1 (** Bitmatch library. *)
2 (* Copyright (C) 2008 Red Hat Inc., Richard W.M. Jones
4 * This library is free software; you can redistribute it and/or
5 * modify it under the terms of the GNU Lesser General Public
6 * License as published by the Free Software Foundation; either
7 * version 2 of the License, or (at your option) any later version.
9 * This library is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 * Lesser General Public License for more details.
14 * You should have received a copy of the GNU Lesser General Public
15 * License along with this library; if not, write to the Free Software
16 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
18 * $Id: bitmatch.mli,v 1.10 2008-04-02 12:29:03 rjones Exp $
22 {{:#reference}Jump straight to the reference section for
23 documentation on types and functions}.
27 Bitmatch adds Erlang-style bitstrings and matching over bitstrings
28 as a syntax extension and library for OCaml. You can use
29 this module to both parse and generate binary formats.
31 {{:http://et.redhat.com/~rjones/bitmatch/}OCaml bitmatch website}
35 A function which can parse IPv4 packets:
42 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
43 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
44 | 4 | IHL |Type of Service| Total Length |
45 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
46 | Identification |Flags| Fragment Offset |
47 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
48 | Time to Live | Protocol | Header Checksum |
49 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
51 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
52 | Destination Address |
53 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
55 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
57 | 4 : 4; hdrlen : 4; tos : 8; length : 16;
58 identification : 16; flags : 3; fragoffset : 13;
59 ttl : 8; protocol : 8; checksum : 16;
62 options : (hdrlen-5)*32 : bitstring;
63 payload : -1 : bitstring ->
66 printf " header length: %d * 32 bit words\n" hdrlen;
67 printf " type of service: %d\n" tos;
68 printf " packet length: %d bytes\n" length;
69 printf " identification: %d\n" identification;
70 printf " flags: %d\n" flags;
71 printf " fragment offset: %d\n" fragoffset;
72 printf " ttl: %d\n" ttl;
73 printf " protocol: %d\n" protocol;
74 printf " checksum: %d\n" checksum;
75 printf " source: %lx dest: %lx\n" source dest;
76 printf " header options + padding:\n";
77 Bitmatch.hexdump_bitstring stdout options;
78 printf " packet payload:\n";
79 Bitmatch.hexdump_bitstring stdout payload
82 eprintf "unknown IP version %d\n" version;
86 eprintf "data is smaller than one nibble:\n";
87 Bitmatch.hexdump_bitstring stderr pkt;
91 A program which can parse
92 {{:http://lxr.linux.no/linux/include/linux/ext3_fs.h}Linux EXT3 filesystem superblocks}:
95 let bits = Bitmatch.bitstring_of_file "tests/ext3_sb"
99 | s_inodes_count : 32 : littleendian; (* Inodes count *)
100 s_blocks_count : 32 : littleendian; (* Blocks count *)
101 s_r_blocks_count : 32 : littleendian; (* Reserved blocks count *)
102 s_free_blocks_count : 32 : littleendian; (* Free blocks count *)
103 s_free_inodes_count : 32 : littleendian; (* Free inodes count *)
104 s_first_data_block : 32 : littleendian; (* First Data Block *)
105 s_log_block_size : 32 : littleendian; (* Block size *)
106 s_log_frag_size : 32 : littleendian; (* Fragment size *)
107 s_blocks_per_group : 32 : littleendian; (* # Blocks per group *)
108 s_frags_per_group : 32 : littleendian; (* # Fragments per group *)
109 s_inodes_per_group : 32 : littleendian; (* # Inodes per group *)
110 s_mtime : 32 : littleendian; (* Mount time *)
111 s_wtime : 32 : littleendian; (* Write time *)
112 s_mnt_count : 16 : littleendian; (* Mount count *)
113 s_max_mnt_count : 16 : littleendian; (* Maximal mount count *)
114 0xef53 : 16 : littleendian -> (* Magic signature *)
116 printf "ext3 superblock:\n";
117 printf " s_inodes_count = %ld\n" s_inodes_count;
118 printf " s_blocks_count = %ld\n" s_blocks_count;
119 printf " s_free_inodes_count = %ld\n" s_free_inodes_count;
120 printf " s_free_blocks_count = %ld\n" s_free_blocks_count
123 eprintf "not an ext3 superblock!\n%!";
127 Constructing packets for a simple binary message
132 +---------------+---------------+--------------------------+
133 | type | subtype | parameter |
134 +---------------+---------------+--------------------------+
135 <-- 16 bits --> <-- 16 bits --> <------- 32 bits -------->
137 All fields are in network byte order.
140 let make_message typ subtype param =
147 {2 Loading, creating bitstrings}
149 The basic data type is the {!bitstring}, a string of bits of
150 arbitrary length. Bitstrings can be any length in bits and
151 operations do not need to be byte-aligned (although they will
152 generally be more efficient if they are byte-aligned).
