task_start.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 kernel

import (
	"fmt"

	"gvisor.dev/gvisor/pkg/abi/linux"
	"gvisor.dev/gvisor/pkg/atomicbitops"
	"gvisor.dev/gvisor/pkg/context"
	"gvisor.dev/gvisor/pkg/errors/linuxerr"
	"gvisor.dev/gvisor/pkg/hostarch"
	"gvisor.dev/gvisor/pkg/sentry/inet"
	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
	"gvisor.dev/gvisor/pkg/sentry/kernel/futex"
	"gvisor.dev/gvisor/pkg/sentry/kernel/sched"
	"gvisor.dev/gvisor/pkg/sentry/usage"
	"gvisor.dev/gvisor/pkg/sentry/vfs"
)

// TaskConfig defines the configuration of a new Task (see below).
type TaskConfig struct {
	// Kernel is the owning Kernel.
	Kernel *Kernel

	// Parent is the new task's parent. Parent may be nil.
	Parent *Task

	// If InheritParent is not nil, use InheritParent's parent as the new
	// task's parent.
	InheritParent *Task

	// ThreadGroup is the ThreadGroup the new task belongs to.
	ThreadGroup *ThreadGroup

	// SignalMask is the new task's initial signal mask.
	SignalMask linux.SignalSet

	// TaskImage is the TaskImage of the new task. Ownership of the
	// TaskImage is transferred to TaskSet.NewTask, whether or not it
	// succeeds.
	TaskImage *TaskImage

	// FSContext is the FSContext of the new task. A reference must be held on
	// FSContext, which is transferred to TaskSet.NewTask whether or not it
	// succeeds.
	FSContext *FSContext

	// FDTable is the FDTableof the new task. A reference must be held on
	// FDMap, which is transferred to TaskSet.NewTask whether or not it
	// succeeds.
	FDTable *FDTable

	// Credentials is the Credentials of the new task.
	Credentials *auth.Credentials

	// Niceness is the niceness of the new task.
	Niceness int

	// NetworkNamespace is the network namespace to be used for the new task.
	NetworkNamespace *inet.Namespace

	// AllowedCPUMask contains the cpus that this task can run on.
	AllowedCPUMask sched.CPUSet

	// UTSNamespace is the UTSNamespace of the new task.
	UTSNamespace *UTSNamespace

	// IPCNamespace is the IPCNamespace of the new task.
	IPCNamespace *IPCNamespace

	// MountNamespace is the MountNamespace of the new task.
	MountNamespace *vfs.MountNamespace

	// RSeqAddr is a pointer to the the userspace linux.RSeq structure.
	RSeqAddr hostarch.Addr

	// RSeqSignature is the signature that the rseq abort IP must be signed
	// with.
	RSeqSignature uint32

	// ContainerID is the container the new task belongs to.
	ContainerID string

	// InitialCgroups are the cgroups the container is initialised to.
	InitialCgroups map[Cgroup]struct{}

	// UserCounters is user resource counters.
	UserCounters *userCounters

	// SessionKeyring is the session keyring associated with the parent task.
	// It may be nil.
	SessionKeyring *auth.Key
}

// NewTask creates a new task defined by cfg.
//
// NewTask does not start the returned task; the caller must call Task.Start.
//
// If successful, NewTask transfers references held by cfg to the new task.
// Otherwise, NewTask releases them.
func (ts *TaskSet) NewTask(ctx context.Context, cfg *TaskConfig) (*Task, error) {
	var err error
	cleanup := func() {
		cfg.TaskImage.release(ctx)
		cfg.FSContext.DecRef(ctx)
		cfg.FDTable.DecRef(ctx)
		cfg.UTSNamespace.DecRef(ctx)
		cfg.IPCNamespace.DecRef(ctx)
		cfg.NetworkNamespace.DecRef(ctx)
		if cfg.MountNamespace != nil {
			cfg.MountNamespace.DecRef(ctx)
		}
	}
	if err := cfg.UserCounters.incRLimitNProc(ctx); err != nil {
		cleanup()
		return nil, err
	}
	t, err := ts.newTask(ctx, cfg)
	if err != nil {
		cfg.UserCounters.decRLimitNProc()
		cleanup()
		return nil, err
	}
	return t, nil
}

