module Bitmatch:Bitmatch library.sig
..end
Bitmatch adds Erlang-style bitstrings and matching over bitstrings as a syntax extension and library for OCaml. You can use this module to both parse and generate binary formats, for example, communications protocols, disk formats and binary files.
A function which can parse IPv4 packets:
let display pkt =
bitmatch pkt with
(* IPv4 packet header
0 1 2 3
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 4 | IHL |Type of Service| Total Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |Flags| Fragment Offset |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time to Live | Protocol | Header Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
*)
| { 4 : 4; hdrlen : 4; tos : 8; length : 16;
identification : 16; flags : 3; fragoffset : 13;
ttl : 8; protocol : 8; checksum : 16;
source : 32;
dest : 32;
options : (hdrlen-5)*32 : bitstring;
payload : -1 : bitstring } ->
printf "IPv4:\n";
printf " header length: %d * 32 bit words\n" hdrlen;
printf " type of service: %d\n" tos;
printf " packet length: %d bytes\n" length;
printf " identification: %d\n" identification;
printf " flags: %d\n" flags;
printf " fragment offset: %d\n" fragoffset;
printf " ttl: %d\n" ttl;
printf " protocol: %d\n" protocol;
printf " checksum: %d\n" checksum;
printf " source: %lx dest: %lx\n" source dest;
printf " header options + padding:\n";
Bitmatch.hexdump_bitstring stdout options;
printf " packet payload:\n";
Bitmatch.hexdump_bitstring stdout payload
| { version : 4 } ->
eprintf "unknown IP version %d\n" version;
exit 1
| { _ } as pkt ->
eprintf "data is smaller than one nibble:\n";
Bitmatch.hexdump_bitstring stderr pkt;
exit 1
A program which can parse Linux EXT3 filesystem superblocks:
let bits = Bitmatch.bitstring_of_file "tests/ext3_sb"
let () =
bitmatch bits with
| { s_inodes_count : 32 : littleendian; (* Inodes count *)
s_blocks_count : 32 : littleendian; (* Blocks count *)
s_r_blocks_count : 32 : littleendian; (* Reserved blocks count *)
s_free_blocks_count : 32 : littleendian; (* Free blocks count *)
s_free_inodes_count : 32 : littleendian; (* Free inodes count *)
s_first_data_block : 32 : littleendian; (* First Data Block *)
s_log_block_size : 32 : littleendian; (* Block size *)
s_log_frag_size : 32 : littleendian; (* Fragment size *)
s_blocks_per_group : 32 : littleendian; (* # Blocks per group *)
s_frags_per_group : 32 : littleendian; (* # Fragments per group *)
s_inodes_per_group : 32 : littleendian; (* # Inodes per group *)
s_mtime : 32 : littleendian; (* Mount time *)
s_wtime : 32 : littleendian; (* Write time *)
s_mnt_count : 16 : littleendian; (* Mount count *)
s_max_mnt_count : 16 : littleendian; (* Maximal mount count *)
0xef53 : 16 : littleendian } -> (* Magic signature *)
printf "ext3 superblock:\n";
printf " s_inodes_count = %ld\n" s_inodes_count;
printf " s_blocks_count = %ld\n" s_blocks_count;
printf " s_free_inodes_count = %ld\n" s_free_inodes_count;
printf " s_free_blocks_count = %ld\n" s_free_blocks_count
| { _ } ->
eprintf "not an ext3 superblock!\n%!";
exit 2
Constructing packets for a simple binary message protocol:
(*
+---------------+---------------+--------------------------+
| type | subtype | parameter |
+---------------+---------------+--------------------------+
<-- 16 bits --> <-- 16 bits --> <------- 32 bits -------->
All fields are in network byte order.
*)
let make_message typ subtype param =
(BITSTRING {
typ : 16;
subtype : 16;
param : 32
}) ;;
The basic data type is the Bitmatch.bitstring
, a string of bits of
arbitrary length. Bitstrings can be any length in bits and
operations do not need to be byte-aligned (although they will
generally be more efficient if they are byte-aligned).
Internally a bitstring is stored as a normal OCaml string
together with an offset and length, where the offset and length are
measured in bits. Thus one can efficiently form substrings of
bitstrings, overlay a bitstring on existing data, and load and save
bitstrings from files or other external sources.
To load a bitstring from a file use Bitmatch.bitstring_of_file
or
Bitmatch.bitstring_of_chan
.
There are also functions to create bitstrings from arbitrary data. See the reference below.
Use the bitmatch
operator (part of the syntax extension) to break
apart a bitstring into its fields. bitmatch
works a lot like the
OCaml match
operator.
