2 files changed, 171 insertions, 0 deletions
diff --git a/pkg/sentry/fs/user/path.go b/pkg/sentry/fs/user/path.go
new file mode 100644
index 000000000..fbd4547a7
--- /dev/null
+++ b/pkg/sentry/fs/user/path.go
@@ -0,0 +1,169 @@
+// Copyright 2020 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 user
+
+import (
+	"fmt"
+	"path"
+	"strings"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/fspath"
+	"gvisor.dev/gvisor/pkg/log"
+	"gvisor.dev/gvisor/pkg/sentry/fs"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// ResolveExecutablePath resolves the given executable name given the working
+// dir and environment.
+func ResolveExecutablePath(ctx context.Context, creds *auth.Credentials, mns *fs.MountNamespace, envv []string, wd, name 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.
+	paths := getPath(envv)
+
+	root := fs.RootFromContext(ctx)
+	if root == nil {
+		// Caller has no root. Don't bother traversing anything.
+		return "", syserror.ENOENT
+	}
+	defer root.DecRef()
+	for _, p := range paths {
+		if !path.IsAbs(p) {
+			// Relative paths aren't safe, no one should be using them.
+			log.Warningf("Skipping relative path %q in $PATH", p)
+			continue
+		}
+
+		binPath := path.Join(p, name)
+		traversals := uint(linux.MaxSymlinkTraversals)
+		d, err := mns.FindInode(ctx, root, nil, binPath, &traversals)
+		if err == syserror.ENOENT || err == syserror.EACCES {
+			// Didn't find it here.
+			continue
+		}
+		if err != nil {
+			return "", err
+		}
+		defer d.DecRef()
+
+		// Check that it is a regular file.
+		if !fs.IsRegular(d.Inode.StableAttr) {
+			continue
+		}
+
+		// 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, name), nil
+	}
+
+	// Couldn't find it.
+	return "", syserror.ENOENT
+}
+
+// ResolveExecutablePathVFS2 resolves the given executable name given the
+// working dir and environment.
+func ResolveExecutablePathVFS2(ctx context.Context, creds *auth.Credentials, mns *vfs.MountNamespace, envv []string, wd, name 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.
+	paths := getPath(envv)
+
+	root := mns.Root()
+	defer root.DecRef()
+	for _, p := range paths {
+		if !path.IsAbs(p) {
+			// Relative paths aren't safe, no one should be using them.
+			log.Warningf("Skipping relative path %q in $PATH", p)
+			continue
+		}
+
+		binPath := path.Join(p, name)
+		pop := &vfs.PathOperation{
+			Root:               root,
+			Start:              root,
+			Path:               fspath.Parse(binPath),
+			FollowFinalSymlink: true,
+		}
+		opts := &vfs.OpenOptions{
+			FileExec: true,
+			Flags:    linux.O_RDONLY,
+		}
+		dentry, err := root.Mount().Filesystem().VirtualFilesystem().OpenAt(ctx, creds, pop, opts)
+		if err == syserror.ENOENT || err == syserror.EACCES {
+			// Didn't find it here.
+			continue
+		}
+		if err != nil {
+			return "", err
+		}
+		dentry.DecRef()
+
+		return binPath, nil
+	}
+
+	// Couldn't find it.
+	return "", syserror.ENOENT
+}
+
+// getPath returns the PATH as a slice of strings given the environment
+// 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/pkg/sentry/fs/user/user.go b/pkg/sentry/fs/user/user.go
index fe7f67c00..f4d525523 100644
--- a/pkg/sentry/fs/user/user.go
+++ b/pkg/sentry/fs/user/user.go
@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// Package user contains methods for resolving filesystem paths based on the
+// user and their environment.
 package user
 
 import (