efi_loader: initialise partition_signature memory

Zero partition_signature in the efi_device_path_hard_drive_path
structure when signature_type is 0 (no signature) as required by the
UEFI specification.

This is required so that efi_dp_match() will work as expected
when doing memcmp() comparisons.  Previously uninitialised memory
would cause it not match nodes when it should have when the signature
type was not GUID.

Corrects a problem where the loaded image protocol would not return a
device path with MEDIA_DEVICE causing the OpenBSD bootloader to fail
on rpi_3 and other targets.

v2: Also handle signature_type 1 (MBR) as described in the specification

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Tested-by: Artturi Alm <artturi.alm@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>

lib/efi_loader/efi_device_path.c

@@ -428,10 +428,27 @@ static void *dp_part_fill(void *buf, struct blk_desc *desc, int part)
 			hddp->partmap_type = 2;
 		else
 			hddp->partmap_type = 1;
-		hddp->signature_type = desc->sig_type;
-		if (hddp->signature_type != 0)
+
+		switch (desc->sig_type) {
+		case SIG_TYPE_NONE:
+		default:
+			hddp->signature_type = 0;
+			memset(hddp->partition_signature, 0,
+			       sizeof(hddp->partition_signature));
+			break;
+		case SIG_TYPE_MBR:
+			hddp->signature_type = 1;
+			memset(hddp->partition_signature, 0,
+			       sizeof(hddp->partition_signature));
+			memcpy(hddp->partition_signature, &desc->mbr_sig,
+			       sizeof(desc->mbr_sig));
+			break;
+		case SIG_TYPE_GUID:
+			hddp->signature_type = 2;
 			memcpy(hddp->partition_signature, &desc->guid_sig,
 			       sizeof(hddp->partition_signature));
+			break;
+		}
 
 		buf = &hddp[1];
 	}