github.com/metacubex/gvisor@v0.0.0-20240320004321-933faba989ec/pkg/sentry/syscalls/linux/sys_thread.go

// Copyright 2018 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package linux

import (
	"github.com/metacubex/gvisor/pkg/abi/linux"
	"github.com/metacubex/gvisor/pkg/errors/linuxerr"
	"github.com/metacubex/gvisor/pkg/fspath"
	"github.com/metacubex/gvisor/pkg/hostarch"
	"github.com/metacubex/gvisor/pkg/marshal/primitive"
	"github.com/metacubex/gvisor/pkg/sentry/arch"
	"github.com/metacubex/gvisor/pkg/sentry/kernel"
	"github.com/metacubex/gvisor/pkg/sentry/kernel/sched"
	"github.com/metacubex/gvisor/pkg/sentry/loader"
	"github.com/metacubex/gvisor/pkg/sentry/seccheck"
	"github.com/metacubex/gvisor/pkg/sentry/vfs"
	"github.com/metacubex/gvisor/pkg/usermem"
)

var (
	// ExecMaxTotalSize is the maximum length of all argv and envv entries.
	//
	// N.B. The behavior here is different than Linux. Linux provides a limit on
	// individual arguments of 32 pages, and an aggregate limit of at least 32 pages
	// but otherwise bounded by min(stack size / 4, 8 MB * 3 / 4). We don't implement
	// any behavior based on the stack size, and instead provide a fixed hard-limit of
	// 2 MB (which should work well given that 8 MB stack limits are common).
	ExecMaxTotalSize = 2 * 1024 * 1024

	// ExecMaxElemSize is the maximum length of a single argv or envv entry.
	ExecMaxElemSize = 32 * hostarch.PageSize
)

// Getppid implements linux syscall getppid(2).
func Getppid(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	parent := t.Parent()
	if parent == nil {
		return 0, nil, nil
	}
	return uintptr(t.PIDNamespace().IDOfThreadGroup(parent.ThreadGroup())), nil, nil
}

// Getpid implements linux syscall getpid(2).
func Getpid(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	return uintptr(t.ThreadGroup().ID()), nil, nil
}

// Gettid implements linux syscall gettid(2).
func Gettid(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	return uintptr(t.ThreadID()), nil, nil
}

// Execve implements linux syscall execve(2).
func Execve(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	pathnameAddr := args[0].Pointer()
	argvAddr := args[1].Pointer()
	envvAddr := args[2].Pointer()
	return execveat(t, linux.AT_FDCWD, pathnameAddr, argvAddr, envvAddr, 0 /* flags */)
}

// Execveat implements linux syscall execveat(2).
func Execveat(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	dirfd := args[0].Int()
	pathnameAddr := args[1].Pointer()
	argvAddr := args[2].Pointer()
	envvAddr := args[3].Pointer()
	flags := args[4].Int()
	return execveat(t, dirfd, pathnameAddr, argvAddr, envvAddr, flags)
}

func execveat(t *kernel.Task, dirfd int32, pathnameAddr, argvAddr, envvAddr hostarch.Addr, flags int32) (uintptr, *kernel.SyscallControl, error) {
	if flags&^(linux.AT_EMPTY_PATH|linux.AT_SYMLINK_NOFOLLOW) != 0 {
		return 0, nil, linuxerr.EINVAL
	}

	pathname, err := t.CopyInString(pathnameAddr, linux.PATH_MAX)
	if err != nil {
		return 0, nil, err
	}
	var argv, envv []string
	if argvAddr != 0 {
		var err error
		argv, err = t.CopyInVector(argvAddr, ExecMaxElemSize, ExecMaxTotalSize)
		if err != nil {
			return 0, nil, err
		}
	}
	if envvAddr != 0 {
		var err error
		envv, err = t.CopyInVector(envvAddr, ExecMaxElemSize, ExecMaxTotalSize)
		if err != nil {
			return 0, nil, err
		}
	}

