syzkaller/prog/encodingexec.go

// Copyright 2015 syzkaller project authors. All rights reserved.
// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.

// This file does serialization of programs for executor binary.
// The format aims at simple parsing: binary and irreversible.

package prog

import (
	"fmt"

	"github.com/google/syzkaller/sys"
)

const (
	ExecInstrEOF = ^uintptr(iota)
	ExecInstrCopyin
	ExecInstrCopyout
)

const (
	ExecArgConst = uintptr(iota)
	ExecArgResult
	ExecArgData
)

const (
	ExecBufferSize = 2 << 20

	ptrSize    = 8
	pageSize   = 4 << 10
	dataOffset = 512 << 20
)

// SerializeForExec serializes program p for execution by process pid into the provided buffer.
// If the provided buffer is too small for the program an error is returned.
func (p *Prog) SerializeForExec(buffer []byte, pid int) error {
	if debug {
		if err := p.validate(); err != nil {
			panic(fmt.Errorf("serializing invalid program: %v", err))
		}
	}
	var instrSeq uintptr
	w := &execContext{
		buf:  buffer,
		eof:  false,
		args: make(map[*Arg]argInfo),
	}
	for _, c := range p.Calls {
		// Calculate checksums.
		csumMap := calcChecksumsCall(c, pid)
		// Calculate arg offsets within structs.
		// Generate copyin instructions that fill in data into pointer arguments.
		foreachArg(c, func(arg, _ *Arg, _ *[]*Arg) {
			if arg.Kind == ArgPointer && arg.Res != nil {
				foreachSubargOffset(arg.Res, func(arg1 *Arg, offset uintptr) {
					if len(arg1.Uses) != 0 {
						w.args[arg1] = argInfo{Offset: offset}
					}
					if !sys.IsPad(arg1.Type) &&
						!(arg1.Kind == ArgData && len(arg1.Data) == 0) &&
						arg1.Type.Dir() != sys.DirOut {
						w.write(ExecInstrCopyin)
						w.write(physicalAddr(arg) + offset)
						w.writeArg(arg1, pid, csumMap)
						instrSeq++
					}
				})
			}
		})
		// Generate the call itself.
		w.write(uintptr(c.Meta.ID))
		w.write(uintptr(len(c.Args)))
		for _, arg := range c.Args {
			w.writeArg(arg, pid, csumMap)
		}
		if len(c.Ret.Uses) != 0 {
			w.args[c.Ret] = argInfo{Idx: instrSeq}
		}
		instrSeq++
		// Generate copyout instructions that persist interesting return values.
		foreachArg(c, func(arg, base *Arg, _ *[]*Arg) {
			if len(arg.Uses) == 0 {
				return
			}
			switch arg.Kind {
			case ArgReturn:
				// Idx is already assigned above.
			case ArgConst, ArgResult:
				// Create a separate copyout instruction that has own Idx.
				if base.Kind != ArgPointer {
					panic("arg base is not a pointer")
				}
				info := w.args[arg]
				info.Idx = instrSeq
				instrSeq++
				w.args[arg] = info
				w.write(ExecInstrCopyout)
				w.write(physicalAddr(base) + info.Offset)
				w.write(arg.Size())
			default:
				panic("bad arg kind in copyout")
			}
		})
	}
	w.write(ExecInstrEOF)
	if w.eof {
		return fmt.Errorf("provided buffer is too small")
	}
	return nil
}

func physicalAddr(arg *Arg) uintptr {
	if arg.Kind != ArgPointer {
		panic("physicalAddr: bad arg kind")
	}
	addr := arg.AddrPage*pageSize + dataOffset
	if arg.AddrOffset >= 0 {
		addr += uintptr(arg.AddrOffset)
	} else {
		addr += pageSize - uintptr(-arg.AddrOffset)
	}
	return addr
}

type execContext struct {
	buf  []byte
	eof  bool
	args map[*Arg]argInfo
}

type argInfo struct {
	Offset uintptr // from base pointer
	Idx    uintptr // instruction index
}

func (w *execContext) write(v uintptr) {
	if len(w.buf) < 8 {
		w.eof = true
		return
	}
	w.buf[0] = byte(v >> 0)
	w.buf[1] = byte(v >> 8)
	w.buf[2] = byte(v >> 16)
	w.buf[3] = byte(v >> 24)
	w.buf[4] = byte(v >> 32)
	w.buf[5] = byte(v >> 40)
	w.buf[6] = byte(v >> 48)
	w.buf[7] = byte(v >> 56)
	w.buf = w.buf[8:]
}

func (w *execContext) writeArg(arg *Arg, pid int, csumMap map[*Arg]*Arg) {
	switch arg.Kind {
	case ArgConst:
		if _, ok := arg.Type.(*sys.CsumType); ok {
			if arg, ok = csumMap[arg]; !ok {
				panic("csum arg is not in csum map")
			}
		}
		w.write(ExecArgConst)
		w.write(arg.Size())
		w.write(arg.Value(pid))
		w.write(arg.Type.BitfieldOffset())
		w.write(arg.Type.BitfieldLength())
	case ArgResult:
		w.write(ExecArgResult)
		w.write(arg.Size())
		w.write(w.args[arg.Res].Idx)
		w.write(arg.OpDiv)
		w.write(arg.OpAdd)
	case ArgPointer:
		w.write(ExecArgConst)
		w.write(arg.Size())
		w.write(physicalAddr(arg))
		w.write(0) // bit field offset
		w.write(0) // bit field length
	case ArgPageSize:
		w.write(ExecArgConst)
		w.write(arg.Size())
		w.write(arg.AddrPage * pageSize)
		w.write(0) // bit field offset
		w.write(0) // bit field length
	case ArgData:
		w.write(ExecArgData)
		w.write(uintptr(len(arg.Data)))
		padded := len(arg.Data)
		if pad := 8 - len(arg.Data)%8; pad != 8 {
			padded += pad
		}
		if len(w.buf) < padded {
			w.eof = true
		} else {
			copy(w.buf, arg.Data)
			w.buf = w.buf[padded:]
		}
	default:
		panic("unknown arg type")
	}
}