powerpc/pseries: Add backward compatibilty to read old kernel oops-log

Older kernels has just length information in their header. Handle it
while reading old kernel oops log from pstore.

Applies on top of powerpc/pseries: Fix buffer overflow when reading from pstore
Signed-off-by: Aruna Balakrishnaiah <aruna@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/pseries/nvram.c

@@ -720,15 +720,25 @@ static ssize_t nvram_pstore_read(u64 *id, enum pstore_type_id *type,
 
 	if (nvram_type_ids[read_type] == PSTORE_TYPE_DMESG) {
 		int length, unzipped_len;
+		size_t hdr_size;
 
 		oops_hdr = (struct oops_log_info *)buff;
-		length = oops_hdr->report_length;
+		if (oops_hdr->version < OOPS_HDR_VERSION) {
+			/* Old format oops header had 2-byte record size */
+			hdr_size = sizeof(u16);
+			length = oops_hdr->version;
+			time->tv_sec = 0;
+			time->tv_nsec = 0;
+		} else {
+			hdr_size = sizeof(*oops_hdr);
+			length = oops_hdr->report_length;
+			time->tv_sec = oops_hdr->timestamp;
+			time->tv_nsec = 0;
+		}
 		*buf = kmalloc(length, GFP_KERNEL);
 		if (*buf == NULL)
 			return -ENOMEM;
-		memcpy(*buf, buff + sizeof(*oops_hdr), length);
-		time->tv_sec = oops_hdr->timestamp;
-		time->tv_nsec = 0;
+		memcpy(*buf, buff + hdr_size, length);
 		kfree(buff);
 
 		if (err_type == ERR_TYPE_KERNEL_PANIC_GZ) {