Re: [kvm-unit-tests PATCH v3 1/7] lib: Add pseudo random functions

["Nicholas Piggin" <npiggin@gmail.com>]

On Fri Jun 21, 2024 at 12:16 AM AEST, Nina Schoetterl-Glausch wrote:
> Add functions for generating pseudo random 32 and 64 bit values.
> The implementation uses SHA-256 and so the randomness should have good
> quality.
> Implement the necessary subset of SHA-256.
> The PRNG algorithm is equivalent to the following python snippet:
>
> def prng32(seed):
>     from hashlib import sha256
>     state = seed.to_bytes(8, byteorder="big")
>     while True:
>         state = sha256(state).digest()
>         for i in range(8):
>             yield int.from_bytes(state[i*4:(i+1)*4], byteorder="big")
>

Nice, could use this for powerpc radom SPR value tests (and
probably other things too).

> Acked-by: Andrew Jones <andrew.jones@linux.dev>
> Signed-off-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
> ---
>
> Notes:
>     Since a PRNG with better quality was asked for I decided to use SHA-256
>     because:
>      * it is a standard, commonly used algorithm
>      * high quality randomness is assured
>      * the implementation can be checked against the spec
>      * the implementation can be easily checked via comparison
>     
>     I tested the implementation in the following way:
>     
>     cat <<'EOF' > rand.py
>     #!/usr/bin/python3
>     
>     def prng32(seed):
>         from hashlib import sha256
>         state = seed.to_bytes(8, byteorder="big")
>         while True:
>             state = sha256(state).digest()
>             for i in range(8):
>                 yield int.from_bytes(state[i*4:(i+1)*4], byteorder="big")
>     
>     r = prng32(0)
>     for i in range(100):
>         print(f"{next(r):08x}")
>     
>     EOF
>     
>     cat <<'EOF' > rand.c
>     #include <stdio.h>
>     #include "rand.h"
>     
>     void main(void)
>     {
>     	prng_state state = prng_init(0);
>     	for (int i = 0; i < 100; i++) {
>     		printf("%08x\n", prng32(&state));
>     	}
>     }
>     EOF
>     cat <<'EOF' > libcflat.h
>     #define ARRAY_SIZE(_a) (sizeof(_a)/sizeof((_a)[0]))
>     EOF
>     chmod +x rand.py
>     ln -s lib/rand.c librand.c
>     gcc -Ilib librand.c rand.c
>     diff <(./a.out) <(./rand.py)

Cool... you made a unit test for the unit tests. We could start a
make check? :)

Acked-by: Nicholas Piggin <npiggin@gmail.com>

Thanks,
Nick