pstore: Handle zero-sized prz in series

ramoops_pstore_read fails to return the next in a prz
series after first zero-sized entry, not venturing to
the next non-zero entry.

Signed-off-by: Mark Salyzyn <salyzyn@android.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>

fs/pstore/ram.c

@@ -164,6 +164,12 @@ static int ramoops_read_kmsg_hdr(char *buffer, struct timespec *time,
 	return header_length;
 }
 
+static bool prz_ok(struct persistent_ram_zone *prz)
+{
+	return !!prz && !!(persistent_ram_old_size(prz) +
+			   persistent_ram_ecc_string(prz, NULL, 0));
+}
+
 static ssize_t ramoops_pstore_read(u64 *id, enum pstore_type_id *type,
 				   int *count, struct timespec *time,
 				   char **buf, bool *compressed,
@@ -178,13 +184,13 @@ static ssize_t ramoops_pstore_read(u64 *id, enum pstore_type_id *type,
 	prz = ramoops_get_next_prz(cxt->przs, &cxt->dump_read_cnt,
 				   cxt->max_dump_cnt, id, type,
 				   PSTORE_TYPE_DMESG, 1);
-	if (!prz)
+	if (!prz_ok(prz))
 		prz = ramoops_get_next_prz(&cxt->cprz, &cxt->console_read_cnt,
 					   1, id, type, PSTORE_TYPE_CONSOLE, 0);
-	if (!prz)
+	if (!prz_ok(prz))
 		prz = ramoops_get_next_prz(&cxt->fprz, &cxt->ftrace_read_cnt,
 					   1, id, type, PSTORE_TYPE_FTRACE, 0);
-	if (!prz)
+	if (!prz_ok(prz))
 		return 0;
 
 	if (!persistent_ram_old(prz))