	root := t.FSContext().RootDirectory()
	defer root.DecRef(t)
	var executable *vfs.FileDescription
	defer func() {
		if executable != nil {
			executable.DecRef(t)
		}
	}()
	closeOnExec := false
	if path := fspath.Parse(pathname); dirfd != linux.AT_FDCWD && !path.Absolute {
		// We must open the executable ourselves since dirfd is used as the
		// starting point while resolving path, but the task working directory
		// is used as the starting point while resolving interpreters (Linux:
		// fs/binfmt_script.c:load_script() => fs/exec.c:open_exec() =>
		// do_open_execat(fd=AT_FDCWD)), and the loader package is currently
		// incapable of handling this correctly.
		if !path.HasComponents() && flags&linux.AT_EMPTY_PATH == 0 {
			return 0, nil, linuxerr.ENOENT
		}
		dirfile, dirfileFlags := t.FDTable().Get(dirfd)
		if dirfile == nil {
			return 0, nil, linuxerr.EBADF
		}
		start := dirfile.VirtualDentry()
		start.IncRef()
		dirfile.DecRef(t)
		closeOnExec = dirfileFlags.CloseOnExec
		file, err := t.Kernel().VFS().OpenAt(t, t.Credentials(), &vfs.PathOperation{
			Root:               root,
			Start:              start,
			Path:               path,
			FollowFinalSymlink: flags&linux.AT_SYMLINK_NOFOLLOW == 0,
		}, &vfs.OpenOptions{
			Flags:    linux.O_RDONLY,
			FileExec: true,
		})
		start.DecRef(t)
		if err != nil {
			return 0, nil, err
		}
		executable = file
		pathname = executable.MappedName(t)
	}

	// Load the new TaskImage.
	wd := t.FSContext().WorkingDirectory()
	defer wd.DecRef(t)
	remainingTraversals := uint(linux.MaxSymlinkTraversals)
	loadArgs := loader.LoadArgs{
		Root:                root,
		WorkingDir:          wd,
		RemainingTraversals: &remainingTraversals,
		ResolveFinal:        flags&linux.AT_SYMLINK_NOFOLLOW == 0,
		Filename:            pathname,
		File:                executable,
		CloseOnExec:         closeOnExec,
		Argv:                argv,
		Envv:                envv,
		Features:            t.Kernel().FeatureSet(),
	}
	if seccheck.Global.Enabled(seccheck.PointExecve) {
		// Retain the first executable file that is opened (which may open
		// multiple executable files while resolving interpreter scripts).
		if executable == nil {
			loadArgs.AfterOpen = func(f *vfs.FileDescription) {
				if executable == nil {
					f.IncRef()
					executable = f
					pathname = executable.MappedName(t)
				}
			}
		}
	}

	image, se := t.Kernel().LoadTaskImage(t, loadArgs)
	if se != nil {
		return 0, nil, se.ToError()
	}

	ctrl, err := t.Execve(image, argv, envv, executable, pathname)
	return 0, ctrl, err
}

// Exit implements linux syscall exit(2).
func Exit(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	status := args[0].Int()
	t.PrepareExit(linux.WaitStatusExit(status & 0xff))
	return 0, kernel.CtrlDoExit, nil
}

// ExitGroup implements linux syscall exit_group(2).
func ExitGroup(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	status := args[0].Int()
	t.PrepareGroupExit(linux.WaitStatusExit(status & 0xff))
	return 0, kernel.CtrlDoExit, nil
}

// clone is used by Clone, Fork, and VFork.
func clone(t *kernel.Task, flags int, stack hostarch.Addr, parentTID hostarch.Addr, childTID hostarch.Addr, tls hostarch.Addr) (uintptr, *kernel.SyscallControl, error) {
	args := linux.CloneArgs{
		Flags:      uint64(uint32(flags) &^ linux.CSIGNAL),
		ChildTID:   uint64(childTID),
		ParentTID:  uint64(parentTID),
		ExitSignal: uint64(flags & linux.CSIGNAL),
		Stack:      uint64(stack),
		TLS:        uint64(tls),
	}
	ntid, ctrl, err := t.Clone(&args)
	return uintptr(ntid), ctrl, err
}

