Given that this code is new, let's add a selftest for it as well. It doesn't rely on fixed sets, instead it picks 1024 numbers and verifies that they're not more correlated than desired.
Link: https://lore.kernel.org/netdev/20200808152628.ga27...@sdf.org/ Cc: George Spelvin <l...@sdf.org> Cc: Amit Klein <aksecur...@gmail.com> Cc: Eric Dumazet <eduma...@google.com> Cc: "Jason A. Donenfeld" <ja...@zx2c4.com> Cc: Andy Lutomirski <l...@kernel.org> Cc: Kees Cook <keesc...@chromium.org> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: ty...@mit.edu Cc: Florian Westphal <f...@strlen.de> Cc: Marc Plumb <lkml.mpl...@gmail.com> Signed-off-by: Willy Tarreau <w...@1wt.eu> --- lib/random32.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 56 insertions(+) diff --git a/lib/random32.c b/lib/random32.c index 7f047844e494..4d0e05e471d7 100644 --- a/lib/random32.c +++ b/lib/random32.c @@ -38,6 +38,7 @@ #include <linux/jiffies.h> #include <linux/random.h> #include <linux/sched.h> +#include <linux/bitops.h> #include <asm/unaligned.h> #include <trace/events/random.h> @@ -556,6 +557,61 @@ static void prandom_timer_start(struct random_ready_callback *unused) mod_timer(&seed_timer, jiffies); } +#ifdef CONFIG_RANDOM32_SELFTEST +/* Principle: True 32-bit random numbers will all have 16 differing bits on + * average. For each 32-bit number, there are 601M numbers differing by 16 + * bits, and 89% of the numbers differ by at least 12 bits. Note that more + * than 16 differing bits also implies a correlation with inverted bits. Thus + * we take 1024 random numbers and compare each of them to the other ones, + * counting the deviation of correlated bits to 16. Constants report 32, + * counters 32-log2(TEST_SIZE), and pure randoms, around 6 or lower. With the + * u32 total, TEST_SIZE may be as large as 4096 samples. + */ +#define TEST_SIZE 1024 +static int __init prandom32_state_selftest(void) +{ + unsigned int x, y, bits, samples; + u32 xor, flip; + u32 total; + u32 *data; + + data = kmalloc(sizeof(*data) * TEST_SIZE, GFP_KERNEL); + if (!data) + return 0; + + for (samples = 0; samples < TEST_SIZE; samples++) + data[samples] = prandom_u32(); + + flip = total = 0; + for (x = 0; x < samples; x++) { + for (y = 0; y < samples; y++) { + if (x == y) + continue; + xor = data[x] ^ data[y]; + flip |= xor; + bits = hweight32(xor); + total += (bits - 16) * (bits - 16); + } + } + + /* We'll return the average deviation as 2*sqrt(corr/samples), which + * is also sqrt(4*corr/samples) which provides a better resolution. + */ + bits = int_sqrt(total / (samples * (samples - 1)) * 4); + if (bits > 6) + pr_warn("prandom32: self test failed (at least %u bits" + " correlated, fixed_mask=%#x fixed_value=%#x\n", + bits, ~flip, data[0] & ~flip); + else + pr_info("prandom32: self test passed (less than %u bits" + " correlated)\n", + bits+1); + kfree(data); + return 0; +} +core_initcall(prandom32_state_selftest); +#endif /* CONFIG_RANDOM32_SELFTEST */ + /* * Start periodic full reseeding as soon as strong * random numbers are available. -- 2.28.0