The general form of bitmatch
is:
bitmatch {
bitstring-expression } with
| {
pattern } ->
code
| {
pattern } ->
code
|
...
As with normal match, the statement attempts to match the
bitstring against each pattern in turn. If none of the patterns
match then the standard library Match_failure
exception is
thrown.
Patterns look a bit different from normal match patterns. They
consist of a list of bitfields separated by ;
where each bitfield
contains a bind variable, the width (in bits) of the field, and
other information. Some example patterns:
bitmatch bits with
| { version : 8; name : 8; param : 8 } -> ...
(* Bitstring of at least 3 bytes. First byte is the version
number, second byte is a field called name, third byte is
a field called parameter. *)
| { flag : 1 } ->
printf "flag is %b\n" flag
(* A single flag bit (mapped into an OCaml boolean). *)
| { len : 4; data : 1+len } ->
printf "len = %d, data = 0x%Lx\n" len data
(* A 4-bit length, followed by 1-16 bits of data, where the
length of the data is computed from len. *)
| { ipv6_source : 128 : bitstring;
ipv6_dest : 128 : bitstring } -> ...
(* IPv6 source and destination addresses. Each is 128 bits
and is mapped into a bitstring type which will be a substring
of the main bitstring expression. *)
You can also add conditional when-clauses:
| { version : 4 }
when version = 4 || version = 6 -> ...
(* Only match and run the code when version is 4 or 6. If
it isn't we will drop through to the next case. *)
Note that the pattern is only compared against the first part of
the bitstring (there may be more data in the bitstring following
the pattern, which is not matched). In terms of regular
expressions you might say that the pattern matches ^pattern
, not
^pattern$
. To ensure that the bitstring contains only the
pattern, add a length -1 bitstring to the end and test that its
length is zero in the when-clause:
| { n : 4;
rest : -1 : bitstring }
when Bitmatch.bitstring_length rest = 0 -> ...
(* Only matches exactly 4 bits. *)
Normally the first part of each field is a binding variable, but you can also match a constant, as in:
| { (4|6) : 4 } -> ...
(* Only matches if the first 4 bits contain either
the integer 4 or the integer 6. *)
One may also match on strings:
| { "MAGIC" : 5*8 : string } -> ...
(* Only matches if the string "MAGIC" appears at the start
of the input. *)
The exact format of each pattern field is:
pattern : length [: qualifier [,qualifier ...]]
pattern
is the pattern, binding variable name, or constant to
match. length
is the length in bits which may be either a
constant or an expression. The length expression is just an OCaml
expression and can use any values defined in the program, and refer
back to earlier fields (but not to later fields).
Integers can only have lengths in the range [1..64] bits. See the integer types section below for how these are mapped to the OCaml int/int32/int64 types. This is checked at compile time if the length expression is constant, otherwise it is checked at runtime and you will get a runtime exception eg. in the case of a computed length expression.
A bitstring field of length -1 matches all the rest of the bitstring (thus this is only useful as the last field in a pattern).
A bitstring field of length 0 matches an empty bitstring (occasionally useful when matching optional subfields).
Qualifiers are a list of identifiers/expressions which control the type, signedness and endianness of the field. Permissible qualifiers are:
int
: field has an integer typestring
: field is a string typebitstring
: field is a bitstring typesigned
: field is signedunsigned
: field is unsignedbigendian
: field is big endian - a.k.a network byte orderlittleendian
: field is little endian - a.k.a Intel byte ordernativeendian
: field is same endianness as the machineendian (expr)
: expr
should be an expression which evaluates to
a Bitmatch.endian
type, ie. LittleEndian
, BigEndian
or NativeEndian
.
The expression is an arbitrary OCaml expression and can use the
value of earlier fields in the bitmatch.int
, unsigned
, bigendian
.
Note that many of these qualifiers cannot be used together, eg. bitstrings do not have endianness. The syntax extension should give you a compile-time error if you use incompatible qualifiers.
As well as a list of fields, it is possible to name the bitstring and/or have a default match case:
| { _ } -> ...
(* Default match case. *)
| { _ } as pkt -> ...
(* Default match case, with 'pkt' bound to the whole bitstring. *)
Bitstrings may be constructed using the BITSTRING
operator (as an
expression). The BITSTRING
operator takes a list of fields,
similar to the list of fields for matching:
let version = 1 ;;
let data = 10 ;;
let bits =
BITSTRING {
version : 4;
data : 12
} ;;
(* Constructs a 16-bit bitstring with the first four bits containing
the integer 1, and the following 12 bits containing the integer 10,
arranged in network byte order. *)
Bitmatch.hexdump_bitstring stdout bits ;;
(* Prints:
00000000 10 0a |.. |
*)
The format of each field is the same as for pattern fields (see Pattern field reference section), and things like computed length fields, fixed value fields, insertion of bitstrings within bitstrings, etc. are all supported.