// Fork implements Linux syscall fork(2).
func Fork(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	// "A call to fork() is equivalent to a call to clone(2) specifying flags
	// as just SIGCHLD." - fork(2)
	return clone(t, int(linux.SIGCHLD), 0, 0, 0, 0)
}

// Vfork implements Linux syscall vfork(2).
func Vfork(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	// """
	// A call to vfork() is equivalent to calling clone(2) with flags specified as:
	//
	//     CLONE_VM | CLONE_VFORK | SIGCHLD
	// """ - vfork(2)
	return clone(t, linux.CLONE_VM|linux.CLONE_VFORK|int(linux.SIGCHLD), 0, 0, 0, 0)
}

// Clone3 implements linux syscall clone3(2).
func Clone3(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	cloneArgsPointer := args[0].Pointer()
	size := args[1].SizeT()

	if int(size) < linux.CLONE_ARGS_SIZE_VER0 || int(size) > linux.CLONE_ARGS_SIZE_VER2 {
		return 0, nil, linuxerr.EINVAL
	}

	var cloneArgs linux.CloneArgs
	if cloneArgsPointer != 0 {
		if _, err := cloneArgs.CopyInN(t, cloneArgsPointer, int(size)); err != nil {
			return 0, nil, err
		}
	}

	ntid, ctrl, err := t.Clone(&cloneArgs)
	if err != nil {
		return 0, nil, err
	}
	return uintptr(ntid), ctrl, err
}

// parseCommonWaitOptions applies the options common to wait4 and waitid to
// wopts.
func parseCommonWaitOptions(wopts *kernel.WaitOptions, options int) error {
	switch options & (linux.WCLONE | linux.WALL) {
	case 0:
		wopts.NonCloneTasks = true
	case linux.WCLONE:
		wopts.CloneTasks = true
	case linux.WALL:
		wopts.NonCloneTasks = true
		wopts.CloneTasks = true
	default:
		return linuxerr.EINVAL
	}
	if options&linux.WCONTINUED != 0 {
		wopts.Events |= kernel.EventGroupContinue
	}
	if options&linux.WNOHANG == 0 {
		wopts.BlockInterruptErr = linuxerr.ERESTARTSYS
	}
	if options&linux.WNOTHREAD == 0 {
		wopts.SiblingChildren = true
	}
	return nil
}

// wait4 waits for the given child process to exit.
func wait4(t *kernel.Task, pid int, statusAddr hostarch.Addr, options int, rusageAddr hostarch.Addr) (uintptr, error) {
	if options&^(linux.WNOHANG|linux.WUNTRACED|linux.WCONTINUED|linux.WNOTHREAD|linux.WALL|linux.WCLONE) != 0 {
		return 0, linuxerr.EINVAL
	}
	wopts := kernel.WaitOptions{
		Events:       kernel.EventExit | kernel.EventTraceeStop,
		ConsumeEvent: true,
	}
	// There are four cases to consider:
	//
	// pid < -1    any child process whose process group ID is equal to the absolute value of pid
	// pid == -1   any child process
	// pid == 0    any child process whose process group ID is equal to that of the calling process
	// pid > 0     the child whose process ID is equal to the value of pid
	switch {
	case pid < -1:
		wopts.SpecificPGID = kernel.ProcessGroupID(-pid)
	case pid == -1:
		// Any process is the default.
	case pid == 0:
		wopts.SpecificPGID = t.PIDNamespace().IDOfProcessGroup(t.ThreadGroup().ProcessGroup())
	default:
		wopts.SpecificTID = kernel.ThreadID(pid)
	}

