Quoting Kent R. Spillner (kspill...@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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