Syscall descriptions syntax

Pseudo-formal grammar of syscall description:

syscallname "(" [arg ["," arg]*] ")" [type]
arg = argname type
argname = identifier
type = typename [ "[" type-options "]" ]
typename = "const" | "intN" | "intptr" | "flags" | "array" | "ptr" |
	   "buffer" | "string" | "strconst" | "filename" | "len" |
	   "bytesize" | "bytesizeN" | "bitsize" | "vma" | "proc"
type-options = [type-opt ["," type-opt]]

common type-options include:

"opt" - the argument is optional (like mmap fd argument, or accept peer argument)

rest of the type-options are type-specific:

"const": integer constant, type-options:
	value, underlying type (one if "intN", "intptr")
"intN"/"intptr": an integer without a particular meaning, type-options:
	optional range of values (e.g. "5:10", or "100:200")
"flags": a set of flags, type-options:
	reference to flags description (see below)
"array": a variable/fixed-length array, type-options:
	type of elements, optional size (fixed "5", or ranged "5:10", boundaries inclusive)
"ptr"/"ptr64": a pointer to an object, type-options:
	type of the object; direction (in/out/inout)
	ptr64 has size of 8 bytes regardless of target pointer size
"buffer": a pointer to a memory buffer (like read/write buffer argument), type-options:
	direction (in/out/inout)
"string": a zero-terminated memory buffer (no pointer indirection implied), type-options:
	either a string value in quotes for constant strings (e.g. "foo"),
	or a reference to string flags (special value `filename` produces file names),
	optionally followed by a buffer size (string values will be padded with \x00 to that size)
"stringnoz": a non-zero-terminated memory buffer (no pointer indirection implied), type-options:
	either a string value in quotes for constant strings (e.g. "foo"),
	or a reference to string flags,
"fmt": a string representation of an integer (not zero-terminated), type-options:
	format (one of "dec", "hex", "oct") and the value (a resource, int, flags, const or proc)
	the resulting data is always fixed-size (formatted as "%020llu", "0x%016llx" or "%023llo", respectively)
"fileoff": offset within a file
"len": length of another field (for array it is number of elements), type-options:
	argname of the object
"bytesize": similar to "len", but always denotes the size in bytes, type-options:
	argname of the object
"bitsize": similar to "len", but always denotes the size in bits, type-options:
	argname of the object
"vma": a pointer to a set of pages (used as input for mmap/munmap/mremap/madvise), type-options:
	optional number of pages (e.g. vma[7]), or a range of pages (e.g. vma[2-4])
"proc": per process int (see description below), type-options:
	value range start, how many values per process, underlying type
"text": machine code of the specified type, type-options:
	text type (x86_real, x86_16, x86_32, x86_64, arm64)
"void": type with static size 0
	mostly useful inside of templates and varlen unions, can't be syscall argument

flags/len/flags also have trailing underlying type type-option when used in structs/unions/pointers.

Flags are described as:

flagname = const ["," const]*

or for string flags as:

flagname = "\"" literal "\"" ["," "\"" literal "\""]*


int8, int16, int32 and int64 denote an integer of the corresponding size. intptr denotes a pointer-sized integer, i.e. C long type.

By appending be suffix (e.g. int16be) integers become big-endian.

It's possible to specify range of values for an integer in the format of int32[0:100].

To denote a bitfield of size N use int64:N.

It's possible to use these various kinds of ints as base types for const, flags, len and proc.

example_struct {
	f0	int8			# random 1-byte integer
	f1	const[0x42, int16be]	# const 2-byte integer with value 0x4200 (big-endian 0x42)
	f2	int32[0:100]		# random 4-byte integer with values from 0 to 100 inclusive
	f3	int64:20		# random 20-bit bitfield


Structs are described as:

structname "{" "\n"
	(fieldname type "\n")+
"}" ("[" attribute* "]")?

