boot: bootutil: Take into account scratch trailer when computing max image size

When using swap-scratch and the trailer is larger than a single sector,
the sector containing the last part of the firmware data might only
contain a very small part of the trailer and some padding might be
necessary between the end of the firmware image and the beginning of the
trailer to ensure the scratch trailer won't be overwritten when copying
that sector to the scratch area.

This commit updates the 'bootutil_max_image_size' routine to take that
padding into account.

Signed-off-by: Thomas Altenbach <[email protected]>

Author: taltenbach

boot/bootutil/src/bootutil_misc.c

@@ -399,12 +399,87 @@ boot_write_enc_key(const struct flash_area *fap, uint8_t slot,
 }
 #endif
 
-uint32_t bootutil_max_image_size(const struct flash_area *fap)
+#ifdef MCUBOOT_SWAP_USING_SCRATCH
+size_t
+boot_get_first_trailer_sector(struct boot_loader_state *state, size_t slot, size_t trailer_sz)
 {
-#if defined(MCUBOOT_SWAP_USING_SCRATCH) || defined(MCUBOOT_SINGLE_APPLICATION_SLOT) || \
-    defined(MCUBOOT_FIRMWARE_LOADER) || defined(MCUBOOT_SINGLE_APPLICATION_SLOT_RAM_LOAD)
+    size_t first_trailer_sector = boot_img_num_sectors(state, slot) - 1;
+    size_t sector_sz = boot_img_sector_size(state, slot, first_trailer_sector);
+    size_t trailer_sector_sz = sector_sz;
+
+    while (trailer_sector_sz < trailer_sz) {
+        /* Consider that the image trailer may span across sectors of different sizes */
+        --first_trailer_sector;
+        sector_sz = boot_img_sector_size(state, slot, first_trailer_sector);
+
+        trailer_sector_sz += sector_sz;
+    }
+
+    return first_trailer_sector;
+}
+
+/**
+ * Returns the offset to the end of the first sector of a given slot that holds image trailer data.
+ *
+ * @param state      Current bootloader's state.
+ * @param slot       The index of the slot to consider.
+ * @param trailer_sz The size of the trailer, in bytes.
+ *
+ * @return The offset to the end of the first sector of the slot that holds image trailer data.
+ */
+static uint32_t
+get_first_trailer_sector_end_off(struct boot_loader_state *state, size_t slot, size_t trailer_sz)
+{
+    size_t first_trailer_sector = boot_get_first_trailer_sector(state, slot, trailer_sz);
+
+    return boot_img_sector_off(state, slot, first_trailer_sector) +
+           boot_img_sector_size(state, slot, first_trailer_sector);
+}
+#endif /* MCUBOOT_SWAP_USING_SCRATCH */
+
+uint32_t bootutil_max_image_size(struct boot_loader_state *state, const struct flash_area *fap)
+{
+#if defined(MCUBOOT_SINGLE_APPLICATION_SLOT) ||      \
+    defined(MCUBOOT_FIRMWARE_LOADER) ||              \
+    defined(MCUBOOT_SINGLE_APPLICATION_SLOT_RAM_LOAD)
+    (void) state;
     return boot_status_off(fap);
+#elif defined(MCUBOOT_SWAP_USING_SCRATCH)
+    size_t slot_trailer_sz = boot_trailer_sz(BOOT_WRITE_SZ(state));
+    size_t slot_trailer_off = flash_area_get_size(fap) - slot_trailer_sz;
+
+    /* If the trailer doesn't fit in the last sector of the primary or secondary slot, some padding
+     * might have to be inserted between the end of the firmware image and the beginning of the
+     * trailer to ensure there is enough space for the trailer in the scratch area when the last
+     * sector of the secondary will be copied to the scratch area.
+     *
+     * The value of the padding depends on the amount of trailer data that is contained in the first
+     * trailer containing part of the trailer in the primary and secondary slot.
+     */
+    size_t trailer_sector_primary_end_off =
+        get_first_trailer_sector_end_off(state, BOOT_PRIMARY_SLOT, slot_trailer_sz);
+    size_t trailer_sector_secondary_end_off =
+        get_first_trailer_sector_end_off(state, BOOT_SECONDARY_SLOT, slot_trailer_sz);
+
+    size_t trailer_sz_in_first_sector;
+
+    if (trailer_sector_primary_end_off > trailer_sector_secondary_end_off) {
+        trailer_sz_in_first_sector = trailer_sector_primary_end_off - slot_trailer_off;
+    } else {
+        trailer_sz_in_first_sector = trailer_sector_secondary_end_off - slot_trailer_off;
+    }
+
+    size_t trailer_padding = 0;
+    size_t scratch_trailer_sz = boot_scratch_trailer_sz(BOOT_WRITE_SZ(state));
+
+    if (scratch_trailer_sz > trailer_sz_in_first_sector) {
+        trailer_padding = scratch_trailer_sz - trailer_sz_in_first_sector;
+    }
+
+    return slot_trailer_off - trailer_padding;
 #elif defined(MCUBOOT_SWAP_USING_MOVE) || defined(MCUBOOT_SWAP_USING_OFFSET)
+    (void) state;
+
     struct flash_sector sector;
     /* get the last sector offset */
     int rc = flash_area_get_sector(fap, boot_status_off(fap), &sector);
@@ -416,10 +491,13 @@ uint32_t bootutil_max_image_size(const struct flash_area *fap)
     }
     return flash_sector_get_off(&sector);
 #elif defined(MCUBOOT_OVERWRITE_ONLY)
+    (void) state;
     return boot_swap_info_off(fap);
 #elif defined(MCUBOOT_DIRECT_XIP)
+    (void) state;
     return boot_swap_info_off(fap);
 #elif defined(MCUBOOT_RAM_LOAD)
+    (void) state;
     return boot_swap_info_off(fap);
 #endif
 }

