3 files changed, 58 insertions, 0 deletions
diff --git a/sys/README.md b/sys/README.md
index 2e5f07ec1..b5b497bc6 100644
--- a/sys/README.md
+++ b/sys/README.md
@@ -133,6 +133,36 @@ accept(fd sock, ...) sock
 listen(fd sock, backlog int32)
 ```
 
+### Length
+
+You can specify length of a particular field in struct or a named argument by using `len` and `bytesize` 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]
+}
+
+```
+
 ### Proc
 
 The `proc` type can be used to denote per process integers.
diff --git a/sys/decl.go b/sys/decl.go
index a495ffd4a..3c98d1038 100644
--- a/sys/decl.go
+++ b/sys/decl.go
@@ -29,6 +29,7 @@ const (
 
 type Type interface {
 	Name() string
+	FieldName() string
 	Dir() Dir
 	Optional() bool
 	Default() uintptr
@@ -49,6 +50,7 @@ func IsPad(t Type) bool {
 
 type TypeCommon struct {
 	TypeName   string
+	FldName    string // for struct fields and named args
 	ArgDir     Dir
 	IsOptional bool
 }
@@ -57,6 +59,10 @@ func (t *TypeCommon) Name() string {
 	return t.TypeName
 }
 
+func (t *TypeCommon) FieldName() string {
+	return t.FldName
+}
+
 func (t *TypeCommon) Optional() bool {
 	return t.IsOptional
 }
diff --git a/sys/test.txt b/sys/test.txt
index e36af549d..0ce7ec819 100644
--- a/sys/test.txt
+++ b/sys/test.txt
@@ -186,6 +186,8 @@ syz_test$length17(a0 ptr[in, syz_length_bytesize2_struct])
 syz_test$length18(a0 ptr[in, syz_length_bytesize3_struct])
 syz_test$length19(a0 ptr[in, syz_length_bf_struct])
 
+syz_test$length20(a0 ptr[in, syz_length_parent2_struct])
+
 syz_length_flags = 0, 1
 
 syz_length_int_struct {
@@ -299,6 +301,26 @@ syz_length_bf_struct {
 	f3	bytesize4[f0, int8]
 }
 
+syz_length_parent2_struct_inner_inner {
+	f1	len[parent, int8]
+	f2	len[syz_length_parent2_struct_inner_inner, int8]
+	f3	len[syz_length_parent2_struct_inner, int8]
+	f4	len[syz_length_parent2_struct, int8]
+}
+
+syz_length_parent2_struct_inner {
+	f0	syz_length_parent2_struct_inner_inner
+	f1	len[parent, int8]
+	f2	len[syz_length_parent2_struct_inner, int8]
+	f3	len[syz_length_parent2_struct, int8]
+}
+
+syz_length_parent2_struct {
+	f0	syz_length_parent2_struct_inner
+	f1	len[parent, int8]
+	f2	len[syz_length_parent2_struct, int8]
+}
+
 # Big endian
 
 syz_test$end0(a0 ptr[in, syz_end_int_struct])