// newTask is a helper for TaskSet.NewTask that only takes ownership of parts
// of cfg if it succeeds.
func (ts *TaskSet) newTask(ctx context.Context, cfg *TaskConfig) (*Task, error) {
	srcT := TaskFromContext(ctx)
	tg := cfg.ThreadGroup
	image := cfg.TaskImage
	t := &Task{
		taskNode: taskNode{
			tg:       tg,
			parent:   cfg.Parent,
			children: make(map[*Task]struct{}),
		},
		runState:       (*runApp)(nil),
		interruptChan:  make(chan struct{}, 1),
		signalMask:     atomicbitops.FromUint64(uint64(cfg.SignalMask)),
		signalStack:    linux.SignalStack{Flags: linux.SS_DISABLE},
		image:          *image,
		fsContext:      cfg.FSContext,
		fdTable:        cfg.FDTable,
		k:              cfg.Kernel,
		ptraceTracees:  make(map[*Task]struct{}),
		allowedCPUMask: cfg.AllowedCPUMask.Copy(),
		ioUsage:        &usage.IO{},
		niceness:       cfg.Niceness,
		utsns:          cfg.UTSNamespace,
		ipcns:          cfg.IPCNamespace,
		mountNamespace: cfg.MountNamespace,
		rseqCPU:        -1,
		rseqAddr:       cfg.RSeqAddr,
		rseqSignature:  cfg.RSeqSignature,
		futexWaiter:    futex.NewWaiter(),
		containerID:    cfg.ContainerID,
		cgroups:        make(map[Cgroup]struct{}),
		userCounters:   cfg.UserCounters,
		sessionKeyring: cfg.SessionKeyring,
	}
	t.netns = cfg.NetworkNamespace
	t.creds.Store(cfg.Credentials)
	t.endStopCond.L = &t.tg.signalHandlers.mu
	t.ptraceTracer.Store((*Task)(nil))
	// We don't construct t.blockingTimer until Task.run(); see that function
	// for justification.

	var (
		cg                 Cgroup
		charged, committed bool
	)

	// Reserve cgroup PIDs controller charge. This is either committed when the
	// new task enters the cgroup below, or rolled back on failure.
	//
	// We may also get here from a non-task context (for example, when
	// creating the init task, or from the exec control command). In these cases
	// we skip charging the pids controller, as non-userspace task creation
	// bypasses pid limits.
	if srcT != nil {
		var err error
		if charged, cg, err = srcT.ChargeFor(t, CgroupControllerPIDs, CgroupResourcePID, 1); err != nil {
			return nil, err
		}
		if charged {
			defer func() {
				if !committed {
					if err := cg.Charge(t, cg.Dentry, CgroupControllerPIDs, CgroupResourcePID, -1); err != nil {
						panic(fmt.Sprintf("Failed to clean up PIDs charge on task creation failure: %v", err))
					}
				}
				// Ref from ChargeFor. Note that we need to drop this outside of
				// TaskSet.mu critical sections.
				cg.DecRef(ctx)
			}()
		}
	}

	// Make the new task (and possibly thread group) visible to the rest of
	// the system atomically.
	ts.mu.Lock()
	defer ts.mu.Unlock()
	tg.signalHandlers.mu.Lock()
	defer tg.signalHandlers.mu.Unlock()
	if tg.exiting || tg.execing != nil {
		// If the caller is in the same thread group, then what we return
		// doesn't matter too much since the caller will exit before it returns
		// to userspace. If the caller isn't in the same thread group, then
		// we're in uncharted territory and can return whatever we want.
		return nil, linuxerr.EINTR
	}
	if err := ts.assignTIDsLocked(t); err != nil {
		return nil, err
	}
	// Below this point, newTask is expected not to fail (there is no rollback
	// of assignTIDsLocked or any of the following).

	// Logging on t's behalf will panic if t.logPrefix hasn't been
	// initialized. This is the earliest point at which we can do so
	// (since t now has thread IDs).
	t.updateInfoLocked()

	if cfg.InheritParent != nil {
		t.parent = cfg.InheritParent.parent
	}
	if t.parent != nil {
		t.parent.children[t] = struct{}{}
	}

	// If InitialCgroups is not nil, the new task will be placed in the
	// specified cgroups. Otherwise, if srcT is not nil, the new task will
	// be placed in the srcT's cgroups. If neither is specified, the new task
	// will be in the root cgroups.
	t.EnterInitialCgroups(srcT, cfg.InitialCgroups)
	committed = true

	if tg.leader == nil {
		// New thread group.
		tg.leader = t
		if parentPG := tg.parentPG(); parentPG == nil {
			tg.createSession()
		} else {
			// Inherit the process group and terminal.
			parentPG.incRefWithParent(parentPG)
			tg.processGroup = parentPG
			tg.tty = t.parent.tg.tty
		}

