diff --git a/android/androidmk.go b/android/androidmk.go
index f3c15e4..dbf3aa8 100644
--- a/android/androidmk.go
+++ b/android/androidmk.go
@@ -323,7 +323,7 @@
 		return
 	}
 
-	err := translateAndroidMk(ctx, transMk.String(), androidMkModulesList)
+	err := translateAndroidMk(ctx, absolutePath(transMk.String()), androidMkModulesList)
 	if err != nil {
 		ctx.Errorf(err.Error())
 	}
@@ -364,8 +364,8 @@
 	}
 
 	// Don't write to the file if it hasn't changed
-	if _, err := os.Stat(mkFile); !os.IsNotExist(err) {
-		if data, err := ioutil.ReadFile(mkFile); err == nil {
+	if _, err := os.Stat(absolutePath(mkFile)); !os.IsNotExist(err) {
+		if data, err := ioutil.ReadFile(absolutePath(mkFile)); err == nil {
 			matches := buf.Len() == len(data)
 
 			if matches {
@@ -383,7 +383,7 @@
 		}
 	}
 
-	return ioutil.WriteFile(mkFile, buf.Bytes(), 0666)
+	return ioutil.WriteFile(absolutePath(mkFile), buf.Bytes(), 0666)
 }
 
 func translateAndroidMkModule(ctx SingletonContext, w io.Writer, mod blueprint.Module) error {
diff --git a/android/config.go b/android/config.go
index 101f457..3c49c1a 100644
--- a/android/config.go
+++ b/android/config.go
@@ -135,12 +135,12 @@
 }
 
 func loadConfig(config *config) error {
-	err := loadFromConfigFile(&config.FileConfigurableOptions, config.ConfigFileName)
+	err := loadFromConfigFile(&config.FileConfigurableOptions, absolutePath(config.ConfigFileName))
 	if err != nil {
 		return err
 	}
 
-	return loadFromConfigFile(&config.productVariables, config.ProductVariablesFileName)
+	return loadFromConfigFile(&config.productVariables, absolutePath(config.ProductVariablesFileName))
 }
 
 // loads configuration options from a JSON file in the cwd.
@@ -204,6 +204,17 @@
 	return nil
 }
 
+// NullConfig returns a mostly empty Config for use by standalone tools like dexpreopt_gen that
+// use the android package.
+func NullConfig(buildDir string) Config {
+	return Config{
+		config: &config{
+			buildDir: buildDir,
+			fs:       pathtools.OsFs,
+		},
+	}
+}
+
 // TestConfig returns a Config object suitable for using for tests
 func TestConfig(buildDir string, env map[string]string, bp string, fs map[string][]byte) Config {
 	envCopy := make(map[string]string)
@@ -320,7 +331,7 @@
 		buildDir:          buildDir,
 		multilibConflicts: make(map[ArchType]bool),
 
-		fs: pathtools.OsFs,
+		fs: pathtools.NewOsFs(absSrcDir),
 	}
 
 	config.deviceConfig = &deviceConfig{
@@ -350,7 +361,7 @@
 	}
 
 	inMakeFile := filepath.Join(buildDir, ".soong.in_make")
-	if _, err := os.Stat(inMakeFile); err == nil {
+	if _, err := os.Stat(absolutePath(inMakeFile)); err == nil {
 		config.inMake = true
 	}
 
@@ -398,6 +409,8 @@
 	return Config{config}, nil
 }
 
+var TestConfigOsFs = map[string][]byte{}
+
 // mockFileSystem replaces all reads with accesses to the provided map of
 // filenames to contents stored as a byte slice.
 func (c *config) mockFileSystem(bp string, fs map[string][]byte) {
@@ -901,8 +914,13 @@
 	return c.productVariables.BootJars
 }
 
