Add //pkg/sentry/seccheck.

This defines common infrastructure for dynamically-configured security checks, including an example usage in the clone(2) path. PiperOrigin-RevId: 394797270
2026-05-22 17:12:49 -07:00 · 2021-09-03 19:12:54 -07:00
parent 2aeab259c4
commit 775a321120
11 changed files with 850 additions and 1 deletions
@@ -268,6 +268,7 @@ go_library(
        "//pkg/sentry/mm",
        "//pkg/sentry/pgalloc",
        "//pkg/sentry/platform",
+        "//pkg/sentry/seccheck",
        "//pkg/sentry/socket/netlink/port",
        "//pkg/sentry/socket/unix/transport",
        "//pkg/sentry/time",
@@ -30,6 +30,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/kernel/sched"
 	ktime "gvisor.dev/gvisor/pkg/sentry/kernel/time"
 	"gvisor.dev/gvisor/pkg/sentry/platform"
+	"gvisor.dev/gvisor/pkg/sentry/seccheck"
 	"gvisor.dev/gvisor/pkg/sentry/usage"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/sync"
@@ -874,3 +875,23 @@ func (t *Task) ResetKcov() {
 		t.kcov = nil
 	}
 }
+
+// Preconditions: The TaskSet mutex must be locked.
+func (t *Task) loadSeccheckInfoLocked(req seccheck.TaskFieldSet, mask *seccheck.TaskFieldSet, info *seccheck.TaskInfo) {
+	if req.Contains(seccheck.TaskFieldThreadID) {
+		info.ThreadID = int32(t.k.tasks.Root.tids[t])
+		mask.Add(seccheck.TaskFieldThreadID)
+	}
+	if req.Contains(seccheck.TaskFieldThreadStartTime) {
+		info.ThreadStartTime = t.startTime
+		mask.Add(seccheck.TaskFieldThreadStartTime)
+	}
+	if req.Contains(seccheck.TaskFieldThreadGroupID) {
+		info.ThreadGroupID = int32(t.k.tasks.Root.tgids[t.tg])
+		mask.Add(seccheck.TaskFieldThreadGroupID)
+	}
+	if req.Contains(seccheck.TaskFieldThreadGroupStartTime) {
+		info.ThreadGroupStartTime = t.tg.leader.startTime
+		mask.Add(seccheck.TaskFieldThreadGroupStartTime)
+	}
+}
@@ -23,6 +23,7 @@ import (
 	"gvisor.dev/gvisor/pkg/errors/linuxerr"
 	"gvisor.dev/gvisor/pkg/hostarch"
 	"gvisor.dev/gvisor/pkg/sentry/inet"
+	"gvisor.dev/gvisor/pkg/sentry/seccheck"
 	"gvisor.dev/gvisor/pkg/usermem"
 )

@@ -235,7 +236,23 @@ func (t *Task) Clone(args *linux.CloneArgs) (ThreadID, *SyscallControl, error) {
 	// nt that it must receive before its task goroutine starts running.
 	tid := nt.k.tasks.Root.IDOfTask(nt)
 	defer nt.Start(tid)
-	t.traceCloneEvent(tid)
+
+	if seccheck.Global.Enabled(seccheck.PointClone) {
+		mask, info := getCloneSeccheckInfo(t, nt, args)
+		if err := seccheck.Global.Clone(t, mask, &info); err != nil {
+			// nt has been visible to the rest of the system since NewTask, so
+			// it may be blocking execve or a group stop, have been notified
+			// for group signal delivery, had children reparented to it, etc.
+			// Thus we can't just drop it on the floor. Instead, instruct the
+			// task goroutine to exit immediately, as quietly as possible.
+			nt.exitTracerNotified = true
+			nt.exitTracerAcked = true
+			nt.exitParentNotified = true
+			nt.exitParentAcked = true
+			nt.runState = (*runExitMain)(nil)
+			return 0, nil, err
+		}
+	}

 	// "If fork/clone and execve are allowed by @prog, any child processes will
 	// be constrained to the same filters and system call ABI as the parent." -
@@ -260,6 +277,7 @@ func (t *Task) Clone(args *linux.CloneArgs) (ThreadID, *SyscallControl, error) {
 		ntid.CopyOut(t, hostarch.Addr(args.ParentTID))
 	}

