crypto: skcipher - clean up initialization of skcipher_walk::flags

- Initialize SKCIPHER_WALK_SLEEP in a consistent way, and check for
  atomic=true at the same time as CRYPTO_TFM_REQ_MAY_SLEEP.  Technically
  atomic=true only needs to apply after the first step, but it is very
  rarely used.  We should optimize for the common case.  So, check
  'atomic' alongside CRYPTO_TFM_REQ_MAY_SLEEP.  This is more efficient.

- Initialize flags other than SKCIPHER_WALK_SLEEP to 0 rather than
  preserving them.  No caller actually initializes the flags, which
  makes it impossible to use their original values for anything.
  Indeed, that does not happen and all meaningful flags get overridden
  anyway.  It may have been thought that just clearing one flag would be
  faster than clearing all flags, but that's not the case as the former
  is a read-write operation whereas the latter is just a write.

- Move the explicit clearing of SKCIPHER_WALK_SLOW, SKCIPHER_WALK_COPY,
  and SKCIPHER_WALK_DIFF into skcipher_walk_done(), since it is now
  only needed on non-first steps.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/skcipher.c

@@ -148,6 +148,8 @@ unmap_src:
 	if (total) {
 		crypto_yield(walk->flags & SKCIPHER_WALK_SLEEP ?
 			     CRYPTO_TFM_REQ_MAY_SLEEP : 0);
+		walk->flags &= ~(SKCIPHER_WALK_SLOW | SKCIPHER_WALK_COPY |
+				 SKCIPHER_WALK_DIFF);
 		return skcipher_walk_next(walk);
 	}
 
@@ -235,9 +237,6 @@ static int skcipher_walk_next(struct skcipher_walk *walk)
 	unsigned int bsize;
 	unsigned int n;
 
-	walk->flags &= ~(SKCIPHER_WALK_SLOW | SKCIPHER_WALK_COPY |
-			 SKCIPHER_WALK_DIFF);
-
 	n = walk->total;
 	bsize = min(walk->stride, max(n, walk->blocksize));
 	n = scatterwalk_clamp(&walk->in, n);
@@ -311,7 +310,6 @@ int skcipher_walk_virt(struct skcipher_walk *walk,
 {
 	struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
 	struct skcipher_alg *alg = crypto_skcipher_alg(tfm);
-	int err = 0;
 
 	might_sleep_if(req->base.flags & CRYPTO_TFM_REQ_MAY_SLEEP);
 
@@ -319,17 +317,17 @@ int skcipher_walk_virt(struct skcipher_walk *walk,
 	walk->nbytes = 0;
 	walk->iv = req->iv;
 	walk->oiv = req->iv;
+	if ((req->base.flags & CRYPTO_TFM_REQ_MAY_SLEEP) && !atomic)
+		walk->flags = SKCIPHER_WALK_SLEEP;
+	else
+		walk->flags = 0;
 
 	if (unlikely(!walk->total))
-		goto out;
+		return 0;
 
 	scatterwalk_start(&walk->in, req->src);
 	scatterwalk_start(&walk->out, req->dst);
 
-	walk->flags &= ~SKCIPHER_WALK_SLEEP;
-	walk->flags |= req->base.flags & CRYPTO_TFM_REQ_MAY_SLEEP ?
-		       SKCIPHER_WALK_SLEEP : 0;
-
 	walk->blocksize = crypto_skcipher_blocksize(tfm);
 	walk->ivsize = crypto_skcipher_ivsize(tfm);
 	walk->alignmask = crypto_skcipher_alignmask(tfm);
@@ -339,11 +337,7 @@ int skcipher_walk_virt(struct skcipher_walk *walk,
 	else
 		walk->stride = alg->walksize;
 
-	err = skcipher_walk_first(walk);
+	return skcipher_walk_first(walk);
-out:
-	walk->flags &= atomic ? ~SKCIPHER_WALK_SLEEP : ~0;
-
-	return err;
 }
 EXPORT_SYMBOL_GPL(skcipher_walk_virt);
 
@@ -351,11 +345,14 @@ static int skcipher_walk_aead_common(struct skcipher_walk *walk,
 				     struct aead_request *req, bool atomic)
 {
 	struct crypto_aead *tfm = crypto_aead_reqtfm(req);
-	int err;
 
 	walk->nbytes = 0;
 	walk->iv = req->iv;
 	walk->oiv = req->iv;
+	if ((req->base.flags & CRYPTO_TFM_REQ_MAY_SLEEP) && !atomic)
+		walk->flags = SKCIPHER_WALK_SLEEP;
+	else
+		walk->flags = 0;
 
 	if (unlikely(!walk->total))
 		return 0;
@@ -369,22 +366,12 @@ static int skcipher_walk_aead_common(struct skcipher_walk *walk,
 	scatterwalk_done(&walk->in, 0, walk->total);
 	scatterwalk_done(&walk->out, 0, walk->total);
 
-	if (req->base.flags & CRYPTO_TFM_REQ_MAY_SLEEP)
-		walk->flags |= SKCIPHER_WALK_SLEEP;
-	else
-		walk->flags &= ~SKCIPHER_WALK_SLEEP;
-
 	walk->blocksize = crypto_aead_blocksize(tfm);
 	walk->stride = crypto_aead_chunksize(tfm);
 	walk->ivsize = crypto_aead_ivsize(tfm);
 	walk->alignmask = crypto_aead_alignmask(tfm);
 
-	err = skcipher_walk_first(walk);
+	return skcipher_walk_first(walk);
-
-	if (atomic)
-		walk->flags &= ~SKCIPHER_WALK_SLEEP;
-
-	return err;
 }
 
 int skcipher_walk_aead_encrypt(struct skcipher_walk *walk,