[linux-block.git] / Documentation / process / magic-number.rst

.. _magicnumbers:

Linux magic numbers
===================

This file is a registry of magic numbers which are in use.  When you
add a magic number to a structure, you should also add it to this
file, since it is best if the magic numbers used by various structures
are unique.

It is a **very** good idea to protect kernel data structures with magic
numbers.  This allows you to check at run time whether (a) a structure
has been clobbered, or (b) you've passed the wrong structure to a
routine.  This last is especially useful --- particularly when you are
passing pointers to structures via a void * pointer.  The tty code,
for example, does this frequently to pass driver-specific and line
discipline-specific structures back and forth.

The way to use magic numbers is to declare them at the beginning of
the structure, like so::

	struct tty_ldisc {
		int	magic;
		...
	};

Please follow this discipline when you are adding future enhancements
to the kernel!  It has saved me countless hours of debugging,
especially in the screwy cases where an array has been overrun and
structures following the array have been overwritten.  Using this
discipline, these cases get detected quickly and safely.

Changelog::

					Theodore Ts'o
					31 Mar 94

  The magic table is current to Linux 2.1.55.

					Michael Chastain
					<mailto:mec@shout.net>
					22 Sep 1997

  Now it should be up to date with Linux 2.1.112. Because
  we are in feature freeze time it is very unlikely that
  something will change before 2.2.x. The entries are
  sorted by number field.

					Krzysztof G. Baranowski
					<mailto: kgb@knm.org.pl>
					29 Jul 1998

  Updated the magic table to Linux 2.5.45. Right over the feature freeze,
  but it is possible that some new magic numbers will sneak into the
  kernel before 2.6.x yet.

					Petr Baudis
					<pasky@ucw.cz>
					03 Nov 2002

  Updated the magic table to Linux 2.5.74.