154 Internally a bitstring is stored as a normal OCaml [string]
155 together with an offset and length, where the offset and length are
156 measured in bits. Thus one can efficiently form substrings of
157 bitstrings, overlay a bitstring on existing data, and load and save
158 bitstrings from files or other external sources.
160 To load a bitstring from a file use {!bitstring_of_file} or
161 {!bitstring_of_chan}.
163 There are also functions to create bitstrings from arbitrary data.
164 See the {{:#reference}reference} below.
166 {2 Matching bitstrings with patterns}
168 Use the [bitmatch] operator (part of the syntax extension) to break
169 apart a bitstring into its fields. [bitmatch] works a lot like the
170 OCaml [match] operator.
172 The general form of [bitmatch] is:
174 [bitmatch] {i bitstring-expression} [with]
176 [|] {i pattern} [->] {i code}
178 [|] {i pattern} [->] {i code}
182 As with normal match, the statement attempts to match the
183 bitstring against each pattern in turn. If none of the patterns
184 match then the standard library [Match_failure] exception is
187 Patterns look a bit different from normal match patterns. The
188 consist of a list of bitfields separated by [;] where each bitfield
189 contains a bind variable, the width (in bits) of the field, and
190 other information. Some example patterns:
195 | version : 8; name : 8; param : 8 -> ...
197 (* Bitstring of at least 3 bytes. First byte is the version
198 number, second byte is a field called name, third byte is
199 a field called parameter. *)
202 printf "flag is %b\n" flag
204 (* A single flag bit (mapped into an OCaml boolean). *)
206 | len : 4; data : 1+len ->
207 printf "len = %d, data = 0x%Lx\n" len data
209 (* A 4-bit length, followed by 1-16 bits of data, where the
210 length of the data is computed from len. *)
212 | ipv6_source : 128 : bitstring;
213 ipv6_dest : 128 : bitstring -> ...
215 (* IPv6 source and destination addresses. Each is 128 bits
216 and is mapped into a bitstring type which will be a substring
217 of the main bitstring expression. *)
220 You can also add conditional when-clauses:
224 when version = 4 || version = 6 -> ...
226 (* Only match and run the code when version is 4 or 6. If
227 it isn't we will drop through to the next case. *)
230 Note that the pattern is only compared against the first part of
231 the bitstring (there may be more data in the bitstring following
232 the pattern, which is not matched). In terms of regular
233 expressions you might say that the pattern matches [^pattern], not
234 [^pattern$]. To ensure that the bitstring contains only the
235 pattern, add a length -1 bitstring to the end and test that its
236 length is zero in the when-clause:
240 rest : -1 : bitstring
241 when Bitmatch.bitstring_length rest = 0 -> ...
243 (* Only matches exactly 4 bits. *)
246 Normally the first part of each field is a binding variable,
247 but you can also match a constant, as in:
252 (* Only matches if the first 4 bits contain the integer 6. *)
255 {3:patternfieldreference Pattern field reference}
257 The exact format of each pattern field is:
259 [pattern : length [: qualifier [,qualifier ...]]]
261 [pattern] is the pattern, binding variable name, or constant to
262 match. [length] is the length in bits which may be either a
263 constant or an expression. The length expression is just an OCaml
264 expression and can use any values defined in the program, and refer
265 back to earlier fields (but not to later fields).
267 Integers can only have lengths in the range \[1..64\] bits. See the
268 {{:#integertypes}integer types} section below for how these are
269 mapped to the OCaml int/int32/int64 types. This is checked
270 at compile time if the length expression is constant, otherwise it is
271 checked at runtime and you will get a runtime exception eg. in
272 the case of a computed length expression.
274 A bitstring field of length -1 matches all the rest of the
275 bitstring (thus this is only useful as the last field in a
278 A bitstring field of length 0 matches an empty bitstring
279 (occasionally useful when matching optional subfields).
281 Qualifiers are a list of identifiers which control the type,
282 signedness and endianness of the field. Permissible qualifiers are:
284 - [int] (field has an integer type)
285 - [bitstring] (field is a bitstring type)
286 - [signed] (field is signed)
287 - [unsigned] (field is unsigned)
288 - [bigendian] (field is big endian - a.k.a network byte order)
289 - [littleendian] (field is little endian - a.k.a Intel byte order)
290 - [nativeendian] (field is same endianness as the machine)
292 The default settings are [int], [unsigned], [bigendian].
294 Note that many of these qualifiers cannot be used together,
295 eg. bitstrings do not have endianness. The syntax extension should
296 give you a compile-time error if you use incompatible qualifiers.
