mtd: trx: use separated buffer for TRX header

We plan to adjust usage of the main buffer to allow reading custom
amount of data for CRC32. This means we need another buffer that will be
always block aligned.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

package/system/mtd/src/trx.c

@@ -152,6 +152,7 @@ mtd_fixtrx(const char *mtd, size_t offset)
 {
 	int fd;
 	struct trx_header *trx;
+	char *first_block;
 	char *buf;
 	ssize_t res;
 	size_t block_offset;
@@ -173,19 +174,19 @@ mtd_fixtrx(const char *mtd, size_t offset)
 		exit(1);
 	}
 
-	buf = malloc(erasesize);
-	if (!buf) {
+	first_block = malloc(erasesize);
+	if (!first_block) {
 		perror("malloc");
 		exit(1);
 	}
 
-	res = pread(fd, buf, erasesize, block_offset);
+	res = pread(fd, first_block, erasesize, block_offset);
 	if (res != erasesize) {
 		perror("pread");
 		exit(1);
 	}
 
-	trx = (struct trx_header *) (buf + offset);
+	trx = (struct trx_header *)(first_block + offset);
 	if (trx->magic != STORE32_LE(0x30524448)) {
 		fprintf(stderr, "No trx magic found\n");
 		exit(1);
@@ -198,6 +199,18 @@ mtd_fixtrx(const char *mtd, size_t offset)
 		return 0;
 	}
 
+	buf = malloc(erasesize);
+	if (!buf) {
+		perror("malloc");
+		exit(1);
+	}
+
+	res = pread(fd, buf, erasesize, block_offset);
+	if (res != erasesize) {
+		perror("pread");
+		exit(1);
+	}
+
 	trx->len = STORE32_LE(erasesize - offset);
 
 	trx->crc32 = STORE32_LE(crc32buf((char*) &trx->flag_version, erasesize - offset - 3*4));
@@ -209,7 +222,7 @@ mtd_fixtrx(const char *mtd, size_t offset)
 	if (quiet < 2)
 		fprintf(stderr, "New crc32: 0x%x, rewriting block\n", trx->crc32);
 
-	if (pwrite(fd, buf, erasesize, block_offset) != erasesize) {
+	if (pwrite(fd, first_block, erasesize, block_offset) != erasesize) {
 		fprintf(stderr, "Error writing block (%s)\n", strerror(errno));
 		exit(1);
 	}