					Fabian Frederick
					<ffrederick@users.sourceforge.net>
					09 Jul 2003


===================== ================ ======================== ==========================================
Magic Name            Number           Structure                File
===================== ================ ======================== ==========================================
PG_MAGIC              'P'              pg_{read,write}_hdr      ``include/linux/pg.h``
CMAGIC                0x0111           user                     ``include/linux/a.out.h``
MKISS_DRIVER_MAGIC    0x04bf           mkiss_channel            ``drivers/net/mkiss.h``
HDLC_MAGIC            0x239e           n_hdlc                   ``drivers/char/n_hdlc.c``
APM_BIOS_MAGIC        0x4101           apm_user                 ``arch/x86/kernel/apm_32.c``
DB_MAGIC              0x4442           fc_info                  ``drivers/net/iph5526_novram.c``
DL_MAGIC              0x444d           fc_info                  ``drivers/net/iph5526_novram.c``
FASYNC_MAGIC          0x4601           fasync_struct            ``include/linux/fs.h``
FF_MAGIC              0x4646           fc_info                  ``drivers/net/iph5526_novram.c``
PTY_MAGIC             0x5001                                    ``drivers/char/pty.c``
PPP_MAGIC             0x5002           ppp                      ``include/linux/if_pppvar.h``
SSTATE_MAGIC          0x5302           serial_state             ``include/linux/serial.h``
SLIP_MAGIC            0x5302           slip                     ``drivers/net/slip.h``
STRIP_MAGIC           0x5303           strip                    ``drivers/net/strip.c``
SIXPACK_MAGIC         0x5304           sixpack                  ``drivers/net/hamradio/6pack.h``
AX25_MAGIC            0x5316           ax_disp                  ``drivers/net/mkiss.h``
TTY_MAGIC             0x5401           tty_struct               ``include/linux/tty.h``
MGSL_MAGIC            0x5401           mgsl_info                ``drivers/char/synclink.c``
TTY_DRIVER_MAGIC      0x5402           tty_driver               ``include/linux/tty_driver.h``
MGSLPC_MAGIC          0x5402           mgslpc_info              ``drivers/char/pcmcia/synclink_cs.c``
USB_SERIAL_MAGIC      0x6702           usb_serial               ``drivers/usb/serial/usb-serial.h``
FULL_DUPLEX_MAGIC     0x6969                                    ``drivers/net/ethernet/dec/tulip/de2104x.c``
USB_BLUETOOTH_MAGIC   0x6d02           usb_bluetooth            ``drivers/usb/class/bluetty.c``
RFCOMM_TTY_MAGIC      0x6d02                                    ``net/bluetooth/rfcomm/tty.c``
USB_SERIAL_PORT_MAGIC 0x7301           usb_serial_port          ``drivers/usb/serial/usb-serial.h``
CG_MAGIC              0x00090255       ufs_cylinder_group       ``include/linux/ufs_fs.h``
LSEMAGIC              0x05091998       lse                      ``drivers/fc4/fc.c``
RIEBL_MAGIC           0x09051990                                ``drivers/net/atarilance.c``
NBD_REQUEST_MAGIC     0x12560953       nbd_request              ``include/linux/nbd.h``
RED_MAGIC2            0x170fc2a5       (any)                    ``mm/slab.c``
BAYCOM_MAGIC          0x19730510       baycom_state             ``drivers/net/baycom_epp.c``
ISDN_X25IFACE_MAGIC   0x1e75a2b9       isdn_x25iface_proto_data ``drivers/isdn/isdn_x25iface.h``
ECP_MAGIC             0x21504345       cdkecpsig                ``include/linux/cdk.h``
LSOMAGIC              0x27091997       lso                      ``drivers/fc4/fc.c``
LSMAGIC               0x2a3b4d2a       ls                       ``drivers/fc4/fc.c``
WANPIPE_MAGIC         0x414C4453       sdla_{dump,exec}         ``include/linux/wanpipe.h``
