getrandom waits for a long time when /dev/random is insufficiently read from

Nikos Mavrogiannopoulos
Fri, 29 Jul 2016 10:24:27 -0000

On Fri, Jul 29, 2016 at 7:40 AM, Stephan Mueller <smueller at chronox.de>
wrote:
> And finally, you have a coding error that is very very common but fatal
when
> reading from /dev/random: you do not account for short reads which implies
> that your loop continues even in the case of short reads.
>
> Fix your code with something like the following:
> int read_random(char *buf, size_t buflen)
> {
>         int fd = 0;
>         ssize_t ret = 0;
>         size_t len = 0;
>
>         fd = open("/dev/random", O_RDONLY|O_CLOEXEC);
>         if(0 > fd)
>                 return fd;
>         do {
>                 ret = read(fd, (buf + len), (buflen - len));
>                 if (0 < ret)
>                         len += ret;
>         } while ((0 < ret || EINTR == errno || ERESTART == errno)
>                  && buflen > len);

Unless there is a documentation error, the same is required when using
getrandom(). It can also return short as well as to be interrupted.

regards,
Nikos