runsc: Support multi-container exec.

We must use a context.Context with a Root Dirent that corresponds to the
container's chroot. Previously we were using the root context, which does not
have a chroot.

Getting the correct context required refactoring some of the path-lookup code.
We can't lookup the path without a context.Context, which requires
kernel.CreateProcArgs, which we only get inside control.Execute.  So we have to
do the path lookup much later than we previously were.

PiperOrigin-RevId: 212064734
Change-Id: I84a5cfadacb21fd9c3ab9c393f7e308a40b9b537

6 files changed, 118 insertions, 79 deletions

diff --git a/pkg/sentry/control/proc.go b/pkg/sentry/control/proc.go
index 6949a3ae5..289b8ba0e 100644
--- a/pkg/sentry/control/proc.go
+++ b/pkg/sentry/control/proc.go
@@ -54,6 +54,11 @@ type ExecArgs struct {
 	// Envv is a list of environment variables.
 	Envv []string `json:"envv"`
 
+	// Root defines the root directory for the new process. A reference on
+	// Root must be held for the lifetime of the ExecArgs. If Root is nil,
+	// it will default to the VFS root.
+	Root *fs.Dirent
+
 	// WorkingDirectory defines the working directory for the new process.
 	WorkingDirectory string `json:"wd"`
 
@@ -99,6 +104,7 @@ func (proc *Proc) Exec(args *ExecArgs, waitStatus *uint32) error {
 		Argv:                    args.Argv,
 		Envv:                    args.Envv,
 		WorkingDirectory:        args.WorkingDirectory,
+		Root:                    args.Root,
 		Credentials:             creds,
 		FDMap:                   fdm,
 		Umask:                   0022,
@@ -109,8 +115,18 @@ func (proc *Proc) Exec(args *ExecArgs, waitStatus *uint32) error {
 		AbstractSocketNamespace: proc.Kernel.RootAbstractSocketNamespace(),
 	}
 	ctx := initArgs.NewContext(proc.Kernel)
-	mounter := fs.FileOwnerFromContext(ctx)
 
+	if initArgs.Filename == "" {
+		// Get the full path to the filename from the PATH env variable.
+		paths := fs.GetPath(initArgs.Envv)
+		f, err := proc.Kernel.RootMountNamespace().ResolveExecutablePath(ctx, initArgs.WorkingDirectory, initArgs.Argv[0], paths)
+		if err != nil {
+			return fmt.Errorf("error finding executable %q in PATH %v: %v", initArgs.Argv[0], paths, err)
+		}
+		initArgs.Filename = f
+	}
+
+	mounter := fs.FileOwnerFromContext(ctx)
 	for appFD, f := range args.FilePayload.Files {
 		enableIoctl := args.StdioIsPty && appFD <= 2
 
diff --git a/pkg/sentry/fs/mounts.go b/pkg/sentry/fs/mounts.go
index 0318f135d..c0a803b2d 100644
--- a/pkg/sentry/fs/mounts.go
+++ b/pkg/sentry/fs/mounts.go
@@ -16,9 +16,13 @@ package fs
 
 import (
 	"fmt"
+	"path"
+	"strings"
 	"sync"
 	"syscall"
 
