exec/proot/proot.go

package proot

import (
	"errors"
	"fmt"
	"io"
	"net/netip"
	"os"
	"path"
	"path/filepath"
	"runtime"
	"strings"

	"sirherobrine23.com.br/go-bds/exec/v2/process"
	prootext "sirherobrine23.com.br/go-bds/exec/v2/proot/extensions/extensions"
)

var (
	_ process.Proc = &Proot{}
	_              = process.RegisterProcess(NewProc, "proot")

	ErrPlatformNotSupported = fmt.Errorf("proot: pure-Go backend is only supported on linux/android current is %s/%s", runtime.GOOS, runtime.GOARCH)
)

// Proot runs a process inside Rootfs without requiring chroot(2) privileges.
//
// This implementation is intentionally pure Go: on Linux/Android it uses
// ptrace(2) to rewrite path based syscalls instead of spawning the external
// proot binary.  It is not a byte-for-byte clone of upstream PRoot yet, but it
// implements the core proot semantics needed by go-bds: rootfs remapping,
// bind mappings, cwd tracking, execve/openat/stat/access/readlink/chdir and
// common mutating path syscalls.
type Proot struct {
	Rootfs string              // Rootfs to expose as guest /
	Qemu   string              // Optional qemu user emulator, e.g. qemu-aarch64-static
	GID    uint                // Fake/target GID; currently kept for API compatibility
	UID    uint                // Fake/target UID; currently kept for API compatibility
	Binds  map[string][]string // Host path -> one or more guest destinations
	// DefaultBinds controls automatic host binds. Nil uses the conservative
	// default (/dev only). /proc and /sys are live host kernel trees; binding
	// them by default makes commands such as `find /` traverse dynamic host
	// pseudo-filesystems that can race disappearing tasks/devices. Add "/proc"
	// and/or "/sys" explicitly when a command really needs them. Set to an
	// empty slice to disable all automatic binds.
	DefaultBinds []string
	Extensions   []prootext.Extension // Ordered runtime extensions applied before process start

	proc *nativeProcess
}

// NewProc returns an empty pure-Go proot process.  Set Rootfs before Start.
func NewProc() (*Proot, error) {
	return &Proot{
		Rootfs:     "/",
		Binds:      map[string][]string{},
		Extensions: []prootext.Extension{},
	}, nil
}

// Append dns server to /etc/resolv.conf.
//
//	Example: pr.AddNameservers(netip.MustParseAddr("8.8.8.8"), netip.MustParseAddr("1.1.1.1"))
func (pr Proot) AddNameservers(addrs ...netip.Addr) error {
	file, err := os.OpenFile(filepath.Join(pr.Rootfs, "etc/resolv.conf"), os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
	if err != nil {
		return err
	}
	defer file.Close()
	fmt.Fprint(file, "\n")
	for _, addr := range addrs {
		if _, err := fmt.Fprintf(file, "nameserver %s\n", addr.String()); err != nil {
			return err
		}
	}
	return nil
}

// Start starts the process under the pure-Go ptrace based proot backend.
func (pr *Proot) Start(options *process.Exec) error {
	if options == nil || len(options.Arguments) == 0 || options.Arguments[0] == "" {
		return errors.New("proot: missing command")
	}
	runtimeConfig, err := pr.prepareExtensions()
	if err != nil {
		return err
	}
	started := false
	defer func() {
		if !started {
			_ = runtimeConfig.cleanup()
		}
	}()