	if err := parseCommonWaitOptions(&wopts, options); err != nil {
		return 0, err
	}
	if options&linux.WUNTRACED != 0 {
		wopts.Events |= kernel.EventChildGroupStop
	}

	wr, err := t.Wait(&wopts)
	if err != nil {
		if err == kernel.ErrNoWaitableEvent {
			return 0, nil
		}
		return 0, err
	}
	if statusAddr != 0 {
		if _, err := primitive.CopyUint32Out(t, statusAddr, uint32(wr.Status)); err != nil {
			return 0, err
		}
	}
	if rusageAddr != 0 {
		ru := getrusage(wr.Task, linux.RUSAGE_BOTH)
		if _, err := ru.CopyOut(t, rusageAddr); err != nil {
			return 0, err
		}
	}
	return uintptr(wr.TID), nil
}

// Wait4 implements linux syscall wait4(2).
func Wait4(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	pid := int(args[0].Int())
	statusAddr := args[1].Pointer()
	options := int(args[2].Uint())
	rusageAddr := args[3].Pointer()

	n, err := wait4(t, pid, statusAddr, options, rusageAddr)
	return n, nil, err
}

// WaitPid implements linux syscall waitpid(2).
func WaitPid(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	pid := int(args[0].Int())
	statusAddr := args[1].Pointer()
	options := int(args[2].Uint())

	n, err := wait4(t, pid, statusAddr, options, 0)
	return n, nil, err
}

// Waitid implements linux syscall waitid(2).
func Waitid(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	idtype := args[0].Int()
	id := args[1].Int()
	infop := args[2].Pointer()
	options := int(args[3].Uint())
	rusageAddr := args[4].Pointer()

	if options&^(linux.WNOHANG|linux.WEXITED|linux.WSTOPPED|linux.WCONTINUED|linux.WNOWAIT|linux.WNOTHREAD|linux.WALL|linux.WCLONE) != 0 {
		return 0, nil, linuxerr.EINVAL
	}
	if options&(linux.WEXITED|linux.WSTOPPED|linux.WCONTINUED) == 0 {
		return 0, nil, linuxerr.EINVAL
	}
	wopts := kernel.WaitOptions{
		Events:       kernel.EventTraceeStop,
		ConsumeEvent: options&linux.WNOWAIT == 0,
	}
	switch idtype {
	case linux.P_ALL:
	case linux.P_PID:
		wopts.SpecificTID = kernel.ThreadID(id)
	case linux.P_PGID:
		wopts.SpecificPGID = kernel.ProcessGroupID(id)
	default:
		return 0, nil, linuxerr.EINVAL
	}

	if err := parseCommonWaitOptions(&wopts, options); err != nil {
		return 0, nil, err
	}
	if options&linux.WEXITED != 0 {
		wopts.Events |= kernel.EventExit
	}
	if options&linux.WSTOPPED != 0 {
		wopts.Events |= kernel.EventChildGroupStop
	}

