block/partitions: detect LUKS-formatted disks

If an entire device is formatted as LUKS, there is a small chance it
maybe be detected as an Atari/AHDI disk - causing the kernel to create
partitions, and confusing other systems.

Detect the LUKS header before the Atari partition table to prevent this
from creating partitions on top of the LUKS volume.

Link: https://unix.stackexchange.com/questions/561745/what-are-the-md-partitions-under-a-mdadm-array
Link: rook/rook#7940
Link: https://bugs.launchpad.net/ubuntu/+source/util-linux/+bug/1531404
Signed-off-by: Robin H. Johnson <[email protected]>

Author: robbat2

block/partitions/Kconfig

@@ -96,6 +96,14 @@ config AMIGA_PARTITION
 	  Say Y here if you would like to use hard disks under Linux which
 	  were partitioned under AmigaOS.
 
+config LUKS_PARTITION
+	bool "LUKS partition support" if PARTITION_ADVANCED
+	default y if ATARI_PARTITION
+	help
+	  Say Y here to detect hard disks which have a LUKS header instead of a
+	  partition table. This is valuable as LUKS header may also be wrongly
+	  detected as other partition types.
+
 config ATARI_PARTITION
 	bool "Atari partition table support" if PARTITION_ADVANCED
 	default y if ATARI

block/partitions/Makefile

@@ -11,6 +11,7 @@ obj-$(CONFIG_AIX_PARTITION) += aix.o
 obj-$(CONFIG_CMDLINE_PARTITION) += cmdline.o
 obj-$(CONFIG_MAC_PARTITION) += mac.o
 obj-$(CONFIG_LDM_PARTITION) += ldm.o
+obj-$(CONFIG_LUKS_PARTITION) += luks.o
 obj-$(CONFIG_MSDOS_PARTITION) += msdos.o
 obj-$(CONFIG_OF_PARTITION) += of.o
 obj-$(CONFIG_OSF_PARTITION) += osf.o

block/partitions/check.h

@@ -60,6 +60,7 @@ int efi_partition(struct parsed_partitions *state);
 int ibm_partition(struct parsed_partitions *);
 int karma_partition(struct parsed_partitions *state);
 int ldm_partition(struct parsed_partitions *state);
+int luks_partition(struct parsed_partitions *state);
 int mac_partition(struct parsed_partitions *state);
 int msdos_partition(struct parsed_partitions *state);
 int of_partition(struct parsed_partitions *state);

block/partitions/core.c

@@ -67,6 +67,9 @@ static int (*const check_part[])(struct parsed_partitions *) = {
 #ifdef CONFIG_AMIGA_PARTITION
 	amiga_partition,
 #endif
+#ifdef CONFIG_LUKS_PARTITION
+	luks_partition, /* this must come before atari */
+#endif
 #ifdef CONFIG_ATARI_PARTITION
 	atari_partition,
 #endif

block/partitions/luks.c

@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *  fs/partitions/luks.c
+ *  LUKS on raw partition; this is important because a LUKS volume may detected
+ *  as a valid Atari partition table, breaking other detection.
+ *
+ *  Copyright (C) 2025 Robin H. Johnson ([email protected])
+ *
+ *  Reference: https://gitlab.com/cryptsetup/LUKS2-docs/blob/master/luks2_doc_wip.pdf
+ *  Page 5, Figure 2: LUKS2 binary header on-disk structure
+ *  This only looks for the Magic & version; and NOT a UUID that starts at
+ *  offset 0xA8.
+ */
+
+#include <linux/ctype.h>
+#include <linux/compiler.h>
+#include "check.h"
+
+#define LUKS_MAGIC_1ST_V1		"LUKS\xba\xbe\x00\x01"
+#define LUKS_MAGIC_1ST_V2		"LUKS\xba\xbe\x00\x02"
+#define LUKS_MAGIC_2ND_V1		"SKUL\xba\xbe\x00\x01"
+#define LUKS_MAGIC_2ND_V2		"SKUL\xba\xbe\x00\x02"
+
+int luks_partition(struct parsed_partitions *state)
+{
+	Sector sect;
+	int ret = 0;
+	unsigned char *data;
+
+	data = read_part_sector(state, 0, &sect);
+
+	if (!data)
+		return -1;
+
+	if (memcmp(data, LUKS_MAGIC_1ST_V1, 8) == 0
+		|| memcmp(data, LUKS_MAGIC_2ND_V1, 8) == 0) {
+		strlcat(state->pp_buf, "LUKSv1\n", PAGE_SIZE);
+		ret = 1;
+	} else if (memcmp(data, LUKS_MAGIC_1ST_V2, 8) == 0
+		|| memcmp(data, LUKS_MAGIC_2ND_V2, 8) == 0) {
+		strlcat(state->pp_buf, "LUKSv2\n", PAGE_SIZE);
+		ret = 1;
+	}
+	put_dev_sector(sect);
+	return ret;
+}