Contents of /alx-src/tags/kernel26-2.6.12-alx-r9/Documentation/binfmt_misc.txt
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Wed Mar 4 11:03:09 2009 UTC (15 years, 6 months ago) by niro
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Tag kernel26-2.6.12-alx-r9
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[0]. Legacy behavior of binfmt_misc is to overwrite the |
41 | original argv[0] 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[0]. |
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/~rguenth/linux/binfmt_misc.html |
115 | |
116 | Richard Günther <rguenth@tat.physik.uni-tuebingen.de> |