[11/17] crypto: ansi_cprng - unroll _get_more_prng_bytes

Commit Message

George Spelvin Dec. 2, 2014, 8:54 a.m. UTC

It's more legible, and the code is 15 bytes smaller (i386).

Signed-off-by: George Spelvin <[e-mail]>
---
 crypto/ansi_cprng.c | 87 ++++++++++++++++++++---------------------------------
 1 file changed, 32 insertions(+), 55 deletions(-)

I'm not really sure why this was implemented this convoluted way
in the first place.  Did crypto_cipher_encrypt_one() used to be
an enormous inline function?

Comments

George Spelvin Dec. 2, 2014, 9:54 p.m. UTC | #1

The order of the v1 patches is somewhat in order of "increasing scale",
reflecting my learning about the code.  But if this unroll is okay,
it would probably make the most sense to reorder things so it's
first and then other changes can be made on top of the simpler code.

Given the unusual implementation choice of a loop and a switch, it's
obviously a deliberate one, so I assume there's a reason for it that
I'm just not seeing.
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Patch

diff --git a/crypto/ansi_cprng.c b/crypto/ansi_cprng.c
index de13e741..09bb1252 100644
--- a/crypto/ansi_cprng.c
+++ b/crypto/ansi_cprng.c
@@ -94,67 +94,44 @@  static int _get_more_prng_bytes(struct prng_context *ctx, bool cont_test)
 	hexdump("Input V: ", ctx->V, DEFAULT_BLK_SZ);
 
 	/*
-	 * This algorithm is a 3 stage state machine
+	 * Start by encrypting the counter value.
+	 * This gives us an intermediate value I.
 	 */
-	for (i = 0; i < 3; i++) {
-		unsigned char const *input;
-		unsigned char *output;
+	crypto_cipher_encrypt_one(ctx->tfm, tmp, ctx->DT);
+	hexdump("input I: ", tmp, DEFAULT_BLK_SZ);
 
-		switch (i) {
-		case 0:
-			/*
-			 * Start by encrypting the counter value
-			 * This gives us an intermediate value I
-			 */
-			input = ctx->DT;
-			output = tmp;
-			hexdump("input stage 0: ", input, DEFAULT_BLK_SZ);
-			break;
-		case 1:
-			/*
-			 * Next xor I with our secret vector V.
-			 * Encrypt that result to obtain our pseudo random
-			 * data which we output.  It is kept temporarily
-			 * in (no longer used) V until we have done the
-			 * anti-repetition compare.
-			 */
-			xor_block(tmp, ctx->V);
-			input = output = ctx->V;
-			hexdump("input stage 1: ", input, DEFAULT_BLK_SZ);
-			break;
-		case 2:
-			/*
-			 * First check that we didn't produce the same
-			 * random data that we did last time around.
-			 */
-			if (!memcmp(ctx->V, ctx->rand_data, DEFAULT_BLK_SZ)) {
-				if (cont_test) {
-					panic("cprng %p Failed repetition check!\n",
-						ctx);
-				}
-
-				printk(KERN_ERR
-					"ctx %p Failed repetition check!\n",
-					ctx);
-
-				ctx->flags |= PRNG_NEED_RESET;
-				return -EINVAL;
-			}
-			memcpy(ctx->rand_data, ctx->V, DEFAULT_BLK_SZ);
+	/*
+	 * Next xor I with our secret vector V.
+	 * Encrypt that result to obtain our pseudo random data which
+	 * we output.  It is kept temporarily in (no longer used)
+	 * V until we have done the anti-repetition compare.
+	 */
+	xor_block(tmp, ctx->V);
+	hexdump("input stage 1: ", ctx->V, DEFAULT_BLK_SZ);
+	crypto_cipher_encrypt_one(ctx->tfm, ctx->V, ctx->V);
 
-			/*
-			 * Lastly xor the random data with I
-			 * and encrypt that to obtain a new secret vector V
-			 */
-			xor_block(tmp, ctx->V);
-			input = output = ctx->V;
-			hexdump("input stage 2: ", input, DEFAULT_BLK_SZ);
-			break;
+	/*
+	 * Check that we didn't produce the same random data
+	 * that we did last time around.
+	 */
+	if (!memcmp(ctx->V, ctx->rand_data, DEFAULT_BLK_SZ)) {
+		if (cont_test) {
+			panic("cprng %p Failed repetition check!\n", ctx);
 		}
 
-		/* do the encryption */
-		crypto_cipher_encrypt_one(ctx->tfm, output, input);
+		printk(KERN_ERR "ctx %p Failed repetition check!\n", ctx);
+		ctx->flags |= PRNG_NEED_RESET;
+		return -EINVAL;
 	}
+	memcpy(ctx->rand_data, ctx->V, DEFAULT_BLK_SZ);
+
+	/*
+	 * Lastly xor the random data with I and encrypt that to
+	 * obtain a new secret vector V
+	 */
+	xor_block(tmp, ctx->V);
+	hexdump("input stage 2: ", ctx->V, DEFAULT_BLK_SZ);
+	crypto_cipher_encrypt_one(ctx->tfm, ctx->V, ctx->V);
 
 	/*
 	 * Now update our DT value