pstore/ram: verify ramoops header before saving record

On some devices the persistent memory contains junk after a cold boot,
and /dev/pstore/dmesg-ramoops-* are created with random data which is
not the result of a kernel crash.

This patch adds a ramoops header check and skips any
persistent_ram_zone that does not have a valid header.

Signed-off-by: Ben Zhang <benzh@chromium.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>

fs/pstore/ram.c

@@ -186,12 +186,34 @@ static ssize_t ramoops_pstore_read(u64 *id, enum pstore_type_id *type,
 	ssize_t size;
 	ssize_t ecc_notice_size;
 	struct ramoops_context *cxt = psi->data;
-	struct persistent_ram_zone *prz;
-	int header_length;
+	struct persistent_ram_zone *prz = NULL;
+	int header_length = 0;
 
+	/* Ramoops headers provide time stamps for PSTORE_TYPE_DMESG, but
+	 * PSTORE_TYPE_CONSOLE and PSTORE_TYPE_FTRACE don't currently have
+	 * valid time stamps, so it is initialized to zero.
+	 */
+	time->tv_sec = 0;
+	time->tv_nsec = 0;
+	*compressed = false;
+
+	/* Find the next valid persistent_ram_zone for DMESG */
+	while (cxt->dump_read_cnt < cxt->max_dump_cnt && !prz) {
 		prz = ramoops_get_next_prz(cxt->przs, &cxt->dump_read_cnt,
 					   cxt->max_dump_cnt, id, type,
 					   PSTORE_TYPE_DMESG, 1);
+		if (!prz_ok(prz))
+			continue;
+		header_length = ramoops_read_kmsg_hdr(persistent_ram_old(prz),
+						      time, compressed);
+		/* Clear and skip this DMESG record if it has no valid header */
+		if (!header_length) {
+			persistent_ram_free_old(prz);
+			persistent_ram_zap(prz);
+			prz = NULL;
+		}
+	}
+
 	if (!prz_ok(prz))
 		prz = ramoops_get_next_prz(&cxt->cprz, &cxt->console_read_cnt,
 					   1, id, type, PSTORE_TYPE_CONSOLE, 0);
@@ -204,13 +226,7 @@ static ssize_t ramoops_pstore_read(u64 *id, enum pstore_type_id *type,
 	if (!prz_ok(prz))
 		return 0;
 
-	if (!persistent_ram_old(prz))
-		return 0;
-
-	size = persistent_ram_old_size(prz);
-	header_length = ramoops_read_kmsg_hdr(persistent_ram_old(prz), time,
-			compressed);
-	size -= header_length;
+	size = persistent_ram_old_size(prz) - header_length;
 
 	/* ECC correction notice */
 	ecc_notice_size = persistent_ram_ecc_string(prz, NULL, 0);