-func (c *config) DexpreoptGlobalConfig() string {
-	return String(c.productVariables.DexpreoptGlobalConfig)
+func (c *config) DexpreoptGlobalConfig(ctx PathContext) ([]byte, error) {
+	if c.productVariables.DexpreoptGlobalConfig == nil {
+		return nil, nil
+	}
+	path := absolutePath(*c.productVariables.DexpreoptGlobalConfig)
+	ctx.AddNinjaFileDeps(path)
+	return ioutil.ReadFile(path)
 }
 
 func (c *config) FrameworksBaseDirExists(ctx PathContext) bool {
diff --git a/android/env.go b/android/env.go
index d9f2db2..46bd3d6 100644
--- a/android/env.go
+++ b/android/env.go
@@ -52,6 +52,17 @@
 	os.Clearenv()
 }
 
+// getenv checks either os.Getenv or originalEnv so that it works before or after the init()
+// function above.  It doesn't add any dependencies on the environment variable, so it should
+// only be used for values that won't change.  For values that might change use ctx.Config().Getenv.
+func getenv(key string) string {
+	if originalEnv == nil {
+		return os.Getenv(key)
+	} else {
+		return originalEnv[key]
+	}
+}
+
 func EnvSingleton() Singleton {
 	return &envSingleton{}
 }
@@ -66,7 +77,12 @@
 		return
 	}
 
-	err := env.WriteEnvFile(envFile.String(), envDeps)
+	data, err := env.EnvFileContents(envDeps)
+	if err != nil {
+		ctx.Errorf(err.Error())
+	}
+
+	err = WriteFileToOutputDir(envFile, data, 0666)
 	if err != nil {
 		ctx.Errorf(err.Error())
 	}
diff --git a/android/makevars.go b/android/makevars.go
index 38a028c..aba4cce 100644
--- a/android/makevars.go
+++ b/android/makevars.go
@@ -23,7 +23,6 @@
 	"strings"
 
 	"github.com/google/blueprint"
-	"github.com/google/blueprint/pathtools"
 	"github.com/google/blueprint/proptools"
 )
 
@@ -41,7 +40,6 @@
 	Config() Config
 	DeviceConfig() DeviceConfig
 	AddNinjaFileDeps(deps ...string)
-	Fs() pathtools.FileSystem
 
 	ModuleName(module blueprint.Module) string
 	ModuleDir(module blueprint.Module) string
@@ -151,7 +149,8 @@
 		return
 	}
 
-	outFile := PathForOutput(ctx, "make_vars"+proptools.String(ctx.Config().productVariables.Make_suffix)+".mk").String()
+	outFile := absolutePath(PathForOutput(ctx,
+		"make_vars"+proptools.String(ctx.Config().productVariables.Make_suffix)+".mk").String())
 
 	if ctx.Failed() {
 		return
@@ -175,15 +174,15 @@
 
 	outBytes := s.writeVars(vars)
 
-	if _, err := os.Stat(outFile); err == nil {
-		if data, err := ioutil.ReadFile(outFile); err == nil {
+	if _, err := os.Stat(absolutePath(outFile)); err == nil {
+		if data, err := ioutil.ReadFile(absolutePath(outFile)); err == nil {
 			if bytes.Equal(data, outBytes) {
 				return
 			}
 		}
 	}
 
-	if err := ioutil.WriteFile(outFile, outBytes, 0666); err != nil {
+	if err := ioutil.WriteFile(absolutePath(outFile), outBytes, 0666); err != nil {
 		ctx.Errorf(err.Error())
 	}
 }
diff --git a/android/module.go b/android/module.go
index c998007..67d1f12 100644
--- a/android/module.go
+++ b/android/module.go
@@ -16,13 +16,13 @@
 
 import (
 	"fmt"
+	"os"
 	"path"
 	"path/filepath"
 	"strings"
 	"text/scanner"
 
 	"github.com/google/blueprint"
-	"github.com/google/blueprint/pathtools"
 	"github.com/google/blueprint/proptools"
 )
 
@@ -91,7 +91,8 @@
 
 	Glob(globPattern string, excludes []string) Paths
 	GlobFiles(globPattern string, excludes []string) Paths
-	Fs() pathtools.FileSystem
+	IsSymlink(path Path) bool
+	Readlink(path Path) string
 }
 
