dm-flakey: make corrupting read bios work

JIRA: https://issues.redhat.com/browse/RHEL-119009
Upstream Status: kernel/git/torvalds/linux.git

commit 13e7907
Author: Benjamin Marzinski <bmarzins@redhat.com>
Date:   Tue Apr 22 19:47:38 2025 -0400

    dm-flakey: make corrupting read bios work

    dm-flakey corrupts the read bios in the endio function.  However, the
    corrupt_bio_* functions checked bio_has_data() to see if there was data
    to corrupt. Since this was the endio function, there was no data left to
    complete, so bio_has_data() was always false. Fix this by saving a copy
    of the bio's bi_iter in flakey_map(), and using this to initialize the
    iter for corrupting the read bios. This patch also skips cloning the bio
    for write bios with no data.

    Reported-by: Kent Overstreet <kent.overstreet@linux.dev>
    Fixes: a399879 ("dm flakey: add corrupt_bio_byte feature")
    Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
    Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>

Author: bmarzins

drivers/md/dm-flakey.c

@@ -47,7 +47,8 @@ enum feature_flag_bits {
 };
 
 struct per_bio_data {
-	bool bio_submitted;
+	bool bio_can_corrupt;
+	struct bvec_iter saved_iter;
 };
 
 static int parse_features(struct dm_arg_set *as, struct flakey_c *fc,
@@ -354,7 +355,8 @@ static void flakey_map_bio(struct dm_target *ti, struct bio *bio)
 }
 
 static void corrupt_bio_common(struct bio *bio, unsigned int corrupt_bio_byte,
-			       unsigned char corrupt_bio_value)
+			       unsigned char corrupt_bio_value,
+			       struct bvec_iter start)
 {
 	struct bvec_iter iter;
 	struct bio_vec bvec;
@@ -363,7 +365,7 @@ static void corrupt_bio_common(struct bio *bio, unsigned int corrupt_bio_byte,
 	 * Overwrite the Nth byte of the bio's data, on whichever page
 	 * it falls.
 	 */
-	bio_for_each_segment(bvec, bio, iter) {
+	__bio_for_each_segment(bvec, bio, iter, start) {
 		if (bio_iter_len(bio, iter) > corrupt_bio_byte) {
 			unsigned char *segment = bvec_kmap_local(&bvec);
 			segment[corrupt_bio_byte] = corrupt_bio_value;
@@ -372,36 +374,31 @@ static void corrupt_bio_common(struct bio *bio, unsigned int corrupt_bio_byte,
 				"(rw=%c bi_opf=%u bi_sector=%llu size=%u)\n",
 				bio, corrupt_bio_value, corrupt_bio_byte,
 				(bio_data_dir(bio) == WRITE) ? 'w' : 'r', bio->bi_opf,
-				(unsigned long long)bio->bi_iter.bi_sector,
-				bio->bi_iter.bi_size);
+				(unsigned long long)start.bi_sector,
+				start.bi_size);
 			break;
 		}
 		corrupt_bio_byte -= bio_iter_len(bio, iter);
 	}
 }
 
-static void corrupt_bio_data(struct bio *bio, struct flakey_c *fc)
+static void corrupt_bio_data(struct bio *bio, struct flakey_c *fc,
+			     struct bvec_iter start)
 {
 	unsigned int corrupt_bio_byte = fc->corrupt_bio_byte - 1;
 
-	if (!bio_has_data(bio))
-		return;
-
-	corrupt_bio_common(bio, corrupt_bio_byte, fc->corrupt_bio_value);
+	corrupt_bio_common(bio, corrupt_bio_byte, fc->corrupt_bio_value, start);
 }
 
-static void corrupt_bio_random(struct bio *bio)
+static void corrupt_bio_random(struct bio *bio, struct bvec_iter start)
 {
 	unsigned int corrupt_byte;
 	unsigned char corrupt_value;
 
-	if (!bio_has_data(bio))
-		return;
-
-	corrupt_byte = get_random_u32() % bio->bi_iter.bi_size;
+	corrupt_byte = get_random_u32() % start.bi_size;
 	corrupt_value = get_random_u8();
 
-	corrupt_bio_common(bio, corrupt_byte, corrupt_value);
+	corrupt_bio_common(bio, corrupt_byte, corrupt_value, start);
 }
 
 static void clone_free(struct bio *clone)
@@ -496,7 +493,7 @@ static int flakey_map(struct dm_target *ti, struct bio *bio)
 	unsigned int elapsed;
 	struct per_bio_data *pb = dm_per_bio_data(bio, sizeof(struct per_bio_data));
 
-	pb->bio_submitted = false;
+	pb->bio_can_corrupt = false;
 
 	if (op_is_zone_mgmt(bio_op(bio)))
 		goto map_bio;
@@ -505,10 +502,11 @@ static int flakey_map(struct dm_target *ti, struct bio *bio)
 	elapsed = (jiffies - fc->start_time) / HZ;
 	if (elapsed % (fc->up_interval + fc->down_interval) >= fc->up_interval) {
 		bool corrupt_fixed, corrupt_random;
-		/*
-		 * Flag this bio as submitted while down.
-		 */
-		pb->bio_submitted = true;
+
+		if (bio_has_data(bio)) {
+			pb->bio_can_corrupt = true;
+			pb->saved_iter = bio->bi_iter;
+		}
 
 		/*
 		 * If ERROR_READS isn't set flakey_end_io() will decide if the
@@ -531,6 +529,8 @@ static int flakey_map(struct dm_target *ti, struct bio *bio)
 			return DM_MAPIO_SUBMITTED;
 		}
 
+		if (!pb->bio_can_corrupt)
+			goto map_bio;
 		/*
 		 * Corrupt matching writes.
 		 */
@@ -550,9 +550,11 @@ static int flakey_map(struct dm_target *ti, struct bio *bio)
 			struct bio *clone = clone_bio(ti, fc, bio);
 			if (clone) {
 				if (corrupt_fixed)
-					corrupt_bio_data(clone, fc);
+					corrupt_bio_data(clone, fc,
+							 clone->bi_iter);
 				if (corrupt_random)
-					corrupt_bio_random(clone);
+					corrupt_bio_random(clone,
+							   clone->bi_iter);
 				submit_bio(clone);
 				return DM_MAPIO_SUBMITTED;
 			}
@@ -574,21 +576,21 @@ static int flakey_end_io(struct dm_target *ti, struct bio *bio,
 	if (op_is_zone_mgmt(bio_op(bio)))
 		return DM_ENDIO_DONE;
 
-	if (!*error && pb->bio_submitted && (bio_data_dir(bio) == READ)) {
+	if (!*error && pb->bio_can_corrupt && (bio_data_dir(bio) == READ)) {
 		if (fc->corrupt_bio_byte) {
 			if ((fc->corrupt_bio_rw == READ) &&
 			    all_corrupt_bio_flags_match(bio, fc)) {
 				/*
 				 * Corrupt successful matching READs while in down state.
 				 */
-				corrupt_bio_data(bio, fc);
+				corrupt_bio_data(bio, fc, pb->saved_iter);
 			}
 		}
 		if (fc->random_read_corrupt) {
 			u64 rnd = get_random_u64();
 			u32 rem = do_div(rnd, PROBABILITY_BASE);
 			if (rem < fc->random_read_corrupt)
-				corrupt_bio_random(bio);
+				corrupt_bio_random(bio, pb->saved_iter);
 		}
 	}