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

2 files changed, 83 insertions, 1 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
+}