Re: [Qemu-devel] [PATCH v3 1/4] Add basic version of bridge helper

[Stefan Hajnoczi]

On Mon, Oct 31, 2011 at 02:36:28PM -0400, Corey Bryant wrote:
A couple of nitpicks regarding error handling:

> +static int has_vnet_hdr(int fd)
> +{
> +    unsigned int features = 0;
> +    struct ifreq ifreq;
> +
> +    if (ioctl(fd, TUNGETFEATURES, &features) == -1) {
> +        return -errno;
> +    }
> +
> +    if (!(features & IFF_VNET_HDR)) {
> +        return -ENOTSUP;
> +    }
> +
> +    if (ioctl(fd, TUNGETIFF, &ifreq) != -1 || errno != EBADFD) {
> +        return -ENOTSUP;
> +    }
> +
> +    return 1;
> +}

This function is strange, it looks like a boolean function but actually
only returns 1 or -errno.  It is used incorrectly in main().  I suggest
changing the return value to bool and returning false on error.

> +    /* open a socket to use to control the network interfaces */
> +    ctlfd = socket(AF_INET, SOCK_STREAM, 0);
> +    if (ctlfd == -1) {
> +        fprintf(stderr, "failed to open control socket\n");
> +        ret = -errno;

It's better to stash away errno before invoking other library functions.
man errno(3) says:

"a function that succeeds is allowed to change errno"

This means fprintf(3) could clobber errno.

I suggest simply printing out errno with the error message and returning
exit code 1 (EXIT_FAILURE).  The same applies for the other error exit
cases in main().

> +cleanup:
> +
> +    close(fd);
> +
> +    close(ctlfd);

ctlfd is an uninitialized variable if opening fd fails.  We also never
close unixfd.

I'd remove this cleanup code and just return without closing any file
descriptors - let the kernel do it.

Stefan