mtd: bcm47xxpart: improve handling TRX partition size

When bcm47xxpart finds a TRX partition (container) it's supposed to jump
to the end of it and keep looking for more partitions. TRX and its
subpartitions are handled by a separate parser.

The problem with old code was relying on the length specified in a TRX
header. That isn't reliable as TRX is commonly modified to have checksum
cover only non-changing subpartitions. Otherwise modifying e.g. a rootfs
would result in CRC32 mismatch and bootloader refusing to boot a
firmware.

Fix it by trying better to figure out a real TRX size. We can securely
assume that TRX has to cover all subpartitions and the last one is at
least of a block size in size. Then compare it with a length field.

This makes code more optimal & reliable thanks to skipping data that
shouldn't be parsed.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>

drivers/mtd/bcm47xxpart.c

@@ -186,6 +186,8 @@ static int bcm47xxpart_parse(struct mtd_info *master,
 		/* TRX */
 		if (buf[0x000 / 4] == TRX_MAGIC) {
 			struct trx_header *trx;
+			uint32_t last_subpart;
+			uint32_t trx_size;
 
 			if (trx_num >= ARRAY_SIZE(trx_parts))
 				pr_warn("No enough space to store another TRX found at 0x%X\n",
@@ -195,11 +197,23 @@ static int bcm47xxpart_parse(struct mtd_info *master,
 			bcm47xxpart_add_part(&parts[curr_part++], "firmware",
 					     offset, 0);
 
-			/* Jump to the end of TRX */
+			/*
+			 * Try to find TRX size. The "length" field isn't fully
+			 * reliable as it could be decreased to make CRC32 cover
+			 * only part of TRX data. It's commonly used as checksum
+			 * can't cover e.g. ever-changing rootfs partition.
+			 * Use offsets as helpers for assuming min TRX size.
+			 */
 			trx = (struct trx_header *)buf;
-			offset = roundup(offset + trx->length, blocksize);
-			/* Next loop iteration will increase the offset */
-			offset -= blocksize;
+			last_subpart = max3(trx->offset[0], trx->offset[1],
+					    trx->offset[2]);
+			trx_size = max(trx->length, last_subpart + blocksize);
+
+			/*
+			 * Skip the TRX data. Decrease offset by block size as
+			 * the next loop iteration will increase it.
+			 */
+			offset += roundup(trx_size, blocksize) - blocksize;
 			continue;
 		}