The BITSTRING
operator may throw a Bitmatch.Construct_failure
exception at runtime.
Runtime errors include:
Integer types are mapped to OCaml types bool
, int
, int32
or
int64
using a system which tries to ensure that (a) the types are
reasonably predictable and (b) the most efficient type is
preferred.
The rules are slightly different depending on whether the bit length expression in the field is a compile-time constant or a computed expression.
Detection of compile-time constants is quite simplistic so only an
simple integer literals and simple expressions (eg. 5*8
) are
recognized as constants.
In any case the bit size of an integer is limited to the range
[1..64]. This is detected as a compile-time error if that is
possible, otherwise a runtime check is added which can throw an
Invalid_argument
exception.
The mapping is thus:
Bit size ---- OCaml type ---- Constant Computed expression 1 bool int64 2..31 int int64 32 int32 int64 33..64 int64 int64
A possible future extension may allow people with 64 bit computers
to specify a more optimal int
type for bit sizes in the range
32..63
. If this was implemented then such code could not even
be compiled on 32 bit platforms, so it would limit portability.
Another future extension may be to allow computed expressions to assert min/max range for the bit size, allowing a more efficient data type than int64 to be used. (Of course under such circumstances there would still need to be a runtime check to enforce the size).
Using the compiler directly you can do:
ocamlc -I +bitmatch \ -pp "camlp4o `ocamlc -where`/bitmatch/pa_bitmatch.cmo" \ bitmatch.cma test.ml -o test
Simpler method using findlib:
ocamlfind ocamlc \ -package bitmatch.syntax -syntax bitmatch.syntax \ -linkpkg test.ml -o test
The main concerns for input are buffer overflows and denial of service.
It is believed that this library is robust against attempted buffer overflows. In addition to OCaml's normal bounds checks, we check that field lengths are >= 0, and many additional checks.
Denial of service attacks are more problematic. We only work forwards through the bitstring, thus computation will eventually terminate. As for computed lengths, code such as this is thought to be secure:
bitmatch bits with
| { len : 64;
buffer : Int64.to_int len : bitstring } ->
The len
field can be set arbitrarily large by an attacker, but
when pattern-matching against the buffer
field this merely causes
a test such as if len <= remaining_size
to fail. Even if the
length is chosen so that buffer
bitstring is allocated, the
allocation of sub-bitstrings is efficient and doesn't involve an
arbitary-sized allocation or any copying.
However the above does not necessarily apply to strings used in
matching, since they may cause the library to use the
Bitmatch.string_of_bitstring
function, which allocates a string.
So you should take care if you use the string
type particularly
with a computed length that is derived from external input.
The main protection against attackers should be to ensure that the main program will only read input bitstrings up to a certain length, which is outside the scope of this library.
As with the input side, computed lengths are believed to be safe. For example:
let len = read_untrusted_source () in
let buffer = allocate_bitstring () in
BITSTRING {
buffer : len : bitstring
}
This code merely causes a check that buffer's length is the same as
len
. However the program function allocate_bitstring
must
refuse to allocate an oversized buffer (but that is outside the
scope of this library).
In bitmatch
statements, fields are evaluated left to right.
Note that the when-clause is evaluated last, so if you are relying on the when-clause to filter cases then your code may do a lot of extra and unncessary pattern-matching work on fields which may never be needed just to evaluate the when-clause. You can usually rearrange the code to do only the first part of the match, followed by the when-clause, followed by a second inner bitmatch.
The current implementation is believed to be fully type-safe, and makes compile and run-time checks where appropriate. If you find a case where a check is missing please submit a bug report or a patch.
These are thought to be the current limits:
Integers: [1..64] bits.
Bitstrings (32 bit platforms): maximum length is limited by the string size, ie. 16 MBytes.
Bitstrings (64 bit platforms): maximum length is thought to be limited by the string size, ie. effectively unlimited.
Bitstrings must be loaded into memory before we can match against them. Thus available memory may be considered a limit for some applications.
type
endian =
| |
BigEndian |
| |
LittleEndian |
| |
NativeEndian |
val string_of_endian : endian -> string
typebitstring =
string * int * int
bitstring
is the basic type used to store bitstrings.
The type contains the underlying data (a string), the current bit offset within the string and the current bit length of the string (counting from the bit offset). Note that the offset and length are in bits, not bytes.
