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 1.15 2008-04-25 12:08:51 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, for
30 example, communications protocols, disk formats and binary files.
32 {{:http://et.redhat.com/~rjones/bitmatch/}OCaml bitmatch website}
36 A function which can parse IPv4 packets:
43 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
44 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
45 | 4 | IHL |Type of Service| Total Length |
46 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
47 | Identification |Flags| Fragment Offset |
48 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
49 | Time to Live | Protocol | Header Checksum |
50 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
52 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
53 | Destination Address |
54 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
56 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
58 | { 4 : 4; hdrlen : 4; tos : 8; length : 16;
59 identification : 16; flags : 3; fragoffset : 13;
60 ttl : 8; protocol : 8; checksum : 16;
63 options : (hdrlen-5)*32 : bitstring;
64 payload : -1 : bitstring } ->
67 printf " header length: %d * 32 bit words\n" hdrlen;
68 printf " type of service: %d\n" tos;
69 printf " packet length: %d bytes\n" length;
70 printf " identification: %d\n" identification;
71 printf " flags: %d\n" flags;
72 printf " fragment offset: %d\n" fragoffset;
73 printf " ttl: %d\n" ttl;
74 printf " protocol: %d\n" protocol;
75 printf " checksum: %d\n" checksum;
76 printf " source: %lx dest: %lx\n" source dest;
77 printf " header options + padding:\n";
78 Bitmatch.hexdump_bitstring stdout options;
79 printf " packet payload:\n";
80 Bitmatch.hexdump_bitstring stdout payload
83 eprintf "unknown IP version %d\n" version;
87 eprintf "data is smaller than one nibble:\n";
88 Bitmatch.hexdump_bitstring stderr pkt;
92 A program which can parse
93 {{:http://lxr.linux.no/linux/include/linux/ext3_fs.h}Linux EXT3 filesystem superblocks}:
96 let bits = Bitmatch.bitstring_of_file "tests/ext3_sb"
100 | { s_inodes_count : 32 : littleendian; (* Inodes count *)
101 s_blocks_count : 32 : littleendian; (* Blocks count *)
102 s_r_blocks_count : 32 : littleendian; (* Reserved blocks count *)
103 s_free_blocks_count : 32 : littleendian; (* Free blocks count *)
104 s_free_inodes_count : 32 : littleendian; (* Free inodes count *)
105 s_first_data_block : 32 : littleendian; (* First Data Block *)
106 s_log_block_size : 32 : littleendian; (* Block size *)
107 s_log_frag_size : 32 : littleendian; (* Fragment size *)
108 s_blocks_per_group : 32 : littleendian; (* # Blocks per group *)
109 s_frags_per_group : 32 : littleendian; (* # Fragments per group *)
110 s_inodes_per_group : 32 : littleendian; (* # Inodes per group *)
111 s_mtime : 32 : littleendian; (* Mount time *)
112 s_wtime : 32 : littleendian; (* Write time *)
113 s_mnt_count : 16 : littleendian; (* Mount count *)
114 s_max_mnt_count : 16 : littleendian; (* Maximal mount count *)
115 0xef53 : 16 : littleendian } -> (* Magic signature *)
117 printf "ext3 superblock:\n";
118 printf " s_inodes_count = %ld\n" s_inodes_count;
119 printf " s_blocks_count = %ld\n" s_blocks_count;
120 printf " s_free_inodes_count = %ld\n" s_free_inodes_count;
121 printf " s_free_blocks_count = %ld\n" s_free_blocks_count
124 eprintf "not an ext3 superblock!\n%!";
128 Constructing packets for a simple binary message
133 +---------------+---------------+--------------------------+
134 | type | subtype | parameter |
135 +---------------+---------------+--------------------------+
136 <-- 16 bits --> <-- 16 bits --> <------- 32 bits -------->
138 All fields are in network byte order.
141 let make_message typ subtype param =
149 {2 Loading, creating bitstrings}
151 The basic data type is the {!bitstring}, a string of bits of
152 arbitrary length. Bitstrings can be any length in bits and
153 operations do not need to be byte-aligned (although they will
154 generally be more efficient if they are byte-aligned).
156 Internally a bitstring is stored as a normal OCaml [string]
157 together with an offset and length, where the offset and length are
158 measured in bits. Thus one can efficiently form substrings of
159 bitstrings, overlay a bitstring on existing data, and load and save
160 bitstrings from files or other external sources.
162 To load a bitstring from a file use {!bitstring_of_file} or
163 {!bitstring_of_chan}.