298 {3 Other cases in bitmatch}
300 As well as a list of fields, it is possible to name the
301 bitstring and/or have a default match case:
306 (* Default match case. *)
310 (* Default match case, with 'pkt' bound to the whole bitstring. *)
313 {2 Constructing bitstrings}
315 Bitstrings may be constructed using the [BITSTRING] operator (as an
316 expression). The [BITSTRING] operator takes a list of fields,
317 similar to the list of fields for matching:
327 (* Constructs a 16-bit bitstring with the first four bits containing
328 the integer 1, and the following 12 bits containing the integer 10,
329 arranged in network byte order. *)
331 Bitmatch.hexdump_bitstring stdout bits ;;
339 The format of each field is the same as for pattern fields (see
340 {{:#patternfieldreference}Pattern field reference section}), and
341 things like computed length fields, fixed value fields, insertion
342 of bitstrings within bitstrings, etc. are all supported.
344 {3 Construction exception}
346 The [BITSTRING] operator may throw a {!Construct_failure}
347 exception at runtime.
349 Runtime errors include:
351 - int field length not in the range \[1..64\]
352 - a bitstring with a length declared which doesn't have the
353 same length at runtime
354 - trying to insert an out of range value into an int field
355 (eg. an unsigned int field which is 2 bits wide can only
356 take values in the range \[0..3\]).
358 {2:integertypes Integer types}
360 Integer types are mapped to OCaml types [bool], [int], [int32] or
361 [int64] using a system which tries to ensure that (a) the types are
362 reasonably predictable and (b) the most efficient type is
365 The rules are slightly different depending on whether the bit
366 length expression in the field is a compile-time constant or a
369 Detection of compile-time constants is quite simplistic so only an
370 immediate, simple integer is recognised as a constant and anything
371 else is considered a computed expression, even expressions such as
372 [5-2] which are obviously (to our eyes) constant.
374 In any case the bit size of an integer is limited to the range
375 \[1..64\]. This is detected as a compile-time error if that is
376 possible, otherwise a runtime check is added which can throw an
377 [Invalid_argument] exception.
382 Bit size ---- OCaml type ----
383 Constant Computed expression
391 A possible future extension may allow people with 64 bit computers
392 to specify a more optimal [int] type for bit sizes in the range
393 [32..63]. If this was implemented then such code {i could not even
394 be compiled} on 32 bit platforms, so it would limit portability.
396 Another future extension may be to allow computed
397 expressions to assert min/max range for the bit size,
398 allowing a more efficient data type than int64 to be
399 used. (Of course under such circumstances there would
400 still need to be a runtime check to enforce the
405 Using the compiler directly you can do:
408 ocamlc -I bitmatch -pp "camlp4o -I bitmatch pa_bitmatch.cmo" foo.ml -o foo
414 ocamlfind ocamlc -package bitmatch.syntax -linkpkg foo.ml -o foo
417 {2 Security and type safety}
419 {3 Security on input}
421 The main concerns for input are buffer overflows and denial
424 It is believed that this library is robust against attempted buffer
425 overflows. In addition to OCaml's normal bounds checks, we check
426 that field lengths are >= 0, and many additional checks.
428 Denial of service attacks are more problematic although we still
429 believe that the library is robust. We only work forwards through
430 the bitstring, thus computation will eventually terminate. As for
431 computed lengths, code such as this is thought to be secure:
436 buffer : Int64.to_int len : bitstring ->
439 The [len] field can be set arbitrarily large by an attacker, but
440 when pattern-matching against the [buffer] field this merely causes
441 a test such as [if len <= remaining_size] to fail. Even if the
442 length is chosen so that [buffer] bitstring is allocated, the
443 allocation of sub-bitstrings is efficient and doesn't involve an
444 arbitary-sized allocation or any copying.
446 The main protection against attackers should therefore be to ensure
447 that the main program will only read input bitstrings up to a
448 certain length, which is outside the scope of this library.
450 {3 Security on output}
452 As with the input side, computed lengths are believed to be
456 let len = read_untrusted_source () in
457 let buffer = allocate_bitstring () in
459 buffer : len : bitstring
462 This code merely causes a check that buffer's length is the same as
463 [len]. However the program function [allocate_bitstring] must
464 refuse to allocate an oversized buffer (but that is outside the
465 scope of this library).
467 {3 Order of evaluation}
469 In [bitmatch] statements, fields are evaluated left to right.
471 Note that the when-clause is evaluated {i last}, so if you are
472 relying on the when-clause to filter cases then your code may do a
473 lot of extra and unncessary pattern-matching work on fields which
474 may never be needed just to evaluate the when-clause. You can
475 usually rearrange the code to do only the first part of the match,
476 followed by the when-clause, followed by a second inner bitmatch.