boot/bootutil/src/bootutil_priv.h

@@ -366,6 +366,20 @@ int boot_read_enc_key(const struct flash_area *fap, uint8_t slot,
                       struct boot_status *bs);
 #endif
 
+#ifdef MCUBOOT_SWAP_USING_SCRATCH
+/**
+ * Finds the first sector of a given slot that holds image trailer data.
+ *
+ * @param state      Current bootloader's state.
+ * @param slot       The index of the slot to consider.
+ * @param trailer_sz The size of the trailer, in bytes.
+ *
+ * @return The index of the first sector of the slot that holds image trailer data.
+ */
+size_t
+boot_get_first_trailer_sector(struct boot_loader_state *state, size_t slot, size_t trailer_sz);
+#endif
+
 /**
  * Checks that a buffer is erased according to what the erase value for the
  * flash device provided in `flash_area` is.
@@ -533,7 +547,7 @@ int boot_load_image_to_sram(struct boot_loader_state *state);
 
 #endif /* MCUBOOT_RAM_LOAD */
 
-uint32_t bootutil_max_image_size(const struct flash_area *fap);
+uint32_t bootutil_max_image_size(struct boot_loader_state *state, const struct flash_area *fap);
 
 int boot_read_image_size(struct boot_loader_state *state, int slot,
                          uint32_t *size);

boot/bootutil/src/image_validate.c

@@ -555,7 +555,7 @@ bootutil_img_validate(struct boot_loader_state *state,
         goto out;
     }
 
-    if (it.tlv_end > bootutil_max_image_size(fap)) {
+    if (it.tlv_end > bootutil_max_image_size(state, fap)) {
         rc = -1;
         goto out;
     }

boot/bootutil/src/swap_scratch.c

@@ -531,33 +531,6 @@ find_swap_count(const struct boot_loader_state *state, uint32_t copy_size)
     return swap_count;
 }
 
-/**
- * Finds the first sector of a given slot that holds image trailer data.
- *
- * @param state      Current bootloader's state.
- * @param slot       The index of the slot to consider.
- * @param trailer_sz The size of the trailer, in bytes.
- *
- * @return The index of the first sector of the slot that holds image trailer data.
- */
-static size_t
-get_first_trailer_sector(struct boot_loader_state *state, size_t slot, size_t trailer_sz)
-{
-    size_t first_trailer_sector = boot_img_num_sectors(state, slot) - 1;
-    size_t sector_sz = boot_img_sector_size(state, slot, first_trailer_sector);
-    size_t trailer_sector_sz = sector_sz;
-
-    while (trailer_sector_sz < trailer_sz) {
-        /* Consider that the image trailer may span across sectors of different sizes */
-        --first_trailer_sector;
-        sector_sz = boot_img_sector_size(state, slot, first_trailer_sector);
-
-        trailer_sector_sz += sector_sz;
-    }
-
-    return first_trailer_sector;
-}
-
 /**
  * Swaps the contents of two flash regions within the two image slots.
  *
@@ -616,7 +589,8 @@ boot_swap_sectors(int idx, uint32_t sz, struct boot_loader_state *state,
      * NOTE: `use_scratch` is a temporary flag (never written to flash) which
      * controls if special handling is needed (swapping the first trailer sector).
      */
-    first_trailer_sector_primary = get_first_trailer_sector(state, BOOT_PRIMARY_SLOT, trailer_sz);
+    first_trailer_sector_primary =
+        boot_get_first_trailer_sector(state, BOOT_PRIMARY_SLOT, trailer_sz);
 
     /* Check if the currently swapped sector(s) contain the trailer or part of it */
     if ((img_off + sz) >
@@ -696,7 +670,7 @@ boot_swap_sectors(int idx, uint32_t sz, struct boot_loader_state *state,
                  * sector(s) containing the beginning of the trailer won't be erased again.
                  */
                 size_t trailer_sector_secondary =
-                    get_first_trailer_sector(state, BOOT_SECONDARY_SLOT, trailer_sz);
+                    boot_get_first_trailer_sector(state, BOOT_SECONDARY_SLOT, trailer_sz);
 
                 uint32_t trailer_sector_offset =
                     boot_img_sector_off(state, BOOT_SECONDARY_SLOT, trailer_sector_secondary);