+	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
+	"gvisor.googlesource.com/gvisor/pkg/log"
 	"gvisor.googlesource.com/gvisor/pkg/refs"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/context"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/kernel/auth"
@@ -509,3 +513,65 @@ func (mns *MountNamespace) SyncAll(ctx context.Context) {
 	defer mns.mu.Unlock()
 	mns.root.SyncAll(ctx)
 }
+
+// ResolveExecutablePath resolves the given executable name given a set of
+// paths that might contain it.
+func (mns *MountNamespace) ResolveExecutablePath(ctx context.Context, wd, name string, paths []string) (string, error) {
+	// Absolute paths can be used directly.
+	if path.IsAbs(name) {
+		return name, nil
+	}
+
+	// Paths with '/' in them should be joined to the working directory, or
+	// to the root if working directory is not set.
+	if strings.IndexByte(name, '/') > 0 {
+		if wd == "" {
+			wd = "/"
+		}
+		if !path.IsAbs(wd) {
+			return "", fmt.Errorf("working directory %q must be absolute", wd)
+		}
+		return path.Join(wd, name), nil
+	}
+
+	// Otherwise, We must lookup the name in the paths, starting from the
+	// calling context's root directory.
+	root := RootFromContext(ctx)
+	if root == nil {
+		// Caller has no root. Don't bother traversing anything.
+		return "", syserror.ENOENT
+	}
+	defer root.DecRef()
+	for _, p := range paths {
+		binPath := path.Join(p, name)
+		d, err := mns.FindInode(ctx, root, nil, binPath, linux.MaxSymlinkTraversals)
+		if err == syserror.ENOENT || err == syserror.EACCES {
+			// Didn't find it here.
+			continue
+		}
+		if err != nil {
+			return "", err
+		}
+		defer d.DecRef()
+
+		// Check whether we can read and execute the found file.
+		if err := d.Inode.CheckPermission(ctx, PermMask{Read: true, Execute: true}); err != nil {
+			log.Infof("Found executable at %q, but user cannot execute it: %v", binPath, err)
+			continue
+		}
+		return path.Join("/", p, name), nil
+	}
+	return "", syserror.ENOENT
+}
+
+// GetPath returns the PATH as a slice of strings given the environemnt
+// variables.
+func GetPath(env []string) []string {
+	const prefix = "PATH="
+	for _, e := range env {
+		if strings.HasPrefix(e, prefix) {
+			return strings.Split(strings.TrimPrefix(e, prefix), ":")
+		}
+	}
+	return nil
+}
diff --git a/runsc/boot/controller.go b/runsc/boot/controller.go
index 45aa255c4..fd5b7cc9e 100644
--- a/runsc/boot/controller.go
+++ b/runsc/boot/controller.go
@@ -224,21 +224,39 @@ func (cm *containerManager) Start(args *StartArgs, _ *struct{}) error {
 	return nil
 }
 
+// ExecArgs contains arguments to Execute.
+type ExecArgs struct {
+	control.ExecArgs
+
+	// CID is the ID of the container to exec in.
+	CID string
+}
+
 // Execute runs a command on a created or running sandbox.
-func (cm *containerManager) Execute(e *control.ExecArgs, waitStatus *uint32) error {
+func (cm *containerManager) Execute(e *ExecArgs, waitStatus *uint32) error {
 	log.Debugf("containerManager.Execute: %+v", *e)
 
-	if e.Filename == "" {
-		rootCtx := cm.l.rootProcArgs.NewContext(cm.l.k)
-		rootMns := cm.l.k.RootMountNamespace()
-		var err error
-		if e.Filename, err = getExecutablePath(rootCtx, rootMns, e.Argv[0], e.Envv); err != nil {
-			return fmt.Errorf("error getting executable path for %q: %v", e.Argv[0], err)
-		}
+	// Get the container Root Dirent from the Task, since we must run this
+	// process with the same Root.
+	cm.l.mu.Lock()
+	tgid, ok := cm.l.containerRootTGIDs[e.CID]
+	cm.l.mu.Unlock()
+	if !ok {
+		return fmt.Errorf("cannot exec in container %q: no such container", e.CID)
+	}
+	t := cm.l.k.TaskSet().Root.TaskWithID(kernel.ThreadID(tgid))
+	if t == nil {
+		return fmt.Errorf("cannot exec in container %q: no thread group with ID %d", e.CID, tgid)
+	}
+	t.WithMuLocked(func(t *kernel.Task) {
+		e.Root = t.FSContext().RootDirectory()
+	})
+	if e.Root != nil {
+		defer e.Root.DecRef()
 	}
 
 	proc := control.Proc{Kernel: cm.l.k}
-	if err := proc.Exec(e, waitStatus); err != nil {
+	if err := proc.Exec(&e.ExecArgs, waitStatus); err != nil {
 		return fmt.Errorf("error executing: %+v: %v", e, err)
 	}
 	return nil
diff --git a/runsc/boot/fs.go b/runsc/boot/fs.go
index 3df276170..5ec9a7d03 100644
--- a/runsc/boot/fs.go
+++ b/runsc/boot/fs.go
@@ -16,7 +16,6 @@ package boot
 
 import (
 	"fmt"
-	"path"
 	"path/filepath"
 	"strconv"
 	"strings"
@@ -683,64 +682,11 @@ func setFileSystemForProcess(procArgs *kernel.CreateProcessArgs, spec *specs.Spe
 	return nil
 }
 
