dm-crypt: Skip encryption of file system-encrypted blocks

File systems can encrypt some of their data blocks with their own
encryption keys, and for those blocks another round of encryption at
the dm-crypt layer may be redundant, depending on the keys being used.

This patch enables dm-crypt to observe the REQ_NOENCRYPT flag as an
indicator that a bio request should bypass the dm-crypt encryption
queue.

By default dm-crypt will ignore this request flag from the file
system.  The user must set the allow_encrypt_override option to enable
this functionality.  Once the dm-crypt has been used with the
allow_encrypt_override option for any given block device, it must
continue to be used with the option to avoid the possibility of data
corruption.

Change-Id: Ie1b4d40f4e4d96f3349ec8970c56230b2de2de1a
Signed-off-by: Michael Halcrow <mhalcrow@google.com>
Git-repo: https://android.googlesource.com/kernel/common/
Git-commit: f9341c4da4c59ef2cf8247c01abdb4a1586c84cc
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>

drivers/md/dm-crypt.c

@@ -113,7 +113,8 @@ struct iv_tcw_private {
  * and encrypts / decrypts at the same time.
  */
 enum flags { DM_CRYPT_SUSPENDED, DM_CRYPT_KEY_VALID,
-	     DM_CRYPT_SAME_CPU, DM_CRYPT_NO_OFFLOAD };
+	     DM_CRYPT_SAME_CPU, DM_CRYPT_NO_OFFLOAD,
+	     DM_CRYPT_ENCRYPT_OVERRIDE };
 
 /*
  * The fields in here must be read only after initialization.
@@ -1858,6 +1859,9 @@ static int crypt_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 			else if (!strcasecmp(opt_string, "submit_from_crypt_cpus"))
 				set_bit(DM_CRYPT_NO_OFFLOAD, &cc->flags);
 
+			else if (!strcasecmp(opt_string, "allow_encrypt_override"))
+				set_bit(DM_CRYPT_ENCRYPT_OVERRIDE, &cc->flags);
+
 			else {
 				ti->error = "Invalid feature arguments";
 				goto bad;
@@ -1918,12 +1922,15 @@ static int crypt_map(struct dm_target *ti, struct bio *bio)
 	struct crypt_config *cc = ti->private;
 
 	/*
-	 * If bio is REQ_PREFLUSH or REQ_OP_DISCARD, just bypass crypt queues.
+	 * If bio is REQ_PREFLUSH, REQ_NOENCRYPT, or REQ_OP_DISCARD,
+	 * just bypass crypt queues.
 	 * - for REQ_PREFLUSH device-mapper core ensures that no IO is in-flight
 	 * - for REQ_OP_DISCARD caller must use flush if IO ordering matters
 	 */
-	if (unlikely(bio->bi_opf & REQ_PREFLUSH ||
-	    bio_op(bio) == REQ_OP_DISCARD)) {
+	if (unlikely(bio->bi_opf & REQ_PREFLUSH) ||
+	    (unlikely(bio->bi_opf & REQ_NOENCRYPT) &&
+	     test_bit(DM_CRYPT_ENCRYPT_OVERRIDE, &cc->flags)) ||
+	    bio_op(bio) == REQ_OP_DISCARD) {
 		bio->bi_bdev = cc->dev->bdev;
 		if (bio_sectors(bio))
 			bio->bi_iter.bi_sector = cc->start +
@@ -1978,6 +1985,7 @@ static void crypt_status(struct dm_target *ti, status_type_t type,
 		num_feature_args += !!ti->num_discard_bios;
 		num_feature_args += test_bit(DM_CRYPT_SAME_CPU, &cc->flags);
 		num_feature_args += test_bit(DM_CRYPT_NO_OFFLOAD, &cc->flags);
+		num_feature_args += test_bit(DM_CRYPT_ENCRYPT_OVERRIDE, &cc->flags);
 		if (num_feature_args) {
 			DMEMIT(" %d", num_feature_args);
 			if (ti->num_discard_bios)
@@ -1986,6 +1994,8 @@ static void crypt_status(struct dm_target *ti, status_type_t type,
 				DMEMIT(" same_cpu_crypt");
 			if (test_bit(DM_CRYPT_NO_OFFLOAD, &cc->flags))
 				DMEMIT(" submit_from_crypt_cpus");
+			if (test_bit(DM_CRYPT_ENCRYPT_OVERRIDE, &cc->flags))
+				DMEMIT(" allow_encrypt_override");
 		}
 
 		break;