mtd: check for Seama magic early when fixing MD5

This avoid long (and unneeded) process of reading all data in case of
running on MTD not containig Seama entity.

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

package/system/mtd/src/seama.c

@@ -53,7 +53,7 @@ ssize_t pwrite(int fd, const void *buf, size_t count, off_t offset);
 int
 seama_fix_md5(char *buf, size_t len)
 {
-	struct seama_hdr *shdr;
+	struct seama_hdr *shdr = (struct seama_hdr *) buf;
 	char *data;
 	size_t msize;
 	size_t isize;
@@ -64,12 +64,6 @@ seama_fix_md5(char *buf, size_t len)
 	if (len < sizeof(struct seama_hdr))
 		return -1;
 
-	shdr = (struct seama_hdr *) buf;
-	if (shdr->magic != htonl(SEAMA_MAGIC)) {
-		fprintf(stderr, "no SEAMA header found\n");
-		return -1;
-	}
-
 	isize = ntohl(shdr->size);
 	msize = ntohs(shdr->metasize);
 	if (isize == 0) {
@@ -115,9 +109,11 @@ int
 mtd_fixseama(const char *mtd, size_t offset)
 {
 	int fd;
+	char *first_block;
 	char *buf;
 	ssize_t res;
 	size_t block_offset;
+	struct seama_hdr *shdr;
 
 	if (quiet < 2)
 		fprintf(stderr, "Trying to fix SEAMA header in %s at 0x%x...\n",
@@ -138,6 +134,24 @@ mtd_fixseama(const char *mtd, size_t offset)
 		exit(1);
 	}
 
+	first_block = malloc(erasesize);
+	if (!first_block) {
+		perror("malloc");
+		exit(1);
+	}
+
+	res = pread(fd, first_block, erasesize, block_offset);
+	if (res != erasesize) {
+		perror("pread");
+		exit(1);
+	}
+
+	shdr = (struct seama_hdr *)first_block;
+	if (shdr->magic != htonl(SEAMA_MAGIC)) {
+		fprintf(stderr, "No SEAMA header found\n");
+		return -1;
+	}
+
 	buf = malloc(mtdsize);
 	if (!buf) {
 		perror("malloc");