+	t.traceCloneEvent(tid)
 	kind := ptraceCloneKindClone
 	if args.Flags&linux.CLONE_VFORK != 0 {
 		kind = ptraceCloneKindVfork
@@ -279,6 +297,22 @@ func (t *Task) Clone(args *linux.CloneArgs) (ThreadID, *SyscallControl, error) {
 	return ntid, nil, nil
 }

+func getCloneSeccheckInfo(t, nt *Task, args *linux.CloneArgs) (seccheck.CloneFieldSet, seccheck.CloneInfo) {
+	req := seccheck.Global.CloneReq()
+	info := seccheck.CloneInfo{
+		Credentials: t.Credentials(),
+		Args:        *args,
+	}
+	var mask seccheck.CloneFieldSet
+	mask.Add(seccheck.CloneFieldCredentials)
+	mask.Add(seccheck.CloneFieldArgs)
+	t.k.tasks.mu.RLock()
+	defer t.k.tasks.mu.RUnlock()
+	t.loadSeccheckInfoLocked(req.Invoker, &mask.Invoker, &info.Invoker)
+	nt.loadSeccheckInfoLocked(req.Created, &mask.Created, &info.Created)
+	return mask, info
+}
+
 // maybeBeginVforkStop checks if a previously-started vfork child is still
 // running and has not yet released its MM, such that its parent t should enter
 // a vforkStop.
@@ -0,0 +1,54 @@
+load("//tools:defs.bzl", "go_library", "go_test")
+load("//tools/go_fieldenum:defs.bzl", "go_fieldenum")
+load("//tools/go_generics:defs.bzl", "go_template_instance")
+
+licenses(["notice"])
+
+go_fieldenum(
+    name = "seccheck_fieldenum",
+    srcs = [
+        "clone.go",
+        "task.go",
+    ],
+    out = "seccheck_fieldenum.go",
+    package = "seccheck",
+)
+
+go_template_instance(
+    name = "seqatomic_checkerslice",
+    out = "seqatomic_checkerslice_unsafe.go",
+    package = "seccheck",
+    suffix = "CheckerSlice",
+    template = "//pkg/sync/seqatomic:generic_seqatomic",
+    types = {
+        "Value": "[]Checker",
+    },
+)
+
+go_library(
+    name = "seccheck",
+    srcs = [
+        "clone.go",
+        "seccheck.go",
+        "seccheck_fieldenum.go",
+        "seqatomic_checkerslice_unsafe.go",
+        "task.go",
+    ],
+    visibility = ["//:sandbox"],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/context",
+        "//pkg/gohacks",
+        "//pkg/sentry/kernel/auth",
+        "//pkg/sentry/kernel/time",
+        "//pkg/sync",
+    ],
+)
+
+go_test(
+    name = "seccheck_test",
+    size = "small",
+    srcs = ["seccheck_test.go"],
+    library = ":seccheck",
+    deps = ["//pkg/context"],
+)
@@ -0,0 +1,53 @@
+// Copyright 2021 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 seccheck
+
+import (
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+)
+
+// CloneInfo contains information used by the Clone checkpoint.
+//
+// +fieldenum Clone
+type CloneInfo struct {
+	// Invoker identifies the invoking thread.
+	Invoker TaskInfo
+
+	// Credentials are the invoking thread's credentials.
+	Credentials *auth.Credentials
+
+	// Args contains the arguments to kernel.Task.Clone().
+	Args linux.CloneArgs
+
+	// Created identifies the created thread.
+	Created TaskInfo
+}
+
+// CloneReq returns fields required by the Clone checkpoint.
+func (s *state) CloneReq() CloneFieldSet {
+	return s.cloneReq.Load()
+}
+
+// Clone is called at the Clone checkpoint.
+func (s *state) Clone(ctx context.Context, mask CloneFieldSet, info *CloneInfo) error {
+	for _, c := range s.getCheckers() {
+		if err := c.Clone(ctx, mask, *info); err != nil {
+			return err
+		}
+	}
+	return nil
+}
@@ -0,0 +1,136 @@
+// Copyright 2021 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 seccheck defines a structure for dynamically-configured security
+// checks in the sentry.
+package seccheck
+
+import (
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sync"
+)
+
+// A Point represents a checkpoint, a point at which a security check occurs.
+type Point uint
+
+// PointX represents the checkpoint X.
+const (
+	PointClone Point = iota
+	// Add new Points above this line.
