[nrf noup] compression: Fix non-PM configurations

If the partition manager is not used, the area between the MCUboot
header and the image is no longer filled with 0xFFs, but with 0x00s.
Align the decompression logic, so the 0x00s are written into the NVM as
part of the image copy operation, fixing the issue with incorrect digest
value in compressed images.

Ref: NCSDK-37697

Signed-off-by: Tomasz Chyrowicz <tomasz.chyrowicz@nordicsemi.no>

Author: tomchy

boot/zephyr/decompression.c

@@ -1089,6 +1089,9 @@ int boot_copy_region_decompress(struct boot_loader_state *state, const struct fl
     TARGET_STATIC uint8_t decomp_buf[DECOMP_BUF_ALLOC_SIZE] __attribute__((aligned(4)));
     TARGET_STATIC struct image_header modified_hdr;
     uint16_t decomp_buf_max_size;
+#ifndef CONFIG_PARTITION_MANAGER_ENABLED
+    uint32_t hdr_write_pos = 0;
+#endif
 
 #if defined(CONFIG_NRF_COMPRESS_ARM_THUMB)
     uint8_t unaligned_data_length = 0;
@@ -1206,6 +1209,40 @@ int boot_copy_region_decompress(struct boot_loader_state *state, const struct fl
         goto finish;
     }
 
+#ifndef CONFIG_PARTITION_MANAGER_ENABLED
+    /* The image digest is calculated on a binary, that has the area between header and the image
+     * binary filled with zeros.
+     * The zeros are a result of building the image with the CONFIG_ROM_START_OFFSET option set to
+     * the ih_hdr_size.
+     * The configuration with partition manager leaves this area uninitialized and signs
+     * the image using the --pad-header option, which fills the gap with the erase value
+     * (usually 0xFF) instead of zeros.
+     */
+
+    /* Reuse decompression buffer to write zeros. */
+    memset(decomp_buf, 0x00, sizeof(decomp_buf));
+    hdr_write_pos = sizeof(modified_hdr);
+
+    while (hdr_write_pos < hdr->ih_hdr_size) {
+        uint32_t set_size = hdr->ih_hdr_size - hdr_write_pos;
+
+        /* Assuming that sizeof(modified_hdr) is always aligned to write block size. */
+        if (set_size > sizeof(modified_hdr)) {
+            set_size = sizeof(modified_hdr);
+        }
+
+        rc = flash_area_write(fap_dst, off_dst + hdr_write_pos, decomp_buf, set_size);
+        if (rc != 0) {
+            BOOT_LOG_ERR("Flash write failed at offset: 0x%x, size: 0x%x, area: %d, rc: %d",
+                         (off_dst + hdr_write_pos), set_size, fap_dst->fa_id, rc);
+            rc = BOOT_EFLASH;
+            goto finish;
+        }
+
+        hdr_write_pos += set_size;
+    }
+#endif
+
     /* Read in, decompress and write out data */
 #ifdef MCUBOOT_ENC_IMAGES
     while (pos < comp_size) {