Structs can have attributes specified in square brackets after the struct. Attributes are:

"packed": the struct does not have paddings and has default alignment 1
"align_N": the struct has alignment N
"size": the struct is padded up to the specified size



Unions are described as:

unionname "[" "\n"
	(fieldname type "\n")+

Unions can have a trailing “varlen” attribute (specified in square brackets after the union), which means that union length is not maximum of all option lengths, but rather length of a particular chosen option.


Resources represent values that need to be passed from output of one syscall to input of another syscall. For example, close syscall requires an input value (fd) previously returned by open or pipe syscall. To achieve this, fd is declared as a resource. Resources are described as:

"resource" identifier "[" underlying_type "]" [ ":" const ("," const)* ]

underlying_type is either one of int8, int16, int32, int64, intptr or another resource (which models inheritance, for example, a socket is a subype of fd). The optional set of constants represent resource special values, for example, 0xffffffffffffffff (-1) for “no fd”, or AT_FDCWD for “the current dir”. Special values are used once in a while as resource values. If no special values specified, special value of 0 is used. Resources can then be used as types, for example:

resource fd[int32]: 0xffffffffffffffff, AT_FDCWD, 1000000
resource sock[fd]
resource sock_unix[sock]

socket(...) sock
accept(fd sock, ...) sock
listen(fd sock, backlog int32)

Type Aliases

Complex types that are often repeated can be given short type aliases using the following syntax:

type identifier underlying_type

For example:

type signalno int32[0:65]
type net_port proc[20000, 4, int16be]

Then, type alias can be used instead of the underlying type in any contexts. Underlying type needs to be described as if it‘s a struct field, that is, with the base type if it’s required. However, type alias can be used as syscall arguments as well. Underlying types are currently restricted to integer types, ptr, ptr64, const, flags and proc types.

There are some builtin type aliases:

type bool8	int8[0:1]
type bool16	int16[0:1]
type bool32	int32[0:1]
type bool64	int64[0:1]
type boolptr	intptr[0:1]

type filename string[filename]

Type Templates

Type templates can be declared as follows:

type buffer[DIR] ptr[DIR, array[int8]]
type fileoff[BASE] BASE
type nlattr[TYPE, PAYLOAD] {
	nla_len		len[parent, int16]
	nla_type	const[TYPE, int16]
	payload		PAYLOAD
} [align_4]

and later used as follows:

syscall(a buffer[in], b fileoff[int64], c ptr[in, nlattr[FOO, int32]])

There is builtin type template optional defined as:

type optional[T] [
	val	T
	void	void
] [varlen]


You can specify length of a particular field in struct or a named argument by using len, bytesize and bitsize types, for example:

write(fd fd, buf buffer[in], count len[buf]) len[buf]

sock_fprog {
	len	len[filter, int16]
	filter	ptr[in, array[sock_filter]]

If len's argument is a pointer (or a buffer), then the length of the pointee argument is used.

To denote the length of a field in N-byte words use bytesizeN, possible values for N are 1, 2, 4 and 8.

To denote the length of the parent struct, you can use len[parent, int8]. To denote the length of the higher level parent when structs are embedded into one another, you can specify the type name of the particular parent:

struct s1 {
    f0      len[s2]  # length of s2

struct s2 {
    f0      s1
    f1      array[int32]


The proc type can be used to denote per process integers. The idea is to have a separate range of values for each executor, so they don't interfere.

The simplest example is a port number. The proc[20000, 4, int16be] type means that we want to generate an int16be integer starting from 20000 and assign 4 values for each process. As a result the executor number n will get values in the [20000 + n * 4, 20000 + (n + 1) * 4) range.

Integer Constants

Integer constants can be specified as decimal literals, as 0x-prefixed hex literals, as '-surrounded char literals, or as symbolic constants extracted from kernel headers or defined by define directives. For example:

foo(a const[10], b const[-10])
foo(a const[0xabcd])
foo(a int8['a':'z'])
foo(a const[PATH_MAX])
foo(a ptr[in, array[int8, MY_PATH_MAX]])


Description files also contain include directives that refer to Linux kernel header files, incdir directives that refer to custom Linux kernel header directories and define directives that define symbolic constant values.