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tracer.go
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tracer.go
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// Copyright 2018 the u-root 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 strace supports tracing programs.
// The basic control of tracing is via a Tracer, which returns raw
// TraceRecords via a chan. The easiest way to create a Tracer is via
// RunTracerFromCommand, which uses a filled out exec.Cmd to start a
// process and produce trace records.
// Forking is not yet supported.
package strace
import (
"encoding/binary"
"fmt"
"io"
"os/exec"
"runtime"
"syscall"
"time"
"golang.org/x/sys/unix"
)
// Debug is a do-nothing function which can be replaced by, e.g., log.Printf
var Debug = func(string, ...interface{}) {}
// TraceRecord has information about a ptrace event.
type TraceRecord struct {
EX EventType
Regs unix.PtraceRegs
Serial int
Pid int
Err error
Errno int
Args SyscallArguments
Ret [2]SyscallArgument
Sysno int
Time time.Duration
Out string
}
// Tracer has information to trace one process. It can be created by
// starting a command, or attaching. Attaching is not supported yet.
type Tracer struct {
Pid int
EX EventType
Records chan *TraceRecord
Count int
Raw bool // Set by the user, it disables pretty printing
Name string
Printer func(t *Tracer, r *TraceRecord)
Last *TraceRecord
// We save the output from the previous Enter so Exit handling
// can both use and adjust it.
output []string
}
// New returns a new Tracer.
func New() (*Tracer, error) {
return &Tracer{Pid: -1, Records: make(chan *TraceRecord, 1), Printer: SysCall}, nil
}
// RunTracerFromCommand runs a Tracer given an exec.Cmd.
// It locks itself down with LockOSThread and will unlock itself
// when it returns, after the command and all its children exit.
func (t *Tracer) RunTracerFromCmd(c *exec.Cmd) {
defer close(t.Records)
if c.SysProcAttr == nil {
c.SysProcAttr = &syscall.SysProcAttr{}
}
c.SysProcAttr.Ptrace = true
// Because the go runtime forks traced processes with PTRACE_TRACEME
// we need to maintain the parent-child relationship for ptrace to work.
// We've learned this the hard way. So we lock down this thread to
// this proc, and start the command here.
// Note this function will block; if you want it to be nonblocking you
// need to use go etc.
runtime.LockOSThread()
defer runtime.UnlockOSThread()
if err := c.Start(); err != nil {
Debug("Start gets err %v", err)
t.Records <- &TraceRecord{Err: err}
return
}
Debug("Start gets pid %v", c.Process.Pid)
if err := c.Wait(); err != nil {
fmt.Printf("Wait returned: %v\n", err)
t.Records <- &TraceRecord{Err: err}
}
t.Pid = c.Process.Pid
t.Name = fmt.Sprintf("%s(%d)", c.Args[0], t.Pid)
t.EX = Exit
Run(t)
}
// NewTracerChild creates a tracer from a tracer.
func NewTracerChild(pid int) (*Tracer, error) {
nt, err := New()
if err != nil {
return nil, err
}
nt.Pid = pid
nt.Name = fmt.Sprintf("%d", pid)
nt.EX = Exit
return nt, nil
}
// Step steps a Tracer by issuing a PtraceSyscall to it and then doing a Wait.
// Note that Step waits for any child to return, not just the one we are stepping.
func (t *Tracer) Step(e EventType) (int, error) {
Debug("Step %d", t.Pid)
if err := unix.PtraceSyscall(t.Pid, 0); err != nil {
r := &TraceRecord{Serial: t.Count, EX: e, Pid: t.Pid}
Debug("ptracesyscall for %d gets %v", t.Pid, err)
r.Err = fmt.Errorf("unix.PtraceSyscall: %d: %s: %v", t.Pid, t.Name, err)
t.Records <- r
return -1, r.Err
}
Debug("Stepped %d, now Wait", t.Pid)
pid, w, err := Wait(-1)
Debug("Wait returns (%d, %v, %v)", pid, w, err)
if err != nil {
r := &TraceRecord{Serial: t.Count, EX: e, Pid: t.Pid}
r.Err = fmt.Errorf("unix.Wait: %d: %s: %v, %v", t.Pid, t.Name, w, err)
Debug("wait4 for %d gets %v, %v", t.Pid, w, err)
t.Records <- r
return -1, r.Err
}
Debug("Step %d: back from wait", pid)
return pid, nil
}
// Run runs a set of processes as defined by a Tracer. Because of Unix restrictions
// around which processes which can trace other processes, Run gets a tad involved.