-// getExecutablePath traverses the *container's* filesystem to resolve exec's
-// absolute path. For example, if the container is being served files by the
-// fsgofer serving /foo/bar as the container root, it will search within
-// /foo/bar, not the host root.
-// TODO: Unit test this.
-func getExecutablePath(ctx context.Context, mns *fs.MountNamespace, filename string, env []string) (string, error) {
-	exec := filepath.Clean(filename)
-
-	// Don't search PATH if exec is a path to a file (absolute or relative).
-	if strings.IndexByte(exec, '/') >= 0 {
-		return exec, nil
-	}
-
-	// Search the PATH for a file whose name matches the one we are looking
-	// for.
-	pathDirs := specutils.GetPath(env)
-	for _, p := range pathDirs {
-		// Try to find the binary inside path p.
-		binPath := path.Join(p, filename)
-		root := fs.RootFromContext(ctx)
-		defer root.DecRef()
-		d, err := mns.FindInode(ctx, root, nil, binPath, linux.MaxSymlinkTraversals)
-		if err == syserror.ENOENT || err == syserror.EACCES {
-			continue
-		}
-		if err != nil {
-			return "", fmt.Errorf("FindInode(%q) failed: %v", binPath, err)
-		}
-		defer d.DecRef()
-
-		// Check whether we can read and execute the found file.
-		if err := d.Inode.CheckPermission(ctx, fs.PermMask{Read: true, Execute: true}); err != nil {
-			log.Infof("Found executable at %q, but user cannot execute it: %v", binPath, err)
-			continue
-		}
-		return path.Join("/", p, exec), nil
-	}
-
-	return "", fmt.Errorf("could not find executable %q in path %v", exec, pathDirs)
-}
-
 // setExecutablePath sets the procArgs.Filename by searching the PATH for an
 // executable matching the procArgs.Argv[0].
 func setExecutablePath(ctx context.Context, mns *fs.MountNamespace, procArgs *kernel.CreateProcessArgs) error {
-	if procArgs.Filename != "" {
-		// Sanity check.
-		if !path.IsAbs(procArgs.Filename) {
-			return fmt.Errorf("filename must be absolute: %q", procArgs.Filename)
-		}
-		// Nothing to set.
-		return nil
-	}
-
-	if len(procArgs.Argv) == 0 {
-		return fmt.Errorf("Argv must not be empty")
-	}
-
-	f, err := getExecutablePath(ctx, mns, procArgs.Argv[0], procArgs.Envv)
+	paths := fs.GetPath(procArgs.Envv)
+	f, err := mns.ResolveExecutablePath(ctx, procArgs.WorkingDirectory, procArgs.Argv[0], paths)
 	if err != nil {
 		return err
 	}
diff --git a/runsc/sandbox/sandbox.go b/runsc/sandbox/sandbox.go
index 697210669..f272496a1 100644
--- a/runsc/sandbox/sandbox.go
+++ b/runsc/sandbox/sandbox.go
@@ -187,11 +187,16 @@ func (s *Sandbox) Execute(cid string, e *control.ExecArgs) (syscall.WaitStatus,
 	}
 	defer conn.Close()
 
+	ea := &boot.ExecArgs{
+		ExecArgs: *e,
+		CID:      cid,
+	}
+
 	// Send a message to the sandbox control server to start the container.
 	var waitStatus uint32
 	// TODO: Pass in the container id (cid) here. The sandbox
 	// should execute in the context of that container.
-	if err := conn.Call(boot.ContainerExecute, e, &waitStatus); err != nil {
+	if err := conn.Call(boot.ContainerExecute, ea, &waitStatus); err != nil {
 		return 0, fmt.Errorf("error executing in sandbox: %v", err)
 	}
 
diff --git a/runsc/specutils/specutils.go b/runsc/specutils/specutils.go
index f3fa8d129..fdc9007e0 100644
--- a/runsc/specutils/specutils.go
+++ b/runsc/specutils/specutils.go
@@ -163,18 +163,6 @@ func ReadSpecFromFile(bundleDir string, specFile *os.File) (*specs.Spec, error)
 	return &spec, nil
 }
 
-// GetPath returns the PATH as a slice of strings given the environemnt
-// variables.
-func GetPath(env []string) []string {
-	const prefix = "PATH="
-	for _, e := range env {
-		if strings.HasPrefix(e, prefix) {
-			return strings.Split(strings.TrimPrefix(e, prefix), ":")
-		}
-	}
-	return nil
-}
-
 // Capabilities takes in spec and returns a TaskCapabilities corresponding to
 // the spec.
 func Capabilities(specCaps *specs.LinuxCapabilities) (*auth.TaskCapabilities, error) {