CS_CARD_MAGIC         0x43525553       cs_card                  ``sound/oss/cs46xx.c``
LABELCL_MAGIC         0x4857434c       labelcl_info_s           ``include/asm/ia64/sn/labelcl.h``
ISDN_ASYNC_MAGIC      0x49344C01       modem_info               ``include/linux/isdn.h``
CTC_ASYNC_MAGIC       0x49344C01       ctc_tty_info             ``drivers/s390/net/ctctty.c``
ISDN_NET_MAGIC        0x49344C02       isdn_net_local_s         ``drivers/isdn/i4l/isdn_net_lib.h``
SAVEKMSG_MAGIC2       0x4B4D5347       savekmsg                 ``arch/*/amiga/config.c``
CS_STATE_MAGIC        0x4c4f4749       cs_state                 ``sound/oss/cs46xx.c``
SLAB_C_MAGIC          0x4f17a36d       kmem_cache               ``mm/slab.c``
COW_MAGIC             0x4f4f4f4d       cow_header_v1            ``arch/um/drivers/ubd_user.c``
I810_CARD_MAGIC       0x5072696E       i810_card                ``sound/oss/i810_audio.c``
TRIDENT_CARD_MAGIC    0x5072696E       trident_card             ``sound/oss/trident.c``
ROUTER_MAGIC          0x524d4157       wan_device               [in ``wanrouter.h`` pre 3.9]
SAVEKMSG_MAGIC1       0x53415645       savekmsg                 ``arch/*/amiga/config.c``
GDA_MAGIC             0x58464552       gda                      ``arch/mips/include/asm/sn/gda.h``
RED_MAGIC1            0x5a2cf071       (any)                    ``mm/slab.c``
EEPROM_MAGIC_VALUE    0x5ab478d2       lanai_dev                ``drivers/atm/lanai.c``
HDLCDRV_MAGIC         0x5ac6e778       hdlcdrv_state            ``include/linux/hdlcdrv.h``
PCXX_MAGIC            0x5c6df104       channel                  ``drivers/char/pcxx.h``
KV_MAGIC              0x5f4b565f       kernel_vars_s            ``arch/mips/include/asm/sn/klkernvars.h``
I810_STATE_MAGIC      0x63657373       i810_state               ``sound/oss/i810_audio.c``
TRIDENT_STATE_MAGIC   0x63657373       trient_state             ``sound/oss/trident.c``
M3_CARD_MAGIC         0x646e6f50       m3_card                  ``sound/oss/maestro3.c``
FW_HEADER_MAGIC       0x65726F66       fw_header                ``drivers/atm/fore200e.h``
SLOT_MAGIC            0x67267321       slot                     ``drivers/hotplug/cpqphp.h``
SLOT_MAGIC            0x67267322       slot                     ``drivers/hotplug/acpiphp.h``
LO_MAGIC              0x68797548       nbd_device               ``include/linux/nbd.h``
M3_STATE_MAGIC        0x734d724d       m3_state                 ``sound/oss/maestro3.c``
VMALLOC_MAGIC         0x87654320       snd_alloc_track          ``sound/core/memory.c``
KMALLOC_MAGIC         0x87654321       snd_alloc_track          ``sound/core/memory.c``
PWC_MAGIC             0x89DC10AB       pwc_device               ``drivers/usb/media/pwc.h``
NBD_REPLY_MAGIC       0x96744668       nbd_reply                ``include/linux/nbd.h``
ENI155_MAGIC          0xa54b872d       midway_eprom	        ``drivers/atm/eni.h``
CODA_MAGIC            0xC0DAC0DA       coda_file_info           ``fs/coda/coda_fs_i.h``
YAM_MAGIC             0xF10A7654       yam_port                 ``drivers/net/hamradio/yam.c``
CCB_MAGIC             0xf2691ad2       ccb                      ``drivers/scsi/ncr53c8xx.c``
QUEUE_MAGIC_FREE      0xf7e1c9a3       queue_entry              ``drivers/scsi/arm/queue.c``
QUEUE_MAGIC_USED      0xf7e1cc33       queue_entry              ``drivers/scsi/arm/queue.c``
HTB_CMAGIC            0xFEFAFEF1       htb_class                ``net/sched/sch_htb.c``
NMI_MAGIC             0x48414d4d455201 nmi_s                    ``arch/mips/include/asm/sn/nmi.h``
===================== ================ ======================== ==========================================