// It is implemented as a simple loop, driving events via ptrace commands to processes;
// and responding to events returned by a Wait.
// It has to handle a few events specially:
// o if a wait fails, the process has exited, and must no longer be commanded
// this is indicated by a wait followed by an error on PtraceGetRegs
// o if a process forks successully, we must add it to our set of traced processes.
// We attach that process, wait for it, then issue a ptrace system call command
// to it. We don't use the Linux SEIZE command as we can do this in a more generic
// Unix way.
// We create a map of our traced processes and run until it is empty.
// The initial value of the map is just the one process we start with.
func Run(root *Tracer) error {
var nextEX EventType
var procs = map[int]*Tracer{
root.Pid: root,
}
Debug("procs %v", procs)
var tm time.Time
var a SyscallArguments
var sysno = syscall.SYS_EXECVE
Debug("Run %v", root.Pid)
pid := root.Pid
var err error
var count int
for len(procs) > 0 {
t := procs[pid]
t.Count++
count++
Debug("Get regs for %d", pid)
x := &TraceRecord{Serial: t.Count, EX: t.EX, Pid: pid, Args: a}
if err := unix.PtraceGetRegs(pid, &x.Regs); err != nil {
Debug("ptracegetregs for %d gets %v", pid, err)
x.Err = fmt.Errorf("ptracegetregs for %d gets %v", pid, err)
t.Records <- x
delete(procs, pid)
pid, _, _ = Wait(-1)
continue
}
Debug("GOT regs for %d", pid)
x.FillArgs()
if t.EX == Exit {
x.FillRet()
x.Sysno = sysno
x.Time = time.Since(tm)
nextEX = Enter
} else {
tm = time.Now()
x.FillArgs()
a = x.Args
sysno = x.Sysno
t.Last = x
nextEX = Exit
}
if !t.Raw {
SysCall(t, x)
}
Debug("Push %v", x)
t.Records <- x
// Was there a clone? Capture the child. Don't forget the child has an exit
// record for the clone too, so don't get confused.
p := int(x.Ret[0].Int())
Debug("Check for new pid: tracer pid %d, ret %d", t.Pid, p)
if x.Sysno == unix.SYS_CLONE && x.EX == Exit && p > 0 && p != t.Pid {
nt, err := NewTracerChild(int(x.Ret[0].Int()))
if err != nil {
Debug("Setting up child: %v", err)
} else {
nt.Records = t.Records
Debug("New child: %v", nt)
// The result of the attach gets picked up by the wait()
if err := unix.PtraceAttach(nt.Pid); err != nil {
r := &TraceRecord{Serial: nt.Count, EX: Enter, Pid: nt.Pid}
Debug("RunTracerChild: attach for %d gets %v", nt.Pid, err)
nt.Records <- r
}
pid, w, err := Wait(nt.Pid)
Debug("Wait returns (%d, %v, %v)", pid, w, err)
if err != nil || pid != nt.Pid {
r := &TraceRecord{Serial: nt.Count, EX: Exit, Pid: nt.Pid}
r.Err = fmt.Errorf("unix.Wait: %d: %s: %v, %v", nt.Pid, nt.Name, w, err)
Debug("wait4 for %d gets %v, %v", nt.Pid, w, err)
t.Records <- r
}
Debug("Step %d", nt.Pid)
if err := unix.PtraceSyscall(nt.Pid, 0); err != nil {
r := &TraceRecord{Serial: nt.Count, EX: Exit, Pid: nt.Pid}
Debug("ptracesyscall for %d gets %v", nt.Pid, err)
r.Err = fmt.Errorf("unix.PtraceSyscall: %d: %s: %v", nt.Pid, nt.Name, err)
t.Records <- r
}
procs[nt.Pid] = nt
}
}
Debug("Step after exit")
t.EX = nextEX
if pid, err = t.Step(t.EX); err != nil {
return err
}
}
Debug("Pushed %d records", count)
return nil
}
// EventType describes whether a record is system call Entry or Exit
type EventType string
const (
Enter EventType = "E"
Exit = "X"
)
// String is a stringer for TraceRecords
// TODO: stringer for Regs.
