Based on kernel version 4.9. Page generated on 2016-12-21 14:28 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 ':' 19 upon your needs) and echo it to /proc/sys/fs/binfmt_misc/register. 20 21 Here is what the fields mean: 22 - 'name' is an identifier string. A new /proc file will be created with this 23 name below /proc/sys/fs/binfmt_misc; cannot contain slashes '/' for obvious 24 reasons. 25 - 'type' is the type of recognition. Give 'M' for magic and 'E' for extension. 26 - 'offset' is the offset of the magic/mask in the file, counted in bytes. This 27 defaults to 0 if you omit it (i.e. you write ':name:type::magic...'). Ignored 28 when using filename extension matching. 29 - 'magic' is the byte sequence binfmt_misc is matching for. The magic string 30 may contain hex-encoded characters like \x0a or \xA4. Note that you must 31 escape any NUL bytes; parsing halts at the first one. In a shell environment 32 you might have to write \\x0a to prevent the shell from eating your \. 33 If you chose filename extension matching, this is the extension to be 34 recognised (without the '.', the \x0a specials are not allowed). Extension 35 matching is case sensitive, and slashes '/' are not allowed! 36 - 'mask' is an (optional, defaults to all 0xff) mask. You can mask out some 37 bits from matching by supplying a string like magic and as long as magic. 38 The mask is anded with the byte sequence of the file. Note that you must 39 escape any NUL bytes; parsing halts at the first one. Ignored when using 40 filename extension matching. 41 - 'interpreter' is the program that should be invoked with the binary as first 42 argument (specify the full path) 43 - 'flags' is an optional field that controls several aspects of the invocation 44 of the interpreter. It is a string of capital letters, each controls a 45 certain aspect. The following flags are supported - 46 'P' - preserve-argv. Legacy behavior of binfmt_misc is to overwrite 47 the original argv with the full path to the binary. When this 48 flag is included, binfmt_misc will add an argument to the argument 49 vector for this purpose, thus preserving the original argv. 50 e.g. If your interp is set to /bin/foo and you run `blah` (which is 51 in /usr/local/bin), then the kernel will execute /bin/foo with 52 argv set to ["/bin/foo", "/usr/local/bin/blah", "blah"]. The 53 interp has to be aware of this so it can execute /usr/local/bin/blah 54 with argv set to ["blah"]. 55 'O' - open-binary. Legacy behavior of binfmt_misc is to pass the full path 56 of the binary to the interpreter as an argument. When this flag is 57 included, binfmt_misc will open the file for reading and pass its 58 descriptor as an argument, instead of the full path, thus allowing 59 the interpreter to execute non-readable binaries. This feature 60 should be used with care - the interpreter has to be trusted not to 61 emit the contents of the non-readable binary. 62 'C' - credentials. Currently, the behavior of binfmt_misc is to calculate 63 the credentials and security token of the new process according to 64 the interpreter. When this flag is included, these attributes are 65 calculated according to the binary. It also implies the 'O' flag. 66 This feature should be used with care as the interpreter 67 will run with root permissions when a setuid binary owned by root 68 is run with binfmt_misc. 69 'F' - fix binary. The usual behaviour of binfmt_misc is to spawn the 70 binary lazily when the misc format file is invoked. However, 71 this doesn't work very well in the face of mount namespaces and 72 changeroots, so the F mode opens the binary as soon as the 73 emulation is installed and uses the opened image to spawn the 74 emulator, meaning it is always available once installed, 75 regardless of how the environment changes. 76 77 78 There are some restrictions: 79 - the whole register string may not exceed 1920 characters 80 - the magic must reside in the first 128 bytes of the file, i.e. 81 offset+size(magic) has to be less than 128 82 - the interpreter string may not exceed 127 characters 83 84 To use binfmt_misc you have to mount it first. You can mount it with 85 "mount -t binfmt_misc none /proc/sys/fs/binfmt_misc" command, or you can add 86 a line "none /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0" to your 87 /etc/fstab so it auto mounts on boot. 88 89 You may want to add the binary formats in one of your /etc/rc scripts during 90 boot-up. Read the manual of your init program to figure out how to do this 91 right. 92 93 Think about the order of adding entries! Later added entries are matched first! 94 95 96 A few examples (assumed you are in /proc/sys/fs/binfmt_misc): 97 98 - enable support for em86 (like binfmt_em86, for Alpha AXP only): 99 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 100 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 101 102 - enable support for packed DOS applications (pre-configured dosemu hdimages): 103 echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register 104 105 - enable support for Windows executables using wine: 106 echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register 107 108 For java support see Documentation/java.txt 109 110 111 You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable) 112 or 1 (to enable) to /proc/sys/fs/binfmt_misc/status or /proc/.../the_name. 113 Catting the file tells you the current status of binfmt_misc/the entry. 114 115 You can remove one entry or all entries by echoing -1 to /proc/.../the_name 116 or /proc/sys/fs/binfmt_misc/status. 117 118 119 HINTS: 120 ====== 121 122 If you want to pass special arguments to your interpreter, you can 123 write a wrapper script for it. See Documentation/java.txt for an 124 example. 125 126 Your interpreter should NOT look in the PATH for the filename; the kernel 127 passes it the full filename (or the file descriptor) to use. Using $PATH can 128 cause unexpected behaviour and can be a security hazard. 129 130 131 Richard Günther <email@example.com>