Based on kernel version 3.13. Page generated on 2014-01-20 22:00 EST.
1 Kernel Support for miscellaneous (your favourite) Binary Formats v1.1 2 ===================================================================== 3 4 This Kernel feature allows you to invoke almost (for restrictions see below) 5 every program by simply typing its name in the shell. 6 This includes for example compiled Java(TM), Python or Emacs programs. 7 8 To achieve this you must tell binfmt_misc which interpreter has to be invoked 9 with which binary. Binfmt_misc recognises the binary-type by matching some bytes 10 at the beginning of the file with a magic byte sequence (masking out specified 11 bits) you have supplied. Binfmt_misc can also recognise a filename extension 12 aka '.com' or '.exe'. 13 14 First you must mount binfmt_misc: 15 mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc 16 17 To actually register a new binary type, you have to set up a string looking like 18 :name:type:offset:magic:mask:interpreter:flags (where you can choose the ':' upon 19 your needs) and echo it to /proc/sys/fs/binfmt_misc/register. 20 Here is what the fields mean: 21 - 'name' is an identifier string. A new /proc file will be created with this 22 name below /proc/sys/fs/binfmt_misc 23 - 'type' is the type of recognition. Give 'M' for magic and 'E' for extension. 24 - 'offset' is the offset of the magic/mask in the file, counted in bytes. This 25 defaults to 0 if you omit it (i.e. you write ':name:type::magic...') 26 - 'magic' is the byte sequence binfmt_misc is matching for. The magic string 27 may contain hex-encoded characters like \x0a or \xA4. In a shell environment 28 you will have to write \\x0a to prevent the shell from eating your \. 29 If you chose filename extension matching, this is the extension to be 30 recognised (without the '.', the \x0a specials are not allowed). Extension 31 matching is case sensitive! 32 - 'mask' is an (optional, defaults to all 0xff) mask. You can mask out some 33 bits from matching by supplying a string like magic and as long as magic. 34 The mask is anded with the byte sequence of the file. 35 - 'interpreter' is the program that should be invoked with the binary as first 36 argument (specify the full path) 37 - 'flags' is an optional field that controls several aspects of the invocation 38 of the interpreter. It is a string of capital letters, each controls a certain 39 aspect. The following flags are supported - 40 'P' - preserve-argv. Legacy behavior of binfmt_misc is to overwrite the 41 original argv with the full path to the binary. When this flag is 42 included, binfmt_misc will add an argument to the argument vector for 43 this purpose, thus preserving the original argv. 44 'O' - open-binary. Legacy behavior of binfmt_misc is to pass the full path 45 of the binary to the interpreter as an argument. When this flag is 46 included, binfmt_misc will open the file for reading and pass its 47 descriptor as an argument, instead of the full path, thus allowing 48 the interpreter to execute non-readable binaries. This feature should 49 be used with care - the interpreter has to be trusted not to emit 50 the contents of the non-readable binary. 51 'C' - credentials. Currently, the behavior of binfmt_misc is to calculate 52 the credentials and security token of the new process according to 53 the interpreter. When this flag is included, these attributes are 54 calculated according to the binary. It also implies the 'O' flag. 55 This feature should be used with care as the interpreter 56 will run with root permissions when a setuid binary owned by root 57 is run with binfmt_misc. 58 59 60 There are some restrictions: 61 - the whole register string may not exceed 255 characters 62 - the magic must reside in the first 128 bytes of the file, i.e. 63 offset+size(magic) has to be less than 128 64 - the interpreter string may not exceed 127 characters 65 66 To use binfmt_misc you have to mount it first. You can mount it with 67 "mount -t binfmt_misc none /proc/sys/fs/binfmt_misc" command, or you can add 68 a line "none /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0" to your 69 /etc/fstab so it auto mounts on boot. 70 71 You may want to add the binary formats in one of your /etc/rc scripts during 72 boot-up. Read the manual of your init program to figure out how to do this 73 right. 74 75 Think about the order of adding entries! Later added entries are matched first! 76 77 78 A few examples (assumed you are in /proc/sys/fs/binfmt_misc): 79 80 - enable support for em86 (like binfmt_em86, for Alpha AXP only): 81 echo ':i386:M::\x7fELF\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x03:\xff\xff\xff\xff\xff\xfe\xfe\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfb\xff\xff:/bin/em86:' > register 82 echo ':i486:M::\x7fELF\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x06:\xff\xff\xff\xff\xff\xfe\xfe\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfb\xff\xff:/bin/em86:' > register 83 84 - enable support for packed DOS applications (pre-configured dosemu hdimages): 85 echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register 86 87 - enable support for Windows executables using wine: 88 echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register 89 90 For java support see Documentation/java.txt 91 92 93 You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable) 94 or 1 (to enable) to /proc/sys/fs/binfmt_misc/status or /proc/.../the_name. 95 Catting the file tells you the current status of binfmt_misc/the entry. 96 97 You can remove one entry or all entries by echoing -1 to /proc/.../the_name 98 or /proc/sys/fs/binfmt_misc/status. 99 100 101 HINTS: 102 ====== 103 104 If you want to pass special arguments to your interpreter, you can 105 write a wrapper script for it. See Documentation/java.txt for an 106 example. 107 108 Your interpreter should NOT look in the PATH for the filename; the kernel 109 passes it the full filename (or the file descriptor) to use. Using $PATH can 110 cause unexpected behaviour and can be a security hazard. 111 112 113 There is a web page about binfmt_misc at 114 http://www.tat.physik.uni-tuebingen.de 115 116 Richard GÃ¼nther <email@example.com>