165 There are also functions to create bitstrings from arbitrary data.
166 See the {{:#reference}reference} below.
168 {2 Matching bitstrings with patterns}
170 Use the [bitmatch] operator (part of the syntax extension) to break
171 apart a bitstring into its fields. [bitmatch] works a lot like the
172 OCaml [match] operator.
174 The general form of [bitmatch] is:
176 [bitmatch {] {i bitstring-expression} [} with]
178 [| {] {i pattern} [} ->] {i code}
180 [| {] {i pattern} [} ->] {i code}
184 As with normal match, the statement attempts to match the
185 bitstring against each pattern in turn. If none of the patterns
186 match then the standard library [Match_failure] exception is
189 Patterns look a bit different from normal match patterns. The
190 consist of a list of bitfields separated by [;] where each bitfield
191 contains a bind variable, the width (in bits) of the field, and
192 other information. Some example patterns:
197 | { version : 8; name : 8; param : 8 } -> ...
199 (* Bitstring of at least 3 bytes. First byte is the version
200 number, second byte is a field called name, third byte is
201 a field called parameter. *)
204 printf "flag is %b\n" flag
206 (* A single flag bit (mapped into an OCaml boolean). *)
208 | { len : 4; data : 1+len } ->
209 printf "len = %d, data = 0x%Lx\n" len data
211 (* A 4-bit length, followed by 1-16 bits of data, where the
212 length of the data is computed from len. *)
214 | { ipv6_source : 128 : bitstring;
215 ipv6_dest : 128 : bitstring } -> ...
217 (* IPv6 source and destination addresses. Each is 128 bits
218 and is mapped into a bitstring type which will be a substring
219 of the main bitstring expression. *)
222 You can also add conditional when-clauses:
226 when version = 4 || version = 6 -> ...
228 (* Only match and run the code when version is 4 or 6. If
229 it isn't we will drop through to the next case. *)
232 Note that the pattern is only compared against the first part of
233 the bitstring (there may be more data in the bitstring following
234 the pattern, which is not matched). In terms of regular
235 expressions you might say that the pattern matches [^pattern], not
236 [^pattern$]. To ensure that the bitstring contains only the
237 pattern, add a length -1 bitstring to the end and test that its
238 length is zero in the when-clause:
242 rest : -1 : bitstring }
243 when Bitmatch.bitstring_length rest = 0 -> ...
245 (* Only matches exactly 4 bits. *)
248 Normally the first part of each field is a binding variable,
249 but you can also match a constant, as in:
252 | { (4|6) : 4 } -> ...
254 (* Only matches if the first 4 bits contain either
255 the integer 4 or the integer 6. *)
258 {3:patternfieldreference Pattern field reference}
260 The exact format of each pattern field is:
262 [pattern : length [: qualifier [,qualifier ...]]]
264 [pattern] is the pattern, binding variable name, or constant to
265 match. [length] is the length in bits which may be either a
266 constant or an expression. The length expression is just an OCaml
267 expression and can use any values defined in the program, and refer
268 back to earlier fields (but not to later fields).
270 Integers can only have lengths in the range \[1..64\] bits. See the
271 {{:#integertypes}integer types} section below for how these are
272 mapped to the OCaml int/int32/int64 types. This is checked
273 at compile time if the length expression is constant, otherwise it is
274 checked at runtime and you will get a runtime exception eg. in
275 the case of a computed length expression.
277 A bitstring field of length -1 matches all the rest of the
278 bitstring (thus this is only useful as the last field in a
281 A bitstring field of length 0 matches an empty bitstring
282 (occasionally useful when matching optional subfields).
284 Qualifiers are a list of identifiers which control the type,
285 signedness and endianness of the field. Permissible qualifiers are:
287 - [int] (field has an integer type)
288 - [bitstring] (field is a bitstring type)
289 - [signed] (field is signed)
290 - [unsigned] (field is unsigned)
291 - [bigendian] (field is big endian - a.k.a network byte order)
292 - [littleendian] (field is little endian - a.k.a Intel byte order)
293 - [nativeendian] (field is same endianness as the machine)
295 The default settings are [int], [unsigned], [bigendian].
297 Note that many of these qualifiers cannot be used together,
298 eg. bitstrings do not have endianness. The syntax extension should
299 give you a compile-time error if you use incompatible qualifiers.
301 {3 Other cases in bitmatch}
303 As well as a list of fields, it is possible to name the
304 bitstring and/or have a default match case:
309 (* Default match case. *)
311 | { _ } as pkt -> ...
313 (* Default match case, with 'pkt' bound to the whole bitstring. *)
316 {2 Constructing bitstrings}
318 Bitstrings may be constructed using the [BITSTRING] operator (as an
319 expression). The [BITSTRING] operator takes a list of fields,
320 similar to the list of fields for matching:
331 (* Constructs a 16-bit bitstring with the first four bits containing
332 the integer 1, and the following 12 bits containing the integer 10,
333 arranged in network byte order. *)
335 Bitmatch.hexdump_bitstring stdout bits ;;
343 The format of each field is the same as for pattern fields (see
344 {{:#patternfieldreference}Pattern field reference section}), and
345 things like computed length fields, fixed value fields, insertion
346 of bitstrings within bitstrings, etc. are all supported.
