Coming back to this side project and answering my own question: Yes, the roundtrip trough the kernel have to be done. But there a library doing the grunt work: CUDA ( thanks, CL).

The documentation feels a bit scarce (or I'm looking at the wrong places) and the provided cuda example is filled with some other stuff fuse-way of handling parameters or buffer handling. So here another example (even more reduced, nothing usefull happens here...):

Device "driver" (cusetest.c):

#define FUSE_USE_VERSION 30
#define _FILE_OFFSET_BITS 64
#include <fuse/cuse_lowlevel.h>
#include <fuse/fuse_opt.h>

#include <stdio.h>

#define LOG(...) do { fprintf(stderr, __VA_ARGS__); puts(""); } while (0)

static void cusetest_open(fuse_req_t req, struct fuse_file_info *fi) {
    LOG("open");
    fuse_reply_open(req, fi);
}

static void cusetest_read(fuse_req_t req, size_t size, off_t off,
                         struct fuse_file_info *fi) {
    LOG("read");
    fuse_reply_buf(req, "Hello", size > 5 ? 5 : size);
}

static void cusetest_write(fuse_req_t req, const char *buf, size_t size,
                          off_t off, struct fuse_file_info *fi) {
    LOG("write (%u bytes)", size);
    fuse_reply_write(req, size);
}

static void cusetest_ioctl(fuse_req_t req, int cmd, void *arg,
                          struct fuse_file_info *fi, unsigned flags,
                          const void *in_buf, size_t in_bufsz, size_t out_bufsz) {
    LOG("ioctl %d: insize: %u outsize: %u", cmd, in_bufsz, out_bufsz);
    switch (cmd) {
    case 23:
        if (in_bufsz == 0) {
            struct iovec iov = { arg, sizeof(int) };
            fuse_reply_ioctl_retry(req, &iov, 1, NULL, 0);
        } else {
            LOG("  got value: %d", *((int*)in_buf));
            fuse_reply_ioctl(req, 0, NULL, 0);
        }
        break;
    case 42:
        if (out_bufsz == 0) {
            struct iovec iov = { arg, sizeof(int) };
            fuse_reply_ioctl_retry(req, NULL, 0, &iov, 1);
        } else {
            LOG("  write back value");
            int v = 42;
            fuse_reply_ioctl(req, 0, &v, sizeof(int));
        }
        break;
    }
}

static const struct cuse_lowlevel_ops cusetest_clop = {
        .open           = cusetest_open,
        .read           = cusetest_read,
        .write          = cusetest_write,
        .ioctl          = cusetest_ioctl,
};

int main(int argc, char** argv) {
    // -f: run in foreground, -d: debug ouput
    // Compile official example and use -h
    const char* cusearg[] = {"test", "-f", "-d"};
    const char* devarg[]  = {"DEVNAME=cusetest" };
    struct cuse_info ci;
    memset(&ci, 0x00, sizeof(ci));
    ci.flags = CUSE_UNRESTRICTED_IOCTL;
    ci.dev_info_argc=1;
    ci.dev_info_argv = devarg;

    return cuse_lowlevel_main(3, (char**) &cusearg, &ci, &cusetest_clop, NULL);
}

Compile and run: gcc -Wall -g -lfuse cusetest.c -o cusetest && sudo ./cusetest

Test program (testcusetest.c):

#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <sys/ioctl.h>

int main() {
    int fd = open("/dev/cusetest", O_RDWR);

    const char* msg = "Fooooo";
    write(fd, msg, strlen(msg));

    int v = 63;
    ioctl(fd, 23, &v);
    fprintf(stderr, "value is now: %d\n", v);
    ioctl(fd, 42, &v);
    fprintf(stderr, "value is now: %d\n", v);
    close(fd);
    return 0;
}

Compile and run: gcc -Wall testcusetest.c -o testcusetest && ./testcusetest

Its much more to do but should be enough to get startet. A pitfall had been the CUSE_UNRESTRICTED_IOCTL. You need to reply with fuse_reply_ioctl_retry(), if the in- or outputbuffer have a zero length. The callback will be called again with the buffer filled in.