	wr, err := t.Wait(&wopts)
	if err != nil {
		if err == kernel.ErrNoWaitableEvent {
			err = nil
			// "If WNOHANG was specified in options and there were no children
			// in a waitable state, then waitid() returns 0 immediately and the
			// state of the siginfo_t structure pointed to by infop is
			// unspecified." - waitid(2). But Linux's waitid actually zeroes
			// out the fields it would set for a successful waitid in this case
			// as well.
			if infop != 0 {
				var si linux.SignalInfo
				_, err = si.CopyOut(t, infop)
			}
		}
		return 0, nil, err
	}
	if rusageAddr != 0 {
		ru := getrusage(wr.Task, linux.RUSAGE_BOTH)
		if _, err := ru.CopyOut(t, rusageAddr); err != nil {
			return 0, nil, err
		}
	}
	if infop == 0 {
		return 0, nil, nil
	}
	si := linux.SignalInfo{
		Signo: int32(linux.SIGCHLD),
	}
	si.SetPID(int32(wr.TID))
	si.SetUID(int32(wr.UID))
	s := wr.Status
	switch {
	case s.Exited():
		si.Code = linux.CLD_EXITED
		si.SetStatus(int32(s.ExitStatus()))
	case s.Signaled():
		if s.CoreDumped() {
			si.Code = linux.CLD_DUMPED
		} else {
			si.Code = linux.CLD_KILLED
		}
		si.SetStatus(int32(s.TerminationSignal()))
	case s.Stopped():
		if wr.Event == kernel.EventTraceeStop {
			si.Code = linux.CLD_TRAPPED
			si.SetStatus(int32(s.PtraceEvent()))
		} else {
			si.Code = linux.CLD_STOPPED
			si.SetStatus(int32(s.StopSignal()))
		}
	case s.Continued():
		si.Code = linux.CLD_CONTINUED
		si.SetStatus(int32(linux.SIGCONT))
	default:
		t.Warningf("waitid got incomprehensible wait status %d", s)
	}
	_, err = si.CopyOut(t, infop)
	return 0, nil, err
}

// SetTidAddress implements linux syscall set_tid_address(2).
func SetTidAddress(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	addr := args[0].Pointer()

	// Always succeed, return caller's tid.
	t.SetClearTID(addr)
	return uintptr(t.ThreadID()), nil, nil
}

// Setns implements linux syscall setns(2).
func Setns(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	fd := args[0].Int()

	file := t.GetFile(fd)
	if file == nil {
		return 0, nil, linuxerr.EBADF
	}
	defer file.DecRef(t)

	flags := args[1].Int()
	return 0, nil, t.Setns(file, flags)
}

// Unshare implements linux syscall unshare(2).
func Unshare(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	flags := args[0].Int()
	// "CLONE_NEWPID automatically implies CLONE_THREAD as well." - unshare(2)
	if flags&linux.CLONE_NEWPID != 0 {
		flags |= linux.CLONE_THREAD
	}
	// "... specifying CLONE_NEWUSER automatically implies CLONE_THREAD. Since
	// Linux 3.9, CLONE_NEWUSER also automatically implies CLONE_FS."
	if flags&linux.CLONE_NEWUSER != 0 {
		flags |= linux.CLONE_THREAD | linux.CLONE_FS
	}
	return 0, nil, t.Unshare(flags)
}

// SchedYield implements linux syscall sched_yield(2).
func SchedYield(t *kernel.Task, sysno uintptr, _ arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	t.Yield()
	return 0, nil, nil
}

// SchedSetaffinity implements linux syscall sched_setaffinity(2).
func SchedSetaffinity(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	tid := args[0].Int()
	size := args[1].SizeT()
	maskAddr := args[2].Pointer()

	var task *kernel.Task
	if tid == 0 {
		task = t
	} else {
		task = t.PIDNamespace().TaskWithID(kernel.ThreadID(tid))
		if task == nil {
			return 0, nil, linuxerr.ESRCH
		}
	}

	mask := sched.NewCPUSet(t.Kernel().ApplicationCores())
	if size > mask.Size() {
		size = mask.Size()
	}
	if _, err := t.CopyInBytes(maskAddr, mask[:size]); err != nil {
		return 0, nil, err
	}
	return 0, nil, task.SetCPUMask(mask)
}

// SchedGetaffinity implements linux syscall sched_getaffinity(2).
func SchedGetaffinity(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	tid := args[0].Int()
	size := args[1].SizeT()
	maskAddr := args[2].Pointer()