+	pointLength
+
+	numPointBitmaskUint32s = (int(pointLength)-1)/32 + 1
+)
+
+// A Checker performs security checks at checkpoints.
+//
+// Each Checker method X is called at checkpoint X; if the method may return a
+// non-nil error and does so, it causes the checked operation to fail
+// immediately (without calling subsequent Checkers) and return the error. The
+// info argument contains information relevant to the check. The mask argument
+// indicates what fields in info are valid; the mask should usually be a
+// superset of fields requested by the Checker's corresponding CheckerReq, but
+// may be missing requested fields in some cases (e.g. if the Checker is
+// registered concurrently with invocations of checkpoints).
+type Checker interface {
+	Clone(ctx context.Context, mask CloneFieldSet, info CloneInfo) error
+}
+
+// CheckerDefaults may be embedded by implementations of Checker to obtain
+// no-op implementations of Checker methods that may be explicitly overridden.
+type CheckerDefaults struct{}
+
+// Clone implements Checker.Clone.
+func (CheckerDefaults) Clone(ctx context.Context, mask CloneFieldSet, info CloneInfo) error {
+	return nil
+}
+
+// CheckerReq indicates what checkpoints a corresponding Checker runs at, and
+// what information it requires at those checkpoints.
+type CheckerReq struct {
+	// Points are the set of checkpoints for which the corresponding Checker
+	// must be called. Note that methods not specified in Points may still be
+	// called; implementations of Checker may embed CheckerDefaults to obtain
+	// no-op implementations of Checker methods.
+	Points []Point
+
+	// All of the following fields indicate what fields in the corresponding
+	// XInfo struct will be requested at the corresponding checkpoint.
+	Clone CloneFields
+}
+
+// Global is the method receiver of all seccheck functions.
+var Global state
+
+// state is the type of global, and is separated out for testing.
+type state struct {
+	// registrationMu serializes all changes to the set of registered Checkers
+	// for all checkpoints.
+	registrationMu sync.Mutex
+
+	// enabledPoints is a bitmask of checkpoints for which at least one Checker
+	// is registered.
+	//
+	// enabledPoints is accessed using atomic memory operations. Mutation of
+	// enabledPoints is serialized by registrationMu.
+	enabledPoints [numPointBitmaskUint32s]uint32
+
+	// registrationSeq supports store-free atomic reads of registeredCheckers.
+	registrationSeq sync.SeqCount
+
+	// checkers is the set of all registered Checkers in order of execution.
+	//
+	// checkers is accessed using instantiations of SeqAtomic functions.
+	// Mutation of checkers is serialized by registrationMu.
+	checkers []Checker
+
+	// All of the following xReq variables indicate what fields in the
+	// corresponding XInfo struct have been requested by any registered
+	// checker, are accessed using atomic memory operations, and are mutated
+	// with registrationMu locked.
+	cloneReq CloneFieldSet
+}
+
+// AppendChecker registers the given Checker to execute at checkpoints. The
+// Checker will execute after all previously-registered Checkers, and only if
+// those Checkers return a nil error.
+func (s *state) AppendChecker(c Checker, req *CheckerReq) {
+	s.registrationMu.Lock()
+	defer s.registrationMu.Unlock()
+	s.cloneReq.AddFieldsLoadable(req.Clone)
+	s.appendCheckerLocked(c)
+	for _, p := range req.Points {
+		word, bit := p/32, p%32
+		atomic.StoreUint32(&s.enabledPoints[word], s.enabledPoints[word]|(uint32(1)<<bit))
+	}
+}
+
+// Enabled returns true if any Checker is registered for the given checkpoint.
+func (s *state) Enabled(p Point) bool {
+	word, bit := p/32, p%32
+	return atomic.LoadUint32