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Diffstat (limited to 'Documentation/binfmt_misc.txt')
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diff --git a/Documentation/binfmt_misc.txt b/Documentation/binfmt_misc.txt new file mode 100644 index 000000000000..d097f09ee15a --- /dev/null +++ b/Documentation/binfmt_misc.txt @@ -0,0 +1,116 @@ + Kernel Support for miscellaneous (your favourite) Binary Formats v1.1 + ===================================================================== + +This Kernel feature allows you to invoke almost (for restrictions see below) +every program by simply typing its name in the shell. +This includes for example compiled Java(TM), Python or Emacs programs. + +To achieve this you must tell binfmt_misc which interpreter has to be invoked +with which binary. Binfmt_misc recognises the binary-type by matching some bytes +at the beginning of the file with a magic byte sequence (masking out specified +bits) you have supplied. Binfmt_misc can also recognise a filename extension +aka '.com' or '.exe'. + +First you must mount binfmt_misc: + mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc + +To actually register a new binary type, you have to set up a string looking like +:name:type:offset:magic:mask:interpreter:flags (where you can choose the ':' upon +your needs) and echo it to /proc/sys/fs/binfmt_misc/register. +Here is what the fields mean: + - 'name' is an identifier string. A new /proc file will be created with this + name below /proc/sys/fs/binfmt_misc + - 'type' is the type of recognition. Give 'M' for magic and 'E' for extension. + - 'offset' is the offset of the magic/mask in the file, counted in bytes. This + defaults to 0 if you omit it (i.e. you write ':name:type::magic...') + - 'magic' is the byte sequence binfmt_misc is matching for. The magic string + may contain hex-encoded characters like \x0a or \xA4. In a shell environment + you will have to write \\x0a to prevent the shell from eating your \. + If you chose filename extension matching, this is the extension to be + recognised (without the '.', the \x0a specials are not allowed). Extension + matching is case sensitive! + - 'mask' is an (optional, defaults to all 0xff) mask. You can mask out some + bits from matching by supplying a string like magic and as long as magic. + The mask is anded with the byte sequence of the file. + - 'interpreter' is the program that should be invoked with the binary as first + argument (specify the full path) + - 'flags' is an optional field that controls several aspects of the invocation + of the interpreter. It is a string of capital letters, each controls a certain + aspect. The following flags are supported - + 'P' - preserve-argv[0]. Legacy behavior of binfmt_misc is to overwrite the + original argv[0] with the full path to the binary. When this flag is + included, binfmt_misc will add an argument to the argument vector for + this purpose, thus preserving the original argv[0]. + 'O' - open-binary. Legacy behavior of binfmt_misc is to pass the full path + of the binary to the interpreter as an argument. When this flag is + included, binfmt_misc will open the file for reading and pass its + descriptor as an argument, instead of the full path, thus allowing + the interpreter to execute non-readable binaries. This feature should + be used with care - the interpreter has to be trusted not to emit + the contents of the non-readable binary. + 'C' - credentials. Currently, the behavior of binfmt_misc is to calculate + the credentials and security token of the new process according to + the interpreter. When this flag is included, these attributes are + calculated according to the binary. It also implies the 'O' flag. + This feature should be used with care as the interpreter + will run with root permissions when a setuid binary owned by root + is run with binfmt_misc. + + +There are some restrictions: + - the whole register string may not exceed 255 characters + - the magic must reside in the first 128 bytes of the file, i.e. + offset+size(magic) has to be less than 128 + - the interpreter string may not exceed 127 characters + +To use binfmt_misc you have to mount it first. You can mount it with +"mount -t binfmt_misc none /proc/sys/fs/binfmt_misc" command, or you can add +a line "none /proc/sys/fs/binfmt_misc binfmt_misc defaults 0 0" to your +/etc/fstab so it auto mounts on boot. + +You may want to add the binary formats in one of your /etc/rc scripts during +boot-up. Read the manual of your init program to figure out how to do this +right. + +Think about the order of adding entries! Later added entries are matched first! + + +A few examples (assumed you are in /proc/sys/fs/binfmt_misc): + +- enable support for em86 (like binfmt_em86, for Alpha AXP only): + 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 + 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 + +- enable support for packed DOS applications (pre-configured dosemu hdimages): + echo ':DEXE:M::\x0eDEX::/usr/bin/dosexec:' > register + +- enable support for Windows executables using wine: + echo ':DOSWin:M::MZ::/usr/local/bin/wine:' > register + +For java support see Documentation/java.txt + + +You can enable/disable binfmt_misc or one binary type by echoing 0 (to disable) +or 1 (to enable) to /proc/sys/fs/binfmt_misc/status or /proc/.../the_name. +Catting the file tells you the current status of binfmt_misc/the entry. + +You can remove one entry or all entries by echoing -1 to /proc/.../the_name +or /proc/sys/fs/binfmt_misc/status. + + +HINTS: +====== + +If you want to pass special arguments to your interpreter, you can +write a wrapper script for it. See Documentation/java.txt for an +example. + +Your interpreter should NOT look in the PATH for the filename; the kernel +passes it the full filename (or the file descriptor) to use. Using $PATH can +cause unexpected behaviour and can be a security hazard. + + +There is a web page about binfmt_misc at +http://www.tat.physik.uni-tuebingen.de/~rguenth/linux/binfmt_misc.html + +Richard Gnther <rguenth@tat.physik.uni-tuebingen.de> |