348 {3 Construction exception}
350 The [BITSTRING] operator may throw a {!Construct_failure}
351 exception at runtime.
353 Runtime errors include:
355 - int field length not in the range \[1..64\]
356 - a bitstring with a length declared which doesn't have the
357 same length at runtime
358 - trying to insert an out of range value into an int field
359 (eg. an unsigned int field which is 2 bits wide can only
360 take values in the range \[0..3\]).
362 {2:integertypes Integer types}
364 Integer types are mapped to OCaml types [bool], [int], [int32] or
365 [int64] using a system which tries to ensure that (a) the types are
366 reasonably predictable and (b) the most efficient type is
369 The rules are slightly different depending on whether the bit
370 length expression in the field is a compile-time constant or a
373 Detection of compile-time constants is quite simplistic so only an
374 immediate, simple integer is recognised as a constant and anything
375 else is considered a computed expression, even expressions such as
376 [5-2] which are obviously (to our eyes) constant.
378 In any case the bit size of an integer is limited to the range
379 \[1..64\]. This is detected as a compile-time error if that is
380 possible, otherwise a runtime check is added which can throw an
381 [Invalid_argument] exception.
386 Bit size ---- OCaml type ----
387 Constant Computed expression
395 A possible future extension may allow people with 64 bit computers
396 to specify a more optimal [int] type for bit sizes in the range
397 [32..63]. If this was implemented then such code {i could not even
398 be compiled} on 32 bit platforms, so it would limit portability.
400 Another future extension may be to allow computed
401 expressions to assert min/max range for the bit size,
402 allowing a more efficient data type than int64 to be
403 used. (Of course under such circumstances there would
404 still need to be a runtime check to enforce the
409 Using the compiler directly you can do:
412 ocamlc -I +bitmatch \
413 -pp "camlp4o `ocamlc -where`/bitmatch/pa_bitmatch.cmo" \
414 bitmatch.cma test.ml -o test
417 Simpler method using findlib:
421 -package bitmatch.syntax -syntax bitmatch.syntax \
422 -linkpkg test.ml -o test
425 {2 Security and type safety}
427 {3 Security on input}
429 The main concerns for input are buffer overflows and denial
432 It is believed that this library is robust against attempted buffer
433 overflows. In addition to OCaml's normal bounds checks, we check
434 that field lengths are >= 0, and many additional checks.
436 Denial of service attacks are more problematic although we still
437 believe that the library is robust. We only work forwards through
438 the bitstring, thus computation will eventually terminate. As for
439 computed lengths, code such as this is thought to be secure:
444 buffer : Int64.to_int len : bitstring } ->
447 The [len] field can be set arbitrarily large by an attacker, but
448 when pattern-matching against the [buffer] field this merely causes
449 a test such as [if len <= remaining_size] to fail. Even if the
450 length is chosen so that [buffer] bitstring is allocated, the
451 allocation of sub-bitstrings is efficient and doesn't involve an
452 arbitary-sized allocation or any copying.
454 The main protection against attackers should therefore be to ensure
455 that the main program will only read input bitstrings up to a
456 certain length, which is outside the scope of this library.
458 {3 Security on output}
460 As with the input side, computed lengths are believed to be
464 let len = read_untrusted_source () in
465 let buffer = allocate_bitstring () in
467 buffer : len : bitstring
471 This code merely causes a check that buffer's length is the same as
472 [len]. However the program function [allocate_bitstring] must
473 refuse to allocate an oversized buffer (but that is outside the
474 scope of this library).
476 {3 Order of evaluation}
478 In [bitmatch] statements, fields are evaluated left to right.
480 Note that the when-clause is evaluated {i last}, so if you are
481 relying on the when-clause to filter cases then your code may do a
482 lot of extra and unncessary pattern-matching work on fields which
483 may never be needed just to evaluate the when-clause. You can
484 usually rearrange the code to do only the first part of the match,
485 followed by the when-clause, followed by a second inner bitmatch.
489 The current implementation is believed to be fully type-safe,
490 and makes compile and run-time checks where appropriate. If
491 you find a case where a check is missing please submit a
492 bug report or a patch.
496 These are thought to be the current limits:
498 Integers: \[1..64\] bits.
500 Bitstrings (32 bit platforms): maximum length is limited
501 by the string size, ie. 16 MBytes.
503 Bitstrings (64 bit platforms): maximum length is thought to be
504 limited by the string size, ie. effectively unlimited.