Note that there are also defined special per-driver magic numbers in sound
memory management. See ``include/sound/sndmagic.h`` for complete list of them. Many
OSS sound drivers have their magic numbers constructed from the soundcard PCI
ID - these are not listed here as well.

HFS is another larger user of magic numbers - you can find them in
``fs/hfs/hfs.h``.
Commit	Line	Data
edba5eec FV	1	.. _magicnumbers:
edba5eec FV	2
9a11e153 MCC	3	Linux magic numbers
	4	===================
	5
1da177e4 LT	6	This file is a registry of magic numbers which are in use. When you
	7	add a magic number to a structure, you should also add it to this
	8	file, since it is best if the magic numbers used by various structures
	9	are unique.
	10
9a11e153	11	It is a very good idea to protect kernel data structures with magic
1da177e4 LT	12	numbers. This allows you to check at run time whether (a) a structure
	13	has been clobbered, or (b) you've passed the wrong structure to a
	14	routine. This last is especially useful --- particularly when you are
	15	passing pointers to structures via a void * pointer. The tty code,
	16	for example, does this frequently to pass driver-specific and line
	17	discipline-specific structures back and forth.
	18
738947f0	19	The way to use magic numbers is to declare them at the beginning of
9a11e153	20	the structure, like so::
1da177e4	21
9a11e153 MCC	22	struct tty_ldisc {
	23	int magic;
	24	...
	25	};
1da177e4 LT	26
	27	Please follow this discipline when you are adding future enhancements
	28	to the kernel! It has saved me countless hours of debugging,
	29	especially in the screwy cases where an array has been overrun and
	30	structures following the array have been overwritten. Using this
	31	discipline, these cases get detected quickly and safely.
	32
9a11e153 MCC	33	Changelog::
9a11e153 MCC	34
1da177e4 LT	35	Theodore Ts'o
	36	31 Mar 94
	37
9a11e153	38	The magic table is current to Linux 2.1.55.
1da177e4 LT	39
	40	Michael Chastain
	41	<mailto:mec@shout.net>
	42	22 Sep 1997
	43
9a11e153 MCC	44	Now it should be up to date with Linux 2.1.112. Because
	45	we are in feature freeze time it is very unlikely that
	46	something will change before 2.2.x. The entries are
	47	sorted by number field.
1da177e4 LT	48
	49	Krzysztof G. Baranowski
	50	<mailto: kgb@knm.org.pl>
	51	29 Jul 1998
	52
9a11e153 MCC	53	Updated the magic table to Linux 2.5.45. Right over the feature freeze,
	54	but it is possible that some new magic numbers will sneak into the
	55	kernel before 2.6.x yet.
1da177e4 LT	56
	57	Petr Baudis
	58	<pasky@ucw.cz>
	59	03 Nov 2002
	60
9a11e153	61	Updated the magic table to Linux 2.5.74.
1da177e4 LT	62
	63	Fabian Frederick
	64	<ffrederick@users.sourceforge.net>
	65	09 Jul 2003
	66
	67
9a11e153 MCC	68	===================== ================ ======================== ==========================================
	69	Magic Name Number Structure File
	70	===================== ================ ======================== ==========================================
	71	PG_MAGIC 'P' pg_{read,write}_hdr ``include/linux/pg.h``
	72	CMAGIC 0x0111 user ``include/linux/a.out.h``
	73	MKISS_DRIVER_MAGIC 0x04bf mkiss_channel ``drivers/net/mkiss.h``
	74	HDLC_MAGIC 0x239e n_hdlc ``drivers/char/n_hdlc.c``
	75	APM_BIOS_MAGIC 0x4101 apm_user ``arch/x86/kernel/apm_32.c``
9a11e153 MCC	76	DB_MAGIC 0x4442 fc_info ``drivers/net/iph5526_novram.c``
	77	DL_MAGIC 0x444d fc_info ``drivers/net/iph5526_novram.c``
	78	FASYNC_MAGIC 0x4601 fasync_struct ``include/linux/fs.h``
	79	FF_MAGIC 0x4646 fc_info ``drivers/net/iph5526_novram.c``
9a11e153 MCC	80	PTY_MAGIC 0x5001 ``drivers/char/pty.c``
9a11e153 MCC	81	PPP_MAGIC 0x5002 ppp ``include/linux/if_pppvar.h``
9a11e153 MCC	82	SSTATE_MAGIC 0x5302 serial_state ``include/linux/serial.h``
	83	SLIP_MAGIC 0x5302 slip ``drivers/net/slip.h``
	84	STRIP_MAGIC 0x5303 strip ``drivers/net/strip.c``
9a11e153 MCC	85	SIXPACK_MAGIC 0x5304 sixpack ``drivers/net/hamradio/6pack.h``
	86	AX25_MAGIC 0x5316 ax_disp ``drivers/net/mkiss.h``
	87	TTY_MAGIC 0x5401 tty_struct ``include/linux/tty.h``
	88	MGSL_MAGIC 0x5401 mgsl_info ``drivers/char/synclink.c``
	89	TTY_DRIVER_MAGIC 0x5402 tty_driver ``include/linux/tty_driver.h``
	90	MGSLPC_MAGIC 0x5402 mgslpc_info ``drivers/char/pcmcia/synclink_cs.c``