Normally you don't need to use the bitstring type directly, since there are functions and syntax extensions which hide the details.
See also Bitmatch.bitstring_of_string
, Bitmatch.bitstring_of_file
,
Bitmatch.hexdump_bitstring
, Bitmatch.bitstring_length
.
exception Construct_failure of string * string * int * int
Construct_failure (message, file, line, char)
may be
raised by the BITSTRING
constructor.
Common reasons are that values are out of range of the fields that contain them, or that computed lengths are impossible (eg. negative length bitfields).
message
is the error message.
file
, line
and char
point to the original source
location of the BITSTRING
constructor that failed.
val empty_bitstring : bitstring
empty_bitstring
is the empty, zero-length bitstring.val create_bitstring : int -> bitstring
create_bitstring n
creates an n
bit bitstring
containing all zeroes.val make_bitstring : int -> char -> bitstring
make_bitstring n c
creates an n
bit bitstring
containing the repeated 8 bit pattern in c
.
For example, make_bitstring 16 '\x5a'
will create
the bitstring 0x5a5a
or in binary 0101 1010 0101 1010
.
Note that the length is in bits, not bytes.
val bitstring_of_string : string -> bitstring
bitstring_of_string str
creates a bitstring
of length String.length str * 8
(bits) containing the
bits in str
.
Note that the bitstring uses str
as the underlying
string (see the representation of Bitmatch.bitstring
) so you
should not change str
after calling this.
val bitstring_of_file : string -> bitstring
bitstring_of_file filename
loads the named file
into a bitstring.val bitstring_of_chan : Pervasives.in_channel -> bitstring
bitstring_of_chan chan
loads the contents of
the input channel chan
as a bitstring.
The length of the final bitstring is determined
by the remaining input in chan
, but will always
be a multiple of 8 bits.
See also Bitmatch.bitstring_of_chan_max
.
val bitstring_of_chan_max : Pervasives.in_channel -> int -> bitstring
bitstring_of_chan_max chan max
works like
Bitmatch.bitstring_of_chan
but will only read up to
max
bytes from the channel (or fewer if the end of input
occurs before that).val bitstring_of_file_descr : Unix.file_descr -> bitstring
bitstring_of_file_descr fd
loads the contents of
the file descriptor fd
as a bitstring.
See also Bitmatch.bitstring_of_chan
, Bitmatch.bitstring_of_file_descr_max
.
val bitstring_of_file_descr_max : Unix.file_descr -> int -> bitstring
bitstring_of_file_descr_max fd max
works like
Bitmatch.bitstring_of_file_descr
but will only read up to
max
bytes from the channel (or fewer if the end of input
occurs before that).val bitstring_length : bitstring -> int
bitstring_length bitstring
returns the length of
the bitstring in bits.val string_of_bitstring : bitstring -> string
string_of_bitstring bitstring
converts a bitstring to a string
(eg. to allow comparison).
This function is inefficient. In the best case when the bitstring
is nicely byte-aligned we do a String.sub
operation. If the
bitstring isn't aligned then this involves a lot of bit twiddling
and is particularly inefficient.
If the bitstring is not a multiple of 8 bits wide then the
final byte of the string contains the high bits set to the
remaining bits and the low bits set to 0.
val bitstring_to_file : bitstring -> string -> unit
bitstring_to_file bits filename
writes the bitstring bits
to the file filename
. It overwrites the output file.
Some restrictions apply, see Bitmatch.bitstring_to_chan
.
val bitstring_to_chan : bitstring -> Pervasives.out_channel -> unit
bitstring_to_file bits filename
writes the bitstring bits
to the channel chan
.
Channels are made up of bytes, bitstrings can be any bit length
including fractions of bytes. So this function only works
if the length of the bitstring is an exact multiple of 8 bits
(otherwise it raises Invalid_argument "bitstring_to_chan"
).
Furthermore the function is efficient only in the case where
the bitstring is stored fully aligned, otherwise it has to
do inefficient bit twiddling like Bitmatch.string_of_bitstring
.
In the common case where the bitstring was generated by the
BITSTRING
operator and is an exact multiple of 8 bits wide,
then this function will always work efficiently.
val hexdump_bitstring : Pervasives.out_channel -> bitstring -> unit
hexdump_bitstring chan bitstring
prints the bitstring
to the output channel in a format similar to the
Unix command hexdump -C
.module Buffer:sig
..end
BITSTRING
constructor, but
may also be useful for end users.
val package : string
"ocaml-bitmatch"
val version : string
val debug : bool Pervasives.ref
pa_bitmatch.ml
file at compile time, otherwise it
does nothing.