 // BaseModuleContext is the same as blueprint.BaseModuleContext except that Config() returns
@@ -1172,6 +1173,22 @@
 	return pathsForModuleSrcFromFullPath(e, ret, false)
 }
 
+func (b *earlyModuleContext) IsSymlink(path Path) bool {
+	fileInfo, err := b.config.fs.Lstat(path.String())
+	if err != nil {
+		b.ModuleErrorf("os.Lstat(%q) failed: %s", path.String(), err)
+	}
+	return fileInfo.Mode()&os.ModeSymlink == os.ModeSymlink
+}
+
+func (b *earlyModuleContext) Readlink(path Path) string {
+	dest, err := b.config.fs.Readlink(path.String())
+	if err != nil {
+		b.ModuleErrorf("os.Readlink(%q) failed: %s", path.String(), err)
+	}
+	return dest
+}
+
 func (e *earlyModuleContext) Module() Module {
 	module, _ := e.EarlyModuleContext.Module().(Module)
 	return module
diff --git a/android/package_ctx.go b/android/package_ctx.go
index d3527fa..a228910 100644
--- a/android/package_ctx.go
+++ b/android/package_ctx.go
@@ -19,7 +19,6 @@
 	"strings"
 
 	"github.com/google/blueprint"
-	"github.com/google/blueprint/pathtools"
 )
 
 // PackageContext is a wrapper for blueprint.PackageContext that adds
@@ -60,10 +59,6 @@
 	e.pctx.AddNinjaFileDeps(deps...)
 }
 
-func (e *configErrorWrapper) Fs() pathtools.FileSystem {
-	return nil
-}
-
 type PackageVarContext interface {
 	PathContext
 	errorfContext
diff --git a/android/paths.go b/android/paths.go
index a03fe17..02f56d0 100644
--- a/android/paths.go
+++ b/android/paths.go
@@ -16,6 +16,8 @@
 
 import (
 	"fmt"
+	"io/ioutil"
+	"os"
 	"path/filepath"
 	"reflect"
 	"sort"
@@ -25,10 +27,11 @@
 	"github.com/google/blueprint/pathtools"
 )
 
+var absSrcDir string
+
 // PathContext is the subset of a (Module|Singleton)Context required by the
 // Path methods.
 type PathContext interface {
-	Fs() pathtools.FileSystem
 	Config() Config
 	AddNinjaFileDeps(deps ...string)
 }
@@ -390,7 +393,7 @@
 		return PathsWithModuleSrcSubDir(ctx, paths, ""), nil
 	} else {
 		p := pathForModuleSrc(ctx, s)
-		if exists, _, err := ctx.Fs().Exists(p.String()); err != nil {
+		if exists, _, err := ctx.Config().fs.Exists(p.String()); err != nil {
 			reportPathErrorf(ctx, "%s: %s", p, err.Error())
 		} else if !exists {
 			reportPathErrorf(ctx, "module source path %q does not exist", p)
@@ -720,7 +723,7 @@
 		var deps []string
 		// We cannot add build statements in this context, so we fall back to
 		// AddNinjaFileDeps
-		files, deps, err = pathtools.Glob(path.String(), nil, pathtools.FollowSymlinks)
+		files, deps, err = ctx.Config().fs.Glob(path.String(), nil, pathtools.FollowSymlinks)
 		ctx.AddNinjaFileDeps(deps...)
 	}
 
@@ -752,7 +755,7 @@
 		if !exists {
 			modCtx.AddMissingDependencies([]string{path.String()})
 		}
-	} else if exists, _, err := ctx.Fs().Exists(path.String()); err != nil {
+	} else if exists, _, err := ctx.Config().fs.Exists(path.String()); err != nil {
 		reportPathErrorf(ctx, "%s: %s", path, err.Error())
 	} else if !exists {
 		reportPathErrorf(ctx, "source path %q does not exist", path)
@@ -1356,7 +1359,6 @@
 	config Config
 }
 
