bootutil: swap-move: Erase secondary trailer during swap

When using the swap-move strategy, the secondary trailer was erased just
after the primary trailer, when moving the sectors up in the primary
slot. This was working because it was assumed the trailer is stored in
dedicated sectors, so no sector could contain both trailer and firmware
data. However, if this assumption is relaxed, it means it is no more
possible to erase the secondary trailer before the last sector holding
firmware data has been copied to the primary slot, since that sector
might also contain trailer data. So the erasure of the secondary trailer
has to occur at the time the last sector containing firmware data is
swapped.

Signed-off-by: Thomas Altenbach <thomas.altenbach@legrand.com>

Author: taltenbach

boot/bootutil/src/swap_move.c

@@ -360,9 +360,14 @@ swap_status_source(struct boot_loader_state *state)
     BOOT_LOG_SWAP_STATE("Secondary image", &state_secondary_slot);
 
     if (state_primary_slot.magic == BOOT_MAGIC_GOOD &&
-            state_primary_slot.copy_done == BOOT_FLAG_UNSET &&
-            state_secondary_slot.magic != BOOT_MAGIC_GOOD) {
-
+            state_primary_slot.copy_done == BOOT_FLAG_UNSET) {
+        /* In this case, either:
+         *   - A swap operation was interrupted and can be resumed from the status stored in the
+         *     primary slot's trailer.
+         *   - No swap was ever made and the initial firmware image has been written with a MCUboot
+         *     trailer. In this case, the status in the primary slot's trailer is empty and there is
+         *     no harm in loading it.
+         */
         source = BOOT_STATUS_SOURCE_PRIMARY_SLOT;
 
         BOOT_LOG_INF("Boot source: primary slot");
@@ -378,8 +383,7 @@ swap_status_source(struct boot_loader_state *state)
  */
 static void
 boot_move_sector_up(size_t idx, uint32_t sz, struct boot_loader_state *state,
-        struct boot_status *bs, const struct flash_area *fap_pri,
-        const struct flash_area *fap_sec)
+        struct boot_status *bs, const struct flash_area *fap_pri)
 {
     uint32_t new_off;
     uint32_t old_off;
@@ -423,12 +427,6 @@ boot_move_sector_up(size_t idx, uint32_t sz, struct boot_loader_state *state,
             copy_sz = bs->swap_size % sz;
             sector_erased_with_trailer = true;
         }
-
-        /* Remove status from secondary slot trailer, in case of device with
-	 * erase requirement this will also prepare traier for write.
-	 */
-        rc = swap_scramble_trailer_sectors(state, fap_sec);
-        assert(rc == 0);
     }
 
     if (!sector_erased_with_trailer) {
@@ -446,7 +444,7 @@ boot_move_sector_up(size_t idx, uint32_t sz, struct boot_loader_state *state,
 }
 
 static void
-boot_swap_sectors(int idx, uint32_t sz, struct boot_loader_state *state,
+boot_swap_sectors(size_t idx, size_t last_idx, uint32_t sz, struct boot_loader_state *state,
         struct boot_status *bs, const struct flash_area *fap_pri,
         const struct flash_area *fap_sec)
 {
@@ -472,10 +470,43 @@ boot_swap_sectors(int idx, uint32_t sz, struct boot_loader_state *state,
     }
 
     if (bs->state == BOOT_STATUS_STATE_1) {
-        rc = boot_erase_region(fap_sec, sec_off, sz);
-        assert(rc == 0);
+        bool sector_erased_with_trailer = false;
+        uint32_t copy_sz = sz;
+
+        if (idx == last_idx) {
+            rc = swap_scramble_trailer_sectors(state, fap_sec);
+            assert(rc == 0);
+
+            size_t first_trailer_sector_pri =
+                boot_get_first_trailer_sector(state, BOOT_PRIMARY_SLOT);
+            size_t first_trailer_sector_sec =
+                boot_get_first_trailer_sector(state, BOOT_SECONDARY_SLOT);
+
+            if (first_trailer_sector_sec == idx - 1) {
+                /* The destination sector was containing part of the trailer and has therefore
+                 * already been erased.
+                 */
+                sector_erased_with_trailer = true;
+            }
+
+            if (first_trailer_sector_pri == idx) {
+                /* The source sector contains both firmware and trailer data, so only the firmware
+                 * data must be copied to the destination sector.
+                 *
+                 * Swap-move => constant sector size, so 'sz' is the size of a sector and 'swap_size
+                 * % sz' gives the number of bytes used by the largest firmware image in the last
+                 * sector to be moved.
+                 */
+                copy_sz = bs->swap_size % sz;
+            }
+        }
+
+        if (!sector_erased_with_trailer) {
+            rc = boot_erase_region(fap_sec, sec_off, sz);
+            assert(rc == 0);
+        }
 
-        rc = boot_copy_region(state, fap_pri, fap_sec, pri_up_off, sec_off, sz);
+        rc = boot_copy_region(state, fap_pri, fap_sec, pri_up_off, sec_off, copy_sz);
         assert(rc == 0);
 
         rc = boot_write_status(state, bs);
@@ -590,7 +621,7 @@ swap_run(struct boot_loader_state *state, struct boot_status *bs,
         idx = last_idx;
         while (idx > 0) {
             if (idx <= (last_idx - bs->idx + 1)) {
-                boot_move_sector_up(idx, sector_sz, state, bs, fap_pri, fap_sec);
+                boot_move_sector_up(idx, sector_sz, state, bs, fap_pri);
             }
             idx--;
         }
@@ -602,7 +633,7 @@ swap_run(struct boot_loader_state *state, struct boot_status *bs,
     idx = 1;
     while (idx <= last_idx) {
         if (idx >= bs->idx) {
-            boot_swap_sectors(idx, sector_sz, state, bs, fap_pri, fap_sec);
+            boot_swap_sectors(idx, last_idx, sector_sz, state, bs, fap_pri, fap_sec);
         }
         idx++;
     }