Documentation/process/magic-number.rst

   1 .. _magicnumbers:
   2
   3 Linux magic numbers
   4 ===================
   5
   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
  11 It is a **very** good idea to protect kernel data structures with magic
  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
  19 The way to use magic numbers is to declare them at the beginning of
  20 the structure, like so::
  21
  22         struct tty_ldisc {
  23                 int     magic;
  24                 ...
  25         };
  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
  33 Changelog::
  34
  35                                         Theodore Ts'o
  36                                         31 Mar 94
  37
  38   The magic table is current to Linux 2.1.55.
  39
  40                                         Michael Chastain
  41                                         <mailto:mec@shout.net>
  42                                         22 Sep 1997
  43
  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.
  48
  49                                         Krzysztof G. Baranowski
  50                                         <mailto: kgb@knm.org.pl>
  51                                         29 Jul 1998
  52
  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.
  56
  57                                         Petr Baudis
  58                                         <pasky@ucw.cz>
  59                                         03 Nov 2002
  60
  61   Updated the magic table to Linux 2.5.74.
  62
  63                                         Fabian Frederick
  64                                         <ffrederick@users.sourceforge.net>
  65                                         09 Jul 2003
  66
  67
  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``
  76 CYCLADES_MAGIC        0x4359           cyclades_port            ``include/linux/cyclades.h``
  77 DB_MAGIC              0x4442           fc_info                  ``drivers/net/iph5526_novram.c``
  78 DL_MAGIC              0x444d           fc_info                  ``drivers/net/iph5526_novram.c``
  79 FASYNC_MAGIC          0x4601           fasync_struct            ``include/linux/fs.h``
  80 FF_MAGIC              0x4646           fc_info                  ``drivers/net/iph5526_novram.c``
  81 ISICOM_MAGIC          0x4d54           isi_port                 ``include/linux/isicom.h``
  82 PTY_MAGIC             0x5001                                    ``drivers/char/pty.c``
  83 PPP_MAGIC             0x5002           ppp                      ``include/linux/if_pppvar.h``
  84 SSTATE_MAGIC          0x5302           serial_state             ``include/linux/serial.h``
  85 SLIP_MAGIC            0x5302           slip                     ``drivers/net/slip.h``
  86 STRIP_MAGIC           0x5303           strip                    ``drivers/net/strip.c``
  87 SIXPACK_MAGIC         0x5304           sixpack                  ``drivers/net/hamradio/6pack.h``
  88 AX25_MAGIC            0x5316           ax_disp                  ``drivers/net/mkiss.h``
  89 TTY_MAGIC             0x5401           tty_struct               ``include/linux/tty.h``
  90 MGSL_MAGIC            0x5401           mgsl_info                ``drivers/char/synclink.c``
  91 TTY_DRIVER_MAGIC      0x5402           tty_driver               ``include/linux/tty_driver.h``
  92 MGSLPC_MAGIC          0x5402           mgslpc_info              ``drivers/char/pcmcia/synclink_cs.c``
  93 TTY_LDISC_MAGIC       0x5403           tty_ldisc                ``include/linux/tty_ldisc.h``
  94 USB_SERIAL_MAGIC      0x6702           usb_serial               ``drivers/usb/serial/usb-serial.h``
  95 FULL_DUPLEX_MAGIC     0x6969                                    ``drivers/net/ethernet/dec/tulip/de2104x.c``
  96 USB_BLUETOOTH_MAGIC   0x6d02           usb_bluetooth            ``drivers/usb/class/bluetty.c``
  97 RFCOMM_TTY_MAGIC      0x6d02                                    ``net/bluetooth/rfcomm/tty.c``
  98 USB_SERIAL_PORT_MAGIC 0x7301           usb_serial_port          ``drivers/usb/serial/usb-serial.h``
  99 CG_MAGIC              0x00090255       ufs_cylinder_group       ``include/linux/ufs_fs.h``
 100 RPORT_MAGIC           0x00525001       r_port                   ``drivers/char/rocket_int.h``
 101 LSEMAGIC              0x05091998       lse                      ``drivers/fc4/fc.c``
 102 RIEBL_MAGIC           0x09051990                                ``drivers/net/atarilance.c``
 103 NBD_REQUEST_MAGIC     0x12560953       nbd_request              ``include/linux/nbd.h``
 104 RED_MAGIC2            0x170fc2a5       (any)                    ``mm/slab.c``
 105 BAYCOM_MAGIC          0x19730510       baycom_state             ``drivers/net/baycom_epp.c``