func (t *TraceRecord) String() string {
pre := fmt.Sprintf("%s %d#%d:", t.EX, t.Pid, t.Serial)
if t.Err != nil {
return fmt.Sprintf("%s(%v)", pre, t.Err)
}
return fmt.Sprintf("%s %v", pre, t.Regs)
}
// A ProcIO is used to implement io.Reader and io.Writer.
// it contains a pid, which is unchanging; and an
// addr and byte count which change as IO proceeds.
type ProcIO struct {
pid int
addr uintptr
bytes int
}
// NewProcReader returns an io.Reader for a ProcIO.
func NewProcReader(pid int, addr uintptr) io.Reader {
return &ProcIO{pid: pid, addr: addr}
}
// Read implements io.Read for a ProcIO.
func (p *ProcIO) Read(b []byte) (int, error) {
n, err := unix.PtracePeekData(p.pid, p.addr, b)
if err != nil {
return n, err
}
p.addr += uintptr(n)
p.bytes += n
return n, nil
}
// NewProcWriter returns an io.Writer for a ProcIO.
func NewProcWriter(pid int, addr uintptr) io.Writer {
return &ProcIO{pid: pid, addr: addr}
}
// Write implements io.Write for a ProcIO.
func (p *ProcIO) Write(b []byte) (int, error) {
n, err := unix.PtracePokeData(p.pid, p.addr, b)
if err != nil {
return n, err
}
p.addr += uintptr(n)
p.bytes += n
return n, nil
}
// Read reads from the process at Addr to the interface{}
// and returns a byte count and error.
func (t *Tracer) Read(addr Addr, v interface{}) (int, error) {
p := NewProcReader(t.Pid, uintptr(addr))
err := binary.Read(p, binary.LittleEndian, v)
return p.(*ProcIO).bytes, err
}
// ReadString reads a null-terminated string from the process
// at Addr and any errors.
func (t *Tracer) ReadString(addr Addr, max int) (string, error) {
if addr == 0 {
return "<nil>", nil
}
var s string
var b [1]byte
for len(s) < max {
if _, err := t.Read(addr, b[:]); err != nil {
return "", err
}
if b[0] == 0 {
break
}
s = s + string(b[:])
addr++
}
return s, nil
}
// ReadStringVector takes an address, max string size, and max number of string to read,
// and returns a string slice or error.
func (t *Tracer) ReadStringVector(addr Addr, maxsize, maxno int) ([]string, error) {
var v []Addr
if addr == 0 {
return []string{}, nil
}
fmt.Printf("read vec at %#x", addr)
// Read in a maximum of maxno addresses
for len(v) < maxno {
var a uint64
n, err := t.Read(addr, &a)
if err != nil {
fmt.Printf("Could not read vec elemtn at %v", addr)
return nil, err
}
if a == 0 {
break
}
addr += Addr(n)
v = append(v, Addr(a))
}
fmt.Printf("Read %v", v)
var vs []string
for _, a := range v {
s, err := t.ReadString(a, maxsize)
if err != nil {
fmt.Printf("Could not read string at %v", a)
return vs, err
}
vs = append(vs, s)
}
return vs, nil
}
// Write writes to the process address sapce and returns a count and error.
func (t *Tracer) Write(addr Addr, v interface{}) (int, error) {
p := NewProcWriter(t.Pid, uintptr(addr))
err := binary.Write(p, binary.LittleEndian, v)
return p.(*ProcIO).bytes, err
}
// CaptureAddress pulls a socket address from the process as a byte slice.
// It returns any errors.
func CaptureAddress(t *Tracer, addr Addr, addrlen uint32) ([]byte, error) {
b := make([]byte, addrlen)
if _, err := t.Read(addr, b); err != nil {
return nil, err
}
return b, nil
}