		// If our parent is a child subreaper, or if it has a child
		// subreaper, then this new thread group does as well.
		if t.parent != nil {
			tg.hasChildSubreaper = t.parent.tg.isChildSubreaper || t.parent.tg.hasChildSubreaper
		}
	}
	tg.tasks.PushBack(t)
	tg.tasksCount++
	tg.liveTasks++
	tg.activeTasks++

	// Propagate external TaskSet stops to the new task.
	t.stopCount = atomicbitops.FromInt32(ts.stopCount)

	t.mu.Lock()
	defer t.mu.Unlock()

	t.cpu = atomicbitops.FromInt32(assignCPU(t.allowedCPUMask, ts.Root.tids[t]))

	t.startTime = t.k.RealtimeClock().Now()

	// As a final step, initialize the platform context. This may require
	// other pieces to be initialized as the task is used the context.
	t.p = cfg.Kernel.Platform.NewContext(t.AsyncContext())

	return t, nil
}

// assignTIDsLocked ensures that new task t is visible in all PID namespaces in
// which it should be visible.
//
// Preconditions: ts.mu must be locked for writing.
func (ts *TaskSet) assignTIDsLocked(t *Task) error {
	type allocatedTID struct {
		ns  *PIDNamespace
		tid ThreadID
	}
	var allocatedTIDs []allocatedTID
	var tid ThreadID
	var err error
	for ns := t.tg.pidns; ns != nil; ns = ns.parent {
		if tid, err = ns.allocateTID(); err != nil {
			break
		}
		if err = ns.addTask(t, tid); err != nil {
			break
		}
		allocatedTIDs = append(allocatedTIDs, allocatedTID{ns, tid})
	}
	if err != nil {
		// Failure. Remove the tids we already allocated in descendant
		// namespaces.
		for _, a := range allocatedTIDs {
			a.ns.deleteTask(t)
		}
		return err
	}
	t.tg.pidWithinNS.Store(int32(t.tg.pidns.tgids[t.tg]))
	return nil
}

// allocateTID returns an unused ThreadID from ns.
//
// Preconditions: ns.owner.mu must be locked for writing.
func (ns *PIDNamespace) allocateTID() (ThreadID, error) {
	if ns.exiting {
		// "In this case, a subsequent fork(2) into this PID namespace will
		// fail with the error ENOMEM; it is not possible to create a new
		// processes [sic] in a PID namespace whose init process has
		// terminated." - pid_namespaces(7)
		return 0, linuxerr.ENOMEM
	}
	tid := ns.last
	for {
		// Next.
		tid++
		if tid > TasksLimit {
			tid = initTID + 1
		}

		// Is it available?
		tidInUse := func() bool {
			if _, ok := ns.tasks[tid]; ok {
				return true
			}
			if _, ok := ns.processGroups[ProcessGroupID(tid)]; ok {
				return true
			}
			if _, ok := ns.sessions[SessionID(tid)]; ok {
				return true
			}
			return false
		}()

		if !tidInUse {
			ns.last = tid
			return tid, nil
		}

		// Did we do a full cycle?
		if tid == ns.last {
			// No tid available.
			return 0, linuxerr.EAGAIN
		}
	}
}

// Start starts the task goroutine. Start must be called exactly once for each
// task returned by NewTask.
//
// 'tid' must be the task's TID in the root PID namespace and it's used for
// debugging purposes only (set as parameter to Task.run to make it visible
// in stack dumps).
func (t *Task) Start(tid ThreadID) {
	// If the task was restored, it may be "starting" after having already exited.
	if t.runState == nil {
		return
	}
	t.goroutineStopped.Add(1)
	t.tg.liveGoroutines.Add(1)
	t.tg.pidns.owner.liveGoroutines.Add(1)
	t.tg.pidns.owner.runningGoroutines.Add(1)

	// Task is now running in system mode.
	t.accountTaskGoroutineLeave(TaskGoroutineNonexistent)

	// Use the task's TID in the root PID namespace to make it visible in stack dumps.
	go t.run(uintptr(tid)) // S/R-SAFE: synchronizes with saving through stops
}