	// This limitation is because linux stores the cpumask
	// in an array of "unsigned long" so the buffer needs to
	// be a multiple of the word size.
	if size&(t.Arch().Width()-1) > 0 {
		return 0, nil, linuxerr.EINVAL
	}

	var task *kernel.Task
	if tid == 0 {
		task = t
	} else {
		task = t.PIDNamespace().TaskWithID(kernel.ThreadID(tid))
		if task == nil {
			return 0, nil, linuxerr.ESRCH
		}
	}

	mask := task.CPUMask()
	// The buffer needs to be big enough to hold a cpumask with
	// all possible cpus.
	if size < mask.Size() {
		return 0, nil, linuxerr.EINVAL
	}
	_, err := t.CopyOutBytes(maskAddr, mask)

	// NOTE: The syscall interface is slightly different than the glibc
	// interface. The raw sched_getaffinity syscall returns the number of
	// bytes used to represent a cpu mask.
	return uintptr(mask.Size()), nil, err
}

// Getcpu implements linux syscall getcpu(2).
func Getcpu(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	cpu := args[0].Pointer()
	node := args[1].Pointer()
	// third argument to this system call is nowadays unused.

	if cpu != 0 {
		if _, err := primitive.CopyInt32Out(t, cpu, t.CPU()); err != nil {
			return 0, nil, err
		}
	}
	// We always return node 0.
	if node != 0 {
		if _, err := t.MemoryManager().ZeroOut(t, node, 4, usermem.IOOpts{
			AddressSpaceActive: true,
		}); err != nil {
			return 0, nil, err
		}
	}
	return 0, nil, nil
}

// Setpgid implements the linux syscall setpgid(2).
func Setpgid(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	// Note that throughout this function, pgid is interpreted with respect
	// to t's namespace, not with respect to the selected ThreadGroup's
	// namespace (which may be different).
	pid := kernel.ThreadID(args[0].Int())
	pgid := kernel.ProcessGroupID(args[1].Int())

	// "If pid is zero, then the process ID of the calling process is used."
	tg := t.ThreadGroup()
	if pid != 0 {
		ot := t.PIDNamespace().TaskWithID(pid)
		if ot == nil {
			return 0, nil, linuxerr.ESRCH
		}
		tg = ot.ThreadGroup()
		if tg.Leader() != ot {
			return 0, nil, linuxerr.EINVAL
		}

		// Setpgid only operates on child threadgroups.
		if tg != t.ThreadGroup() && (tg.Leader().Parent() == nil || tg.Leader().Parent().ThreadGroup() != t.ThreadGroup()) {
			return 0, nil, linuxerr.ESRCH
		}
	}

	// "If pgid is zero, then the PGID of the process specified by pid is made
	// the same as its process ID."
	defaultPGID := kernel.ProcessGroupID(t.PIDNamespace().IDOfThreadGroup(tg))
	if pgid == 0 {
		pgid = defaultPGID
	} else if pgid < 0 {
		return 0, nil, linuxerr.EINVAL
	}

	// If the pgid is the same as the group, then create a new one. Otherwise,
	// we attempt to join an existing process group.
	if pgid == defaultPGID {
		// For convenience, errors line up with Linux syscall API.
		if err := tg.CreateProcessGroup(); err != nil {
			// Is the process group already as expected? If so,
			// just return success. This is the same behavior as
			// Linux.
			if t.PIDNamespace().IDOfProcessGroup(tg.ProcessGroup()) == defaultPGID {
				return 0, nil, nil
			}
			return 0, nil, err
		}
	} else {
		// Same as CreateProcessGroup, above.
		if err := tg.JoinProcessGroup(t.PIDNamespace(), pgid, tg != t.ThreadGroup()); err != nil {
			// See above.
			if t.PIDNamespace().IDOfProcessGroup(tg.ProcessGroup()) == pgid {
				return 0, nil, nil
			}
			return 0, nil, err
		}
	}

	// Success.
	return 0, nil, nil
}

// Getpgrp implements the linux syscall getpgrp(2).
func Getpgrp(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	return uintptr(t.PIDNamespace().IDOfProcessGroup(t.ThreadGroup().ProcessGroup())), nil, nil
}