-func (x *testPathContext) Fs() pathtools.FileSystem   { return x.config.fs }
 func (x *testPathContext) Config() Config             { return x.config }
 func (x *testPathContext) AddNinjaFileDeps(...string) {}
 
@@ -1402,3 +1404,16 @@
 	}
 	return rel, true, nil
 }
+
+// Writes a file to the output directory.  Attempting to write directly to the output directory
+// will fail due to the sandbox of the soong_build process.
+func WriteFileToOutputDir(path WritablePath, data []byte, perm os.FileMode) error {
+	return ioutil.WriteFile(absolutePath(path.String()), data, perm)
+}
+
+func absolutePath(path string) string {
+	if filepath.IsAbs(path) {
+		return path
+	}
+	return filepath.Join(absSrcDir, path)
+}
diff --git a/android/paths_test.go b/android/paths_test.go
index ec5e598..46e3e1f 100644
--- a/android/paths_test.go
+++ b/android/paths_test.go
@@ -21,7 +21,6 @@
 	"strings"
 	"testing"
 
-	"github.com/google/blueprint/pathtools"
 	"github.com/google/blueprint/proptools"
 )
 
@@ -207,10 +206,6 @@
 	inRoot         bool
 }
 
-func (moduleInstallPathContextImpl) Fs() pathtools.FileSystem {
-	return pathtools.MockFs(nil)
-}
-
 func (m moduleInstallPathContextImpl) Config() Config {
 	return m.baseModuleContext.config
 }
diff --git a/android/register.go b/android/register.go
index b5defec..b48d3d1 100644
--- a/android/register.go
+++ b/android/register.go
@@ -84,7 +84,9 @@
 }
 
 func NewContext() *Context {
-	return &Context{blueprint.NewContext()}
+	ctx := &Context{blueprint.NewContext()}
+	ctx.SetSrcDir(absSrcDir)
+	return ctx
 }
 
 func (ctx *Context) Register() {
diff --git a/android/sandbox.go b/android/sandbox.go
new file mode 100644
index 0000000..ed022fb
--- /dev/null
+++ b/android/sandbox.go
@@ -0,0 +1,47 @@
+// Copyright 2020 Google Inc. All rights reserved.
+//
+// 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 android
+
+import (
+	"fmt"
+	"os"
+)
+
+func init() {
+	// Stash the working directory in a private variable and then change the working directory
+	// to "/", which will prevent untracked accesses to files by Go Soong plugins. The
+	// SOONG_SANDBOX_SOONG_BUILD environment variable is set by soong_ui, and is not
+	// overrideable on the command line.
+
+	orig, err := os.Getwd()
+	if err != nil {
+		panic(fmt.Errorf("failed to get working directory: %s", err))
+	}
+	absSrcDir = orig
+
+	if getenv("SOONG_SANDBOX_SOONG_BUILD") == "true" {
+		err = os.Chdir("/")
+		if err != nil {
+			panic(fmt.Errorf("failed to change working directory to '/': %s", err))
+		}
+	}
+}
+
+// DO NOT USE THIS FUNCTION IN NEW CODE.
+// Deprecated: This function will be removed as soon as the existing use cases that use it have been
+// replaced.
+func AbsSrcDirForExistingUseCases() string {
+	return absSrcDir
+}
diff --git a/android/singleton.go b/android/singleton.go
index 5519ca0..91268ad 100644
--- a/android/singleton.go
+++ b/android/singleton.go
@@ -16,7 +16,6 @@
 
 import (
 	"github.com/google/blueprint"
-	"github.com/google/blueprint/pathtools"
 )
 
 // SingletonContext
@@ -74,8 +73,6 @@
 	// builder whenever a file matching the pattern as added or removed, without rerunning if a
 	// file that does not match the pattern is added to a searched directory.
 	GlobWithDeps(pattern string, excludes []string) ([]string, error)
-
-	Fs() pathtools.FileSystem
 }
 
 type singletonAdaptor struct {