UDP Failures and RNGs

| categories: fedora

Upgrades to a new kernel inevitably break something. 4.7 managed to break a Ruby unit test based on a bug report. A test to send an empty UDP packet was now timing out. I've never worked with Ruby much but the test itself was easy enough to get a general idea what was going on. I grabbed a copy of the ruby source on a rawhide machine and did a mockbuild

# fedpkg co -a ruby
# cd ruby
# dnf builddep ruby.spec
# fedpkg mockbuild

and saw that it was failing in the same way as the report. This is a very large amount of work for what amounts to just one failure and I still wanted to rule out other problems besides the kernel. Fortunately, it could easily be narrowed down to a smaller test case:

$ cat weird_ruby.rb
#!/usr/bin/env ruby

  require "socket"
  require "io/nonblock"
  require "io/wait"
rescue LoadErrror

  def test_udp_recvfrom_nonblock
    u1 = UDPSocket.new
    u2 = UDPSocket.new
    u1.bind("", 0)
    u2.send("", 0, u1.getsockname)
    IO.select [u1]
    u1.close if u1
    u2.close if u2


Ruby devs are probably cringing at the style but the important part is that this was a test case that worked on my 4.6 based machine and failed on the 4.7 based machine. This was a great candidate for bisection. Because I'm lazy and don't like rebooting machines, I used my buildroot setup (I had to recompile my buildroot image to put Ruby in it).

The first bisect in buildroot was useless, it gave me 4.6 as the first bad commit which I knew to be false. I tried 4.5. Still bad. I knew the test case was definitely passing on other environments in 4.6. strace is (still) my favorite userspace tool for 'opaque program, what are you doing' so I decided to give it a try:

brk(0x2560000)                          = 0x2560000
brk(0x255c000)                          = 0x255c000
brk(0x2580000)                          = 0x2580000
brk(0x257c000)                          = 0x257c000
brk(0x25a0000)                          = 0x25a0000
brk(0x259c000)                          = 0x259c000
clock_gettime(CLOCK_PROCESS_CPUTIME_ID, {0, 1882167}) = 0
brk(0x25c1000)                          = 0x25c1000

getrandom waitwhathuh?

# cat /proc/1084/stack
[<ffffffff813f3ac8>] SyS_getrandom+0xd8/0x140
[<ffffffff818e99f2>] entry_SYSCALL_64_fastpath+0x1a/0xa4
[<ffffffffffffffff>] 0xffffffffffffffff

The corresponding code showed it waiting on an event for nonblocking_pool.initialized. So the Ruby environment was internally trying to get a random number but the nonblocking pool wasn't yet initialized. The buildroot environment I run is very minimalist, there isn't much in the way of devices or drivers. The buildroot init system writes a set of random data to initialize the RNG but the RNG code doesn't actually update the calculation of entropy. This means that another call to add entropy must update the calculation. Usually the system uses things like device interrupts or input timing to generate entropy. In a minimal system like buildroot, most of these aren't present. The most reliable source of entropy is typing on the keyboard and, yes, if I keyboard enough the pool will eventually initialize. I should probably put a big blinking HACK sign here before I state my solution. In the buildroot init file, right before the write to the RNG with random data I did

for i in `seq 0 1024`; do

The idea ws to spawn a bunch of different processes in hopes that it would give a bump of entropy to the system. It did indeed work. This is not endorsed as a solution for anything other than 'let me run my Ruby program plx'. I'm certain there are better ideas out there. It's worth noting this limitation is no longer present on newer kernels. There has been some reworking of the random number generator to mitigate problems like these.

Once the Ruby problem was fixed, I could actually bisect and found a very promising and relevant commit. Reporting a kernel bug with a Ruby program still leaves a lot open to wonder about, especially after hitting unexpected problems with the RNG. When submitting kernel problems, it's best to submit C when possible. So with the help of strace (again!) to see what exactly Ruby was doing, I ported the Ruby code to some roughly corresponding C code:

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <string.h>
#include <netdb.h>
#include <errno.h>

int main()
    int fd1, fd2;
    struct sockaddr_in addr1;
    unsigned int len1;
    int ret;
    fd_set rfds;


    if (fd1 < 0 || fd2 < 0) {
        printf("socket fail");

    len1 = sizeof(addr1);

    memset(&addr1, 0, sizeof(addr1));
    addr1.sin_family = AF_INET;
    addr1.sin_addr.s_addr = inet_addr("");
    addr1.sin_port = htons(0);
    ret = bind(fd1, (struct sockaddr *)&addr1, len1);
    if (ret < 0) {
        printf("bind fail %d\n", errno);

    ret = getsockname(fd1, (struct sockaddr *)&addr1, &len1);
    if (ret < 0) {
        printf("getsockname failed %d\n", errno);
    ret = sendto(fd2, "", 0, 0, (struct sockaddr *)&addr1, len1);
    if (ret < 0) {
        printf("sendto failed %d\n", errno);

    FD_SET(fd1, &rfds);
    select(fd1+1, &rfds, NULL, NULL, NULL);

Again, not the prettiest code but it also showed the problem in the same way as the Ruby code. Once I submitted the bug, the turn around time was very fast and the bug fix got merged. There was still some discussion about whether the test case is valid but it sounds like it is supposed to work.

Lessons here: reduce your test cases, bisection is still fantastic, and hacks are what you get for being lazy.