506 Bitstrings must be loaded into memory before we can match against
507 them. Thus available memory may be considered a limit for some
510 {2:reference Reference}
514 type bitstring = string * int * int
515 (** [bitstring] is the basic type used to store bitstrings.
517 The type contains the underlying data (a string),
518 the current bit offset within the string and the
519 current bit length of the string (counting from the
520 bit offset). Note that the offsets are bits, not bytes.
522 Normally you don't need to use the bitstring type
523 directly, since there are functions and syntax
524 extensions which hide the details.
525 See {!bitstring_of_file}, {!hexdump_bitstring},
529 (** {3 Exceptions} *)
531 exception Construct_failure of string * string * int * int
532 (** [Construct_failure (message, file, line, char)] may be
533 raised by the [BITSTRING] constructor.
535 Common reasons are that values are out of range of
536 the fields that contain them, or that computed lengths
537 are impossible (eg. negative length bitfields).
539 [message] is the error message.
541 [file], [line] and [char] point to the original source
542 location of the [BITSTRING] constructor that failed.
545 (** {3 Bitstrings} *)
547 val empty_bitstring : bitstring
548 (** [empty_bitstring] is the empty, zero-length bitstring. *)
550 val create_bitstring : int -> bitstring
551 (** [create_bitstring n] creates an [n] bit bitstring
552 containing all zeroes. *)
554 val make_bitstring : int -> char -> bitstring
555 (** [make_bitstring n c] creates an [n] bit bitstring
556 containing the repeated 8 bit pattern in [c].
558 For example, [make_bitstring 16 '\x5a'] will create
559 the bitstring [0x5a5a] or in binary [0101 1010 0101 1010].
561 Note that the length is in bits, not bytes. *)
563 val bitstring_of_chan : in_channel -> bitstring
564 (** [bitstring_of_chan chan] loads the contents of
565 the input channel [chan] as a bitstring.
567 The length of the final bitstring is determined
568 by the remaining input in [chan], but will always
569 be a multiple of 8 bits. *)
571 val bitstring_of_file : string -> bitstring
572 (** [bitstring_of_file filename] loads the named file
575 val hexdump_bitstring : out_channel -> bitstring -> unit
576 (** [hexdump_bitstring chan bitstring] prints the bitstring
577 to the output channel in a format similar to the
578 Unix command [hexdump -C]. *)
580 val bitstring_length : bitstring -> int
581 (** [bitstring_length bitstring] returns the length of
582 the bitstring in bits. *)
584 val string_of_bitstring : bitstring -> string
585 (** [string_of_bitstring bitstring] converts a bitstring to a string
586 (eg. to allow comparison).
588 This function is inefficient. In the best case when the bitstring
589 is nicely byte-aligned we do a [String.sub] operation. If the
590 bitstring isn't aligned then this involves a lot of bit twiddling
591 and is particularly inefficient.
593 XXX This function wouldn't be needed so much if the [bitmatch]
594 operator allowed us to pattern-match on strings. *)
596 (** {3 Bitstring buffer} *)
600 val create : unit -> t
601 val contents : t -> bitstring
602 val add_bits : t -> string -> int -> unit
603 val add_bit : t -> bool -> unit
604 val add_byte : t -> int -> unit
606 (** Buffers are mainly used by the [BITSTRING] constructor, but
607 may also be useful for end users. They work much like the
608 standard library [Buffer] module. *)
610 (** {3 Miscellaneous} *)
613 (** Set this variable to true to enable extended debugging.
614 This only works if debugging was also enabled in the
615 [pa_bitmatch.ml] file at compile time, otherwise it
620 (* Private functions, called from generated code. Do not use
621 * these directly - they are not safe.
624 val extract_bitstring : string -> int -> int -> int -> bitstring * int * int
626 val extract_remainder : string -> int -> int -> bitstring * int * int
628 val extract_bit : string -> int -> int -> int -> bool * int * int
630 val extract_char_unsigned : string -> int -> int -> int -> int * int * int
632 val extract_int_be_unsigned : string -> int -> int -> int -> int * int * int
634 val extract_int_le_unsigned : string -> int -> int -> int -> int * int * int
636 val extract_int32_be_unsigned : string -> int -> int -> int -> int32 * int * int
638 val extract_int32_le_unsigned : string -> int -> int -> int -> int32 * int * int
640 val extract_int64_be_unsigned : string -> int -> int -> int -> int64 * int * int
642 val construct_bit : Buffer.t -> bool -> int -> unit
644 val construct_char_unsigned : Buffer.t -> int -> int -> exn -> unit
646 val construct_int_be_unsigned : Buffer.t -> int -> int -> exn -> unit
648 val construct_int64_be_unsigned : Buffer.t -> int64 -> int -> exn -> unit