seedrng: import SeedRNG utility for kernel RNG seed files

The RNG can't actually be seeded from a shell script, due to the
reliance on ioctls and the fact that entropy written into the
unprivileged /dev/urandom device is not immediately mixed in, making
subsequent seed reads dangerous. For this reason, the seedrng project
provides a basic "C script" meant to be copy and pasted into projects
like Busybox and tweaked as needed: <https://git.zx2c4.com/seedrng/about/>.

The SeedRNG construction has been part of systemd's seeder since
January, and recently was added to Android, OpenRC, and Void's Runit,
with more integrations on their way depending on context. Virtually
every single Busybox-based distro I have seen seeds things in wrong,
incomplete, or otherwise dangerous way. For example, fixing this issue
in Buildroot requires first for Busybox to have this fix.

This commit imports it into Busybox and wires up the basic config. The
utility itself is tiny, and unlike the example code from the SeedRNG
project, we can re-use libbb's existing hash functions, rather than
having to ship a standalone BLAKE2s, which makes this even smaller.

function                                             old     new   delta
seedrng_main                                           -    1463   +1463
.rodata                                           107858  108665    +807
seed_from_file_if_exists                               -     697    +697
packed_usage                                       34414   34519    +105
static.longopts                                        -      38     +38
static.seedrng_prefix                                  -      26     +26
seed_dir                                               -       8      +8
non_creditable_seed                                    -       8      +8
lock_file                                              -       8      +8
creditable_seed                                        -       8      +8
applet_names                                        2747    2755      +8
applet_main                                         3192    3200      +8
------------------------------------------------------------------------------
(add/remove: 9/0 grow/shrink: 4/0 up/down: 3184/0)           Total: 3184 bytes
   text	   data	    bss	    dec	    hex	filename
 973776	   4219	   1816	 979811	  ef363	busybox_old
 977035	   4227	   1848	 983110	  f0046	busybox_unstripped

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

0 files changed, 0 insertions, 0 deletions