[lxc-devel] [PATCH] hwaddr templating

[Serge Hallyn]

Quoting Kent R. Spillner (kspillner at acm.org):
> On Mon, Jan 06, 2014 at 01:54:14PM +0100, Guillaume ZITTA wrote:
> > This chage introduce also a common randinit() function that could be
> > used to initialize random generator.
> 
> Is there any reason to always prefer libc rand() over /dev/urandom?
> I realize the strength of the random numbers in this particular
> case probably isn't that important but if you want this randinit()
> to be more generally useful then perhaps it makes sense to change a
> few things now:
> 
> > +void randinit(void)
> > +{
> > +    /*
> > +    srand pre-seed function based on /dev/urandom
> > +    */
> > +    FILE *f;
> > +    process_lock();
> > +    f = fopen("/dev/urandom", "r");
> > +    process_unlock();
> > +    if (f) {
> 
> When will this ever fail on Linux?  Does Android provide /dev/urandom?

For one thing, when you're in a nested container and not allowed
to read /dev/urandom :)

> > +        unsigned int seed;
> > +        int ret = fread(&seed, sizeof(seed), 1, f);
> > +        if (ret != 1)
> > +            seed = time(NULL);
> > +        process_lock();
> > +        fclose(f);
> > +        process_unlock();
> > +        srand(seed);
> > +    } else
> > +        srand(time(NULL));
> > +}
> 
> When reading this diff it just felt a little strange that when /dev/urandom
> is working we still only read one byte from it just to seed libc rand().
> What if instead of randinit() you introduced a new function that fills a
> buffer with the requested number of random ints, e.g.:

Do we need to worry about draining the entropy pool?  Keeping in
mind that unprivileged containers are now a reality...

> int lxc_randints(int *buf, size_t count)
> {
>     FILE *f;
> 
>     f = fopen("/dev/urandom", "r");
> 
>     if (f) {
>         int ret;
> 
>         ret = fread(buf, sizeof(int), count, f);
>         /* check ret, handle errors, etc. */
>     } else {
>         srand(time(NULL));
>         do { buf[count] = rand(); } while (count--);
>         /* handle errors, etc. */
>     }
> 
>     return 0;
> }
> 
> And then rand_complete_hwaddr becomes something like:
> 
> static int rand_complete_hwaddr(char *hwaddr)
> {
>     const char hex[] = "0123456789abcdev";
> #define MAC_ADDRESS_HEX_DIGITS 12
>     char buf[MAC_ADDRESS_HEX_DIGITS], *curs = hwaddr;
>     int i = 0;
> 
>     lxc_randints(buf, MAC_ADDRESS_HEX_DIGITS);
> 
>     while (*curs != '\0' && i < MAC_ADDRESS_HEX_DIGITS) {
>         if (*curs == 'x' || *curs == 'X') {
>             if (curs - hwaddr == 1)
>                 *curs = hex[buf[i] & 0x0E];
>             else
>                 *curs = hex[buf[i] & 0x0F];
>         }
> 
>         curs++;
>         i++;
>     }
> 
>     return 0;
> }
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