// Getpgid implements the linux syscall getpgid(2).
func Getpgid(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	tid := kernel.ThreadID(args[0].Int())
	if tid == 0 {
		return Getpgrp(t, sysno, args)
	}

	target := t.PIDNamespace().TaskWithID(tid)
	if target == nil {
		return 0, nil, linuxerr.ESRCH
	}

	return uintptr(t.PIDNamespace().IDOfProcessGroup(target.ThreadGroup().ProcessGroup())), nil, nil
}

// Setsid implements the linux syscall setsid(2).
func Setsid(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	sid, err := t.ThreadGroup().CreateSession()
	if err != nil {
		return 0, nil, err
	}
	return uintptr(sid), nil, nil
}

// Getsid implements the linux syscall getsid(2).
func Getsid(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	tid := kernel.ThreadID(args[0].Int())
	if tid == 0 {
		return uintptr(t.PIDNamespace().IDOfSession(t.ThreadGroup().Session())), nil, nil
	}

	target := t.PIDNamespace().TaskWithID(tid)
	if target == nil {
		return 0, nil, linuxerr.ESRCH
	}

	return uintptr(t.PIDNamespace().IDOfSession(target.ThreadGroup().Session())), nil, nil
}

// Getpriority pretends to implement the linux syscall getpriority(2).
//
// This is a stub; real priorities require a full scheduler.
func Getpriority(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	which := args[0].Int()
	who := kernel.ThreadID(args[1].Int())

	switch which {
	case linux.PRIO_PROCESS:
		// Look for who, return ESRCH if not found.
		var task *kernel.Task
		if who == 0 {
			task = t
		} else {
			task = t.PIDNamespace().TaskWithID(who)
		}

		if task == nil {
			return 0, nil, linuxerr.ESRCH
		}

		// From kernel/sys.c:getpriority:
		// "To avoid negative return values, 'getpriority()'
		// will not return the normal nice-value, but a negated
		// value that has been offset by 20"
		return uintptr(20 - task.Niceness()), nil, nil
	case linux.PRIO_USER:
		fallthrough
	case linux.PRIO_PGRP:
		// PRIO_USER and PRIO_PGRP have no further implementation yet.
		return 0, nil, nil
	default:
		return 0, nil, linuxerr.EINVAL
	}
}

// Setpriority pretends to implement the linux syscall setpriority(2).
//
// This is a stub; real priorities require a full scheduler.
func Setpriority(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	which := args[0].Int()
	who := kernel.ThreadID(args[1].Int())
	niceval := int(args[2].Int())

	// In the kernel's implementation, values outside the range
	// of [-20, 19] are truncated to these minimum and maximum
	// values.
	if niceval < -20 /* min niceval */ {
		niceval = -20
	} else if niceval > 19 /* max niceval */ {
		niceval = 19
	}

	switch which {
	case linux.PRIO_PROCESS:
		// Look for who, return ESRCH if not found.
		var task *kernel.Task
		if who == 0 {
			task = t
		} else {
			task = t.PIDNamespace().TaskWithID(who)
		}

		if task == nil {
			return 0, nil, linuxerr.ESRCH
		}

		task.SetNiceness(niceval)
	case linux.PRIO_USER:
		fallthrough
	case linux.PRIO_PGRP:
		// PRIO_USER and PRIO_PGRP have no further implementation yet.
		return 0, nil, nil
	default:
		return 0, nil, linuxerr.EINVAL
	}

	return 0, nil, nil
}

// Ptrace implements linux system call ptrace(2).
func Ptrace(t *kernel.Task, sysno uintptr, args arch.SyscallArguments) (uintptr, *kernel.SyscallControl, error) {
	req := args[0].Int64()
	pid := kernel.ThreadID(args[1].Int())
	addr := args[2].Pointer()
	data := args[3].Pointer()

	return 0, nil, t.Ptrace(req, pid, addr, data)
}