	if runtimeConfig.config.Rootfs == "" {
		return errors.New("proot: Rootfs is required")
	}
	if options.Environment == nil {
		options.Environment = process.Env{}
	}
	// A guest process must not inherit the host's absolute PATH. In Termux that
	// would make /bin/sh search /data/data/com.termux/files/usr/bin inside the
	// guest rootfs, so otherwise valid guest commands such as cat, ln and id
	// appear to be missing. Preserve an explicitly supplied guest PATH.
	if _, ok := options.Environment["PATH"]; !ok {
		options.Environment["PATH"] = defaultGuestSearchPath
	}
	// Do not leak host-only absolute directories into the guest. Termux sets
	// TMPDIR to /data/data/com.termux/files/usr/tmp; guest maintainer scripts
	// then pass that path explicitly to tools such as mktemp, where it is
	// correctly interpreted inside the rootfs and therefore does not exist.
	// Preserve values explicitly supplied by the caller.
	guestEnvDefaults := map[string]string{
		"TMPDIR": "/tmp",
		"TMP":    "/tmp",
		"TEMP":   "/tmp",
		"HOME":   "/root",
		"LANG":   "C.UTF-8",
	}
	for name, value := range guestEnvDefaults {
		if _, ok := options.Environment[name]; !ok {
			options.Environment[name] = value
		}
	}
	if err := pr.resolveExecutable(options, runtimeConfig.config); err != nil {
		return err
	}
	if runtime.GOOS == "android" {
		// Termux commonly injects LD_PRELOAD; guest dynamic linkers should not see it.
		options.Environment["LD_PRELOAD"] = ""
	}

	p, err := startNative(pr, runtimeConfig.config, runtimeConfig.cleanups, options)
	if err != nil {
		return err
	}
	started = true
	pr.proc = p
	return nil
}

const defaultGuestSearchPath = "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/system/bin:/system/xbin"

func (pr *Proot) resolveExecutable(options *process.Exec, config prootext.Config) error {
	cmd := options.Arguments[0]
	if path.IsAbs(cmd) || strings.ContainsRune(cmd, '/') {
		return nil
	}

	pm, err := newPathMapper(config.Rootfs, config.PathResolvers)
	if err != nil {
		return err
	}

	cwd := options.Cwd
	if cwd == "" {
		cwd = "/"
	}
	searchPath := guestSearchPath(options.Environment)
	for _, dir := range splitGuestSearchPath(searchPath) {
		guest := path.Join(dir, cmd)
		if !path.IsAbs(guest) {
			guest = path.Join(cleanGuestPath(cwd), guest)
		}
		if isExecutable(pm.GuestToHost(guest)) {
			options.Arguments[0] = cleanGuestPath(guest)
			return nil
		}
	}

	return fmt.Errorf("proot: executable %q not found in PATH %q", cmd, searchPath)
}

func guestSearchPath(env process.Env) string {
	if env != nil {
		if value, ok := env["PATH"]; ok {
			return value
		}
	}
	return defaultGuestSearchPath
}

func splitGuestSearchPath(searchPath string) []string {
	dirs := strings.Split(searchPath, ":")
	for i, dir := range dirs {
		if dir == "" {
			dirs[i] = "."
		}
	}
	return dirs
}

func isExecutable(name string) bool {
	st, err := os.Stat(name)
	if err != nil || st.IsDir() {
		return false
	}
	return st.Mode().Perm()&0111 != 0
}

func (pr *Proot) Kill() error {
	if pr.proc == nil {
		return process.ErrNoProcess
	}
	return pr.proc.Kill()
}

func (pr *Proot) Close() error {
	if pr.proc == nil {
		return nil
	}
	return pr.proc.Signal(os.Interrupt)
}

func (pr *Proot) Wait() error {
	if pr.proc == nil {
		return process.ErrNoProcess
	}
	return pr.proc.Wait()
}

func (pr *Proot) Signal(sig os.Signal) error {
	if pr.proc == nil {
		return process.ErrNoProcess
	}
	return pr.proc.Signal(sig)
}

func (pr *Proot) ExitCode() (int, error) {
	if pr.proc == nil {
		return -1, process.ErrNoProcess
	}
	return pr.proc.ExitCode()
}

func (pr *Proot) AttachStdin(r io.Reader) error {
	if pr.proc == nil {
		pr.proc = &nativeProcess{}
	}
	pr.proc.stdin = r
	return nil
}

func (pr *Proot) AttachStdout(w io.Writer) error {
	if pr.proc == nil {
		pr.proc = &nativeProcess{}
	}
	pr.proc.stdout = w
	return nil
}

func (pr *Proot) AttachStderr(w io.Writer) error {
	if pr.proc == nil {
		pr.proc = &nativeProcess{}
	}
	pr.proc.stderr = w
	return nil
}