[PATCH v6 02/15] crypto: testmgr - generate power-of-2 lengths more often

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.com>

Implementations of hash functions often have special cases when lengths
are a multiple of the hash function's internal block size (e.g. 64 for
SHA-256, 128 for SHA-512).  Currently, when the fuzz testing code
generates lengths, it doesn't prefer any length mod 64 over any other.
This limits the coverage of these special cases.

Therefore, this patch updates the fuzz testing code to generate
power-of-2 lengths and divide messages exactly in half a bit more often.

Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 crypto/testmgr.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/crypto/testmgr.c b/crypto/testmgr.c
index a780b615f8c6..f02cb075bd68 100644
--- a/crypto/testmgr.c
+++ b/crypto/testmgr.c
@@ -914,18 +914,24 @@ static unsigned int generate_random_length(struct rnd_state *rng,
 {
 	unsigned int len = prandom_u32_below(rng, max_len + 1);
 
 	switch (prandom_u32_below(rng, 4)) {
 	case 0:
-		return len % 64;
+		len %= 64;
+		break;
 	case 1:
-		return len % 256;
+		len %= 256;
+		break;
 	case 2:
-		return len % 1024;
+		len %= 1024;
+		break;
 	default:
-		return len;
+		break;
 	}
+	if (len && prandom_u32_below(rng, 4) == 0)
+		len = rounddown_pow_of_two(len);
+	return len;
 }
 
 /* Flip a random bit in the given nonempty data buffer */
 static void flip_random_bit(struct rnd_state *rng, u8 *buf, size_t size)
 {
@@ -1017,10 +1023,12 @@ static char *generate_random_sgl_divisions(struct rnd_state *rng,
 		unsigned int this_len;
 		const char *flushtype_str;
 
 		if (div == &divs[max_divs - 1] || prandom_bool(rng))
 			this_len = remaining;
+		else if (prandom_u32_below(rng, 4) == 0)
+			this_len = (remaining + 1) / 2;
 		else
 			this_len = prandom_u32_inclusive(rng, 1, remaining);
 		div->proportion_of_total = this_len;
 
 		if (prandom_u32_below(rng, 4) == 0)
-- 
2.45.2