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 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``
  80 PTY_MAGIC             0x5001                                    ``drivers/char/pty.c``
  81 PPP_MAGIC             0x5002           ppp                      ``include/linux/if_pppvar.h``
  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``
  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``
  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``
  97 LSEMAGIC              0x05091998       lse                      ``drivers/fc4/fc.c``
  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``
 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``
 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 ===================== ================ ======================== ==========================================
 147
 148 Note that there are also defined special per-driver magic numbers in sound
 149 memory management. See ``include/sound/sndmagic.h`` for complete list of them. Many
 150 OSS sound drivers have their magic numbers constructed from the soundcard PCI
 151 ID - these are not listed here as well.
 152
 153 HFS is another larger user of magic numbers - you can find them in
 154 ``fs/hfs/hfs.h``.