9a11e153 MCC	91	USB_SERIAL_MAGIC 0x6702 usb_serial ``drivers/usb/serial/usb-serial.h``
	92	FULL_DUPLEX_MAGIC 0x6969 ``drivers/net/ethernet/dec/tulip/de2104x.c``
	93	USB_BLUETOOTH_MAGIC 0x6d02 usb_bluetooth ``drivers/usb/class/bluetty.c``
	94	RFCOMM_TTY_MAGIC 0x6d02 ``net/bluetooth/rfcomm/tty.c``
	95	USB_SERIAL_PORT_MAGIC 0x7301 usb_serial_port ``drivers/usb/serial/usb-serial.h``
	96	CG_MAGIC 0x00090255 ufs_cylinder_group ``include/linux/ufs_fs.h``
9a11e153	97	LSEMAGIC 0x05091998 lse ``drivers/fc4/fc.c``
9a11e153 MCC	98	RIEBL_MAGIC 0x09051990 ``drivers/net/atarilance.c``
	99	NBD_REQUEST_MAGIC 0x12560953 nbd_request ``include/linux/nbd.h``
	100	RED_MAGIC2 0x170fc2a5 (any) ``mm/slab.c``
	101	BAYCOM_MAGIC 0x19730510 baycom_state ``drivers/net/baycom_epp.c``
	102	ISDN_X25IFACE_MAGIC 0x1e75a2b9 isdn_x25iface_proto_data ``drivers/isdn/isdn_x25iface.h``
	103	ECP_MAGIC 0x21504345 cdkecpsig ``include/linux/cdk.h``
	104	LSOMAGIC 0x27091997 lso ``drivers/fc4/fc.c``
	105	LSMAGIC 0x2a3b4d2a ls ``drivers/fc4/fc.c``
	106	WANPIPE_MAGIC 0x414C4453 sdla_{dump,exec} ``include/linux/wanpipe.h``
	107	CS_CARD_MAGIC 0x43525553 cs_card ``sound/oss/cs46xx.c``
	108	LABELCL_MAGIC 0x4857434c labelcl_info_s ``include/asm/ia64/sn/labelcl.h``
	109	ISDN_ASYNC_MAGIC 0x49344C01 modem_info ``include/linux/isdn.h``
	110	CTC_ASYNC_MAGIC 0x49344C01 ctc_tty_info ``drivers/s390/net/ctctty.c``
	111	ISDN_NET_MAGIC 0x49344C02 isdn_net_local_s ``drivers/isdn/i4l/isdn_net_lib.h``
	112	SAVEKMSG_MAGIC2 0x4B4D5347 savekmsg ``arch/*/amiga/config.c``
	113	CS_STATE_MAGIC 0x4c4f4749 cs_state ``sound/oss/cs46xx.c``
	114	SLAB_C_MAGIC 0x4f17a36d kmem_cache ``mm/slab.c``
	115	COW_MAGIC 0x4f4f4f4d cow_header_v1 ``arch/um/drivers/ubd_user.c``
	116	I810_CARD_MAGIC 0x5072696E i810_card ``sound/oss/i810_audio.c``
	117	TRIDENT_CARD_MAGIC 0x5072696E trident_card ``sound/oss/trident.c``
	118	ROUTER_MAGIC 0x524d4157 wan_device [in ``wanrouter.h`` pre 3.9]
	119	SAVEKMSG_MAGIC1 0x53415645 savekmsg ``arch/*/amiga/config.c``
	120	GDA_MAGIC 0x58464552 gda ``arch/mips/include/asm/sn/gda.h``
	121	RED_MAGIC1 0x5a2cf071 (any) ``mm/slab.c``
	122	EEPROM_MAGIC_VALUE 0x5ab478d2 lanai_dev ``drivers/atm/lanai.c``
	123	HDLCDRV_MAGIC 0x5ac6e778 hdlcdrv_state ``include/linux/hdlcdrv.h``
	124	PCXX_MAGIC 0x5c6df104 channel ``drivers/char/pcxx.h``
	125	KV_MAGIC 0x5f4b565f kernel_vars_s ``arch/mips/include/asm/sn/klkernvars.h``
	126	I810_STATE_MAGIC 0x63657373 i810_state ``sound/oss/i810_audio.c``
	127	TRIDENT_STATE_MAGIC 0x63657373 trient_state ``sound/oss/trident.c``
	128	M3_CARD_MAGIC 0x646e6f50 m3_card ``sound/oss/maestro3.c``
	129	FW_HEADER_MAGIC 0x65726F66 fw_header ``drivers/atm/fore200e.h``
	130	SLOT_MAGIC 0x67267321 slot ``drivers/hotplug/cpqphp.h``
	131	SLOT_MAGIC 0x67267322 slot ``drivers/hotplug/acpiphp.h``
	132	LO_MAGIC 0x68797548 nbd_device ``include/linux/nbd.h``
9a11e153 MCC	133	M3_STATE_MAGIC 0x734d724d m3_state ``sound/oss/maestro3.c``
	134	VMALLOC_MAGIC 0x87654320 snd_alloc_track ``sound/core/memory.c``
	135	KMALLOC_MAGIC 0x87654321 snd_alloc_track ``sound/core/memory.c``
	136	PWC_MAGIC 0x89DC10AB pwc_device ``drivers/usb/media/pwc.h``
	137	NBD_REPLY_MAGIC 0x96744668 nbd_reply ``include/linux/nbd.h``
	138	ENI155_MAGIC 0xa54b872d midway_eprom ``drivers/atm/eni.h``
	139	CODA_MAGIC 0xC0DAC0DA coda_file_info ``fs/coda/coda_fs_i.h``
9a11e153 MCC	140	YAM_MAGIC 0xF10A7654 yam_port ``drivers/net/hamradio/yam.c``
	141	CCB_MAGIC 0xf2691ad2 ccb ``drivers/scsi/ncr53c8xx.c``
	142	QUEUE_MAGIC_FREE 0xf7e1c9a3 queue_entry ``drivers/scsi/arm/queue.c``
	143	QUEUE_MAGIC_USED 0xf7e1cc33 queue_entry ``drivers/scsi/arm/queue.c``
	144	HTB_CMAGIC 0xFEFAFEF1 htb_class ``net/sched/sch_htb.c``
	145	NMI_MAGIC 0x48414d4d455201 nmi_s ``arch/mips/include/asm/sn/nmi.h``
	146	===================== ================ ======================== ==========================================
1da177e4 LT	147
1da177e4 LT	148	Note that there are also defined special per-driver magic numbers in sound
9a11e153	149	memory management. See ``include/sound/sndmagic.h`` for complete list of them. Many
1da177e4 LT	150	OSS sound drivers have their magic numbers constructed from the soundcard PCI
	151	ID - these are not listed here as well.
	152
1da177e4	153	HFS is another larger user of magic numbers - you can find them in
9a11e153	154	``fs/hfs/hfs.h``.