util/cstruct: add package for decoding padded C structures (#5429)

I was working on my "dump iptables rules using only syscalls" branch and
had a bunch of C structure decoding to do. Rather than manually
calculating the padding or using unsafe trickery to actually cast
variable-length structures to Go types, I'd rather use a helper package
that deals with padding for me.

Padding rules were taken from the following article:
  http://www.catb.org/esr/structure-packing/

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>

util/cstruct/cstruct.go

@@ -0,0 +1,179 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package cstruct provides a helper for decoding binary data that is in the
+// form of a padded C structure.
+package cstruct
+
+import (
+	"errors"
+	"io"
+
+	"tailscale.com/util/endian"
+)
+
+// Size of a pointer-typed value, in bits
+const pointerSize = 32 << (^uintptr(0) >> 63)
+
+// We assume that non-64-bit platforms are 32-bit; we don't expect Go to run on
+// a 16- or 8-bit architecture any time soon.
+const is64Bit = pointerSize == 64
+
+// Decoder reads and decodes padded fields from a slice of bytes. All fields
+// are decoded with native endianness.
+//
+// Methods of a Decoder do not return errors, but rather store any error within
+// the Decoder. The first error can be obtained via the Err method; after the
+// first error, methods will return the zero value for their type.
+type Decoder struct {
+	b    []byte
+	off  int
+	err  error
+	dbuf [8]byte // for decoding
+}
+
+// NewDecoder creates a Decoder from a byte slice.
+func NewDecoder(b []byte) *Decoder {
+	return &Decoder{b: b}
+}
+
+var errUnsupportedSize = errors.New("unsupported size")
+
+func padBytes(offset, size int) int {
+	if offset == 0 || size == 1 {
+		return 0
+	}
+	remainder := offset % size
+	return size - remainder
+}
+
+func (d *Decoder) getField(b []byte) error {
+	size := len(b)
+
+	// We only support fields that are multiples of 2 (or 1-sized)
+	if size != 1 && size&1 == 1 {
+		return errUnsupportedSize
+	}
+
+	// Fields are aligned to their size
+	padBytes := padBytes(d.off, size)
+	if d.off+size+padBytes > len(d.b) {
+		return io.EOF
+	}
+	d.off += padBytes
+
+	copy(b, d.b[d.off:d.off+size])
+	d.off += size
+	return nil
+}
+
+// Err returns the first error that was encountered by this Decoder.
+func (d *Decoder) Err() error {
+	return d.err
+}
+
+// Offset returns the current read offset for data in the buffer.
+func (d *Decoder) Offset() int {
+	return d.off
+}
+
+// Byte returns a single byte from the buffer.
+func (d *Decoder) Byte() byte {
+	if d.err != nil {
+		return 0
+	}
+
+	if err := d.getField(d.dbuf[0:1]); err != nil {
+		d.err = err
+		return 0
+	}
+	return d.dbuf[0]
+}
+
+// Byte returns a number of bytes from the buffer based on the size of the
+// input slice. No padding is applied.
+//
+// If an error is encountered or this Decoder has previously encountered an
+// error, no changes are made to the provided buffer.
+func (d *Decoder) Bytes(b []byte) {
+	if d.err != nil {
+		return
+	}
+
+	// No padding for byte slices
+	size := len(b)
+	if d.off+size >= len(d.b) {
+		d.err = io.EOF
+		return
+	}
+	copy(b, d.b[d.off:d.off+size])
+	d.off += size
+}
+
+// Uint16 returns a uint16 decoded from the buffer.
+func (d *Decoder) Uint16() uint16 {
+	if d.err != nil {
+		return 0
+	}
+
+	if err := d.getField(d.dbuf[0:2]); err != nil {
+		d.err = err
+		return 0
+	}
+	return endian.Native.Uint16(d.dbuf[0:2])
+}
+
+// Uint32 returns a uint32 decoded from the buffer.
+func (d *Decoder) Uint32() uint32 {
+	if d.err != nil {
+		return 0
+	}
+
+	if err := d.getField(d.dbuf[0:4]); err != nil {
+		d.err = err
+		return 0
+	}
+	return endian.Native.Uint32(d.dbuf[0:4])
+}
+
+// Uint64 returns a uint64 decoded from the buffer.
+func (d *Decoder) Uint64() uint64 {
+	if d.err != nil {
+		return 0
+	}
+
+	if err := d.getField(d.dbuf[0:8]); err != nil {
+		d.err = err
+		return 0
+	}
+	return endian.Native.Uint64(d.dbuf[0:8])
+}
+
+// Uintptr returns a uintptr decoded from the buffer.
+func (d *Decoder) Uintptr() uintptr {
+	if d.err != nil {
+		return 0
+	}
+
+	if is64Bit {
+		return uintptr(d.Uint64())
+	} else {
+		return uintptr(d.Uint32())
+	}
+}
+
+// Int16 returns a int16 decoded from the buffer.
+func (d *Decoder) Int16() int16 {
+	return int16(d.Uint16())
+}
+
+// Int32 returns a int32 decoded from the buffer.
+func (d *Decoder) Int32() int32 {
+	return int32(d.Uint32())
+}
+
+// Int64 returns a int64 decoded from the buffer.
+func (d *Decoder) Int64() int64 {
+	return int64(d.Uint64())
+}