480 The current implementation is believed to be fully type-safe,
481 and makes compile and run-time checks where appropriate. If
482 you find a case where a check is missing please submit a
483 bug report or a patch.
487 These are thought to be the current limits:
489 Integers: \[1..64\] bits.
491 Bitstrings (32 bit platforms): maximum length is limited
492 by the string size, ie. 16 MBytes.
494 Bitstrings (64 bit platforms): maximum length is thought to be
495 limited by the string size, ie. effectively unlimited.
497 Bitstrings must be loaded into memory before we can match against
498 them. Thus available memory may be considered a limit for some
501 {2:reference Reference}
505 type bitstring = string * int * int
506 (** [bitstring] is the basic type used to store bitstrings.
508 The type contains the underlying data (a string),
509 the current bit offset within the string and the
510 current bit length of the string (counting from the
511 bit offset). Note that the offsets are bits, not bytes.
513 Normally you don't need to use the bitstring type
514 directly, since there are functions and syntax
515 extensions which hide the details.
516 See {!bitstring_of_file}, {!hexdump_bitstring},
520 (** {3 Exceptions} *)
522 exception Construct_failure of string * string * int * int
523 (** [Construct_failure (message, file, line, char)] may be
524 raised by the [BITSTRING] constructor.
526 Common reasons are that values are out of range of
527 the fields that contain them, or that computed lengths
528 are impossible (eg. negative length bitfields).
530 [message] is the error message.
532 [file], [line] and [char] point to the original source
533 location of the [BITSTRING] constructor that failed.
536 (** {3 Bitstrings} *)
538 val empty_bitstring : bitstring
539 (** [empty_bitstring] is the empty, zero-length bitstring. *)
541 val create_bitstring : int -> bitstring
542 (** [create_bitstring n] creates an [n] bit bitstring
543 containing all zeroes. *)
545 val make_bitstring : int -> char -> bitstring
546 (** [make_bitstring n c] creates an [n] bit bitstring
547 containing the repeated 8 bit pattern in [c].
549 For example, [make_bitstring 16 '\x5a'] will create
550 the bitstring [0x5a5a] or in binary [0101 1010 0101 1010].
552 Note that the length is in bits, not bytes. *)
554 val bitstring_of_chan : in_channel -> bitstring
555 (** [bitstring_of_chan chan] loads the contents of
556 the input channel [chan] as a bitstring.
558 The length of the final bitstring is determined
559 by the remaining input in [chan], but will always
560 be a multiple of 8 bits. *)
562 val bitstring_of_file : string -> bitstring
563 (** [bitstring_of_file filename] loads the named file
566 val hexdump_bitstring : out_channel -> bitstring -> unit
567 (** [hexdump_bitstring chan bitstring] prints the bitstring
568 to the output channel in a format similar to the
569 Unix command [hexdump -C]. *)
571 val bitstring_length : bitstring -> int
572 (** [bitstring_length bitstring] returns the length of
573 the bitstring in bits. *)
575 (** {3 Bitstring buffer} *)
579 val create : unit -> t
580 val contents : t -> bitstring
581 val add_bits : t -> string -> int -> unit
582 val add_bit : t -> bool -> unit
583 val add_byte : t -> int -> unit
585 (** Buffers are mainly used by the [BITSTRING] constructor, but
586 may also be useful for end users. They work much like the
587 standard library [Buffer] module. *)
589 (** {3 Miscellaneous} *)
592 (** Set this variable to true to enable extended debugging.
593 This only works if debugging was also enabled in the
594 [pa_bitmatch.ml] file at compile time, otherwise it
599 (* Private functions, called from generated code. Do not use
600 * these directly - they are not safe.
603 val extract_bitstring : string -> int -> int -> int -> bitstring * int * int
605 val extract_remainder : string -> int -> int -> bitstring * int * int
607 val extract_bit : string -> int -> int -> int -> bool * int * int
609 val extract_char_unsigned : string -> int -> int -> int -> int * int * int
611 val extract_int_be_unsigned : string -> int -> int -> int -> int * int * int
613 val extract_int_le_unsigned : string -> int -> int -> int -> int * int * int
615 val extract_int32_be_unsigned : string -> int -> int -> int -> int32 * int * int
617 val extract_int32_le_unsigned : string -> int -> int -> int -> int32 * int * int
619 val extract_int64_be_unsigned : string -> int -> int -> int -> int64 * int * int
621 val construct_bit : Buffer.t -> bool -> int -> unit
623 val construct_char_unsigned : Buffer.t -> int -> int -> exn -> unit
625 val construct_int_be_unsigned : Buffer.t -> int -> int -> exn -> unit
627 val construct_int64_be_unsigned : Buffer.t -> int64 -> int -> exn -> unit