diff --git a/qemu-img.c b/qemu-img.c
index 034122eba5..028c34a2cc 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3244,70 +3244,58 @@ static int img_rebase(int argc, char **argv)
      * the image is the same as the original one at any time.
      */
     if (!unsafe) {
-        int64_t num_sectors;
-        int64_t old_backing_num_sectors;
-        int64_t new_backing_num_sectors = 0;
-        uint64_t sector;
-        int n;
-        int64_t count;
+        int64_t size;
+        int64_t old_backing_size;
+        int64_t new_backing_size = 0;
+        uint64_t offset;
+        int64_t n;
         float local_progress = 0;

         buf_old = blk_blockalign(blk, IO_BUF_SIZE);
         buf_new = blk_blockalign(blk, IO_BUF_SIZE);

-        num_sectors = blk_nb_sectors(blk);
-        if (num_sectors < 0) {
+        size = blk_getlength(blk);
+        if (size < 0) {
             error_report("Could not get size of '%s': %s",
-                         filename, strerror(-num_sectors));
+                         filename, strerror(-size));
             ret = -1;
             goto out;
         }
-        old_backing_num_sectors = blk_nb_sectors(blk_old_backing);
-        if (old_backing_num_sectors < 0) {
+        old_backing_size = blk_getlength(blk_old_backing);
+        if (old_backing_size < 0) {
             char backing_name[PATH_MAX];

             bdrv_get_backing_filename(bs, backing_name, sizeof(backing_name));
             error_report("Could not get size of '%s': %s",
-                         backing_name, strerror(-old_backing_num_sectors));
+                         backing_name, strerror(-old_backing_size));
             ret = -1;
             goto out;
         }
         if (blk_new_backing) {
-            new_backing_num_sectors = blk_nb_sectors(blk_new_backing);
-            if (new_backing_num_sectors < 0) {
+            new_backing_size = blk_getlength(blk_new_backing);
+            if (new_backing_size < 0) {
                 error_report("Could not get size of '%s': %s",
-                             out_baseimg, strerror(-new_backing_num_sectors));
+                             out_baseimg, strerror(-new_backing_size));
                 ret = -1;
                 goto out;
             }
         }

-        if (num_sectors != 0) {
-            local_progress = (float)100 /
-                (num_sectors / MIN(num_sectors, IO_BUF_SIZE / 512));
+        if (size != 0) {
+            local_progress = (float)100 / (size / MIN(size, IO_BUF_SIZE));
         }

-        for (sector = 0; sector < num_sectors; sector += n) {
-
-            /* How many sectors can we handle with the next read? */
-            if (sector + (IO_BUF_SIZE / 512) <= num_sectors) {
-                n = (IO_BUF_SIZE / 512);
-            } else {
-                n = num_sectors - sector;
-            }
+        for (offset = 0; offset < size; offset += n) {
+            /* How many bytes can we handle with the next read? */
+            n = MIN(IO_BUF_SIZE, size - offset);

             /* If the cluster is allocated, we don't need to take action */
-            ret = bdrv_is_allocated(bs, sector << BDRV_SECTOR_BITS,
-                                    n << BDRV_SECTOR_BITS, &count);
+            ret = bdrv_is_allocated(bs, offset, n, &n);
             if (ret < 0) {
                 error_report("error while reading image metadata: %s",
                              strerror(-ret));
                 goto out;
             }
-            /* TODO relax this once bdrv_is_allocated does not enforce
-             * sector alignment */
-            assert(QEMU_IS_ALIGNED(count, BDRV_SECTOR_SIZE));
-            n = count >> BDRV_SECTOR_BITS;
             if (ret) {
                 continue;
             }
@@ -3316,30 +3304,28 @@ static int img_rebase(int argc, char **argv)
              * Read old and new backing file and take into consideration that
              * backing files may be smaller than the COW image.
              */
-            if (sector >= old_backing_num_sectors) {
-                memset(buf_old, 0, n * BDRV_SECTOR_SIZE);
+            if (offset >= old_backing_size) {
+                memset(buf_old, 0, n);
             } else {
-                if (sector + n > old_backing_num_sectors) {
-                    n = old_backing_num_sectors - sector;
+                if (offset + n > old_backing_size) {
+                    n = old_backing_size - offset;
                 }

-                ret = blk_pread(blk_old_backing, sector << BDRV_SECTOR_BITS,
-                                buf_old, n << BDRV_SECTOR_BITS);
+                ret = blk_pread(blk_old_backing, offset, buf_old, n);
                 if (ret < 0) {
                     error_report("error while reading from old backing file");
                     goto out;
                 }
             }

-            if (sector >= new_backing_num_sectors || !blk_new_backing) {
-                memset(buf_new, 0, n * BDRV_SECTOR_SIZE);
+            if (offset >= new_backing_size || !blk_new_backing) {
+                memset(buf_new, 0, n);
             } else {
-                if (sector + n > new_backing_num_sectors) {
-                    n = new_backing_num_sectors - sector;
+                if (offset + n > new_backing_size) {
+                    n = new_backing_size - offset;
                 }

-                ret = blk_pread(blk_new_backing, sector << BDRV_SECTOR_BITS,
-                                buf_new, n << BDRV_SECTOR_BITS);
+                ret = blk_pread(blk_new_backing, offset, buf_new, n);
                 if (ret < 0) {
                     error_report("error while reading from new backing file");
                     goto out;
@@ -3349,15 +3335,13 @@ static int img_rebase(int argc, char **argv)
             /* If they differ, we need to write to the COW file */
             uint64_t written = 0;

-            while (written < n * BDRV_SECTOR_SIZE) {
+            while (written < n) {
                 int64_t pnum;

-                if (compare_buffers(buf_old + written,
-                                    buf_new + written,
-                                    n * BDRV_SECTOR_SIZE - written, &pnum))
+                if (compare_buffers(buf_old + written, buf_new + written,
+                                    n - written, &pnum))
                 {
-                    ret = blk_pwrite(blk,
-                                     (sector << BDRV_SECTOR_BITS) + written,
+                    ret = blk_pwrite(blk, offset + written,
                                      buf_old + written, pnum, 0);
                     if (ret < 0) {
                         error_report("Error while writing to COW image: %s",