 106 ISDN_X25IFACE_MAGIC   0x1e75a2b9       isdn_x25iface_proto_data ``drivers/isdn/isdn_x25iface.h``
 107 ECP_MAGIC             0x21504345       cdkecpsig                ``include/linux/cdk.h``
 108 LSOMAGIC              0x27091997       lso                      ``drivers/fc4/fc.c``
 109 LSMAGIC               0x2a3b4d2a       ls                       ``drivers/fc4/fc.c``
 110 WANPIPE_MAGIC         0x414C4453       sdla_{dump,exec}         ``include/linux/wanpipe.h``
 111 CS_CARD_MAGIC         0x43525553       cs_card                  ``sound/oss/cs46xx.c``
 112 LABELCL_MAGIC         0x4857434c       labelcl_info_s           ``include/asm/ia64/sn/labelcl.h``
 113 ISDN_ASYNC_MAGIC      0x49344C01       modem_info               ``include/linux/isdn.h``
 114 CTC_ASYNC_MAGIC       0x49344C01       ctc_tty_info             ``drivers/s390/net/ctctty.c``
 115 ISDN_NET_MAGIC        0x49344C02       isdn_net_local_s         ``drivers/isdn/i4l/isdn_net_lib.h``
 116 SAVEKMSG_MAGIC2       0x4B4D5347       savekmsg                 ``arch/*/amiga/config.c``
 117 CS_STATE_MAGIC        0x4c4f4749       cs_state                 ``sound/oss/cs46xx.c``
 118 SLAB_C_MAGIC          0x4f17a36d       kmem_cache               ``mm/slab.c``
 119 COW_MAGIC             0x4f4f4f4d       cow_header_v1            ``arch/um/drivers/ubd_user.c``
 120 I810_CARD_MAGIC       0x5072696E       i810_card                ``sound/oss/i810_audio.c``
 121 TRIDENT_CARD_MAGIC    0x5072696E       trident_card             ``sound/oss/trident.c``
 122 ROUTER_MAGIC          0x524d4157       wan_device               [in ``wanrouter.h`` pre 3.9]
 123 SAVEKMSG_MAGIC1       0x53415645       savekmsg                 ``arch/*/amiga/config.c``
 124 GDA_MAGIC             0x58464552       gda                      ``arch/mips/include/asm/sn/gda.h``
 125 RED_MAGIC1            0x5a2cf071       (any)                    ``mm/slab.c``
 126 EEPROM_MAGIC_VALUE    0x5ab478d2       lanai_dev                ``drivers/atm/lanai.c``
 127 HDLCDRV_MAGIC         0x5ac6e778       hdlcdrv_state            ``include/linux/hdlcdrv.h``
 128 PCXX_MAGIC            0x5c6df104       channel                  ``drivers/char/pcxx.h``
 129 KV_MAGIC              0x5f4b565f       kernel_vars_s            ``arch/mips/include/asm/sn/klkernvars.h``
 130 I810_STATE_MAGIC      0x63657373       i810_state               ``sound/oss/i810_audio.c``
 131 TRIDENT_STATE_MAGIC   0x63657373       trient_state             ``sound/oss/trident.c``
 132 M3_CARD_MAGIC         0x646e6f50       m3_card                  ``sound/oss/maestro3.c``
 133 FW_HEADER_MAGIC       0x65726F66       fw_header                ``drivers/atm/fore200e.h``
 134 SLOT_MAGIC            0x67267321       slot                     ``drivers/hotplug/cpqphp.h``
 135 SLOT_MAGIC            0x67267322       slot                     ``drivers/hotplug/acpiphp.h``
 136 LO_MAGIC              0x68797548       nbd_device               ``include/linux/nbd.h``
 137 M3_STATE_MAGIC        0x734d724d       m3_state                 ``sound/oss/maestro3.c``
 138 VMALLOC_MAGIC         0x87654320       snd_alloc_track          ``sound/core/memory.c``
 139 KMALLOC_MAGIC         0x87654321       snd_alloc_track          ``sound/core/memory.c``
 140 PWC_MAGIC             0x89DC10AB       pwc_device               ``drivers/usb/media/pwc.h``
 141 NBD_REPLY_MAGIC       0x96744668       nbd_reply                ``include/linux/nbd.h``
 142 ENI155_MAGIC          0xa54b872d       midway_eprom             ``drivers/atm/eni.h``
 143 CODA_MAGIC            0xC0DAC0DA       coda_file_info           ``fs/coda/coda_fs_i.h``
 144 YAM_MAGIC             0xF10A7654       yam_port                 ``drivers/net/hamradio/yam.c``
 145 CCB_MAGIC             0xf2691ad2       ccb                      ``drivers/scsi/ncr53c8xx.c``
 146 QUEUE_MAGIC_FREE      0xf7e1c9a3       queue_entry              ``drivers/scsi/arm/queue.c``
 147 QUEUE_MAGIC_USED      0xf7e1cc33       queue_entry              ``drivers/scsi/arm/queue.c``
 148 HTB_CMAGIC            0xFEFAFEF1       htb_class                ``net/sched/sch_htb.c``
 149 NMI_MAGIC             0x48414d4d455201 nmi_s                    ``arch/mips/include/asm/sn/nmi.h``
 150 ===================== ================ ======================== ==========================================
 151
 152 Note that there are also defined special per-driver magic numbers in sound
 153 memory management. See ``include/sound/sndmagic.h`` for complete list of them. Many
 154 OSS sound drivers have their magic numbers constructed from the soundcard PCI
 155 ID - these are not listed here as well.
 156
 157 HFS is another larger user of magic numbers - you can find them in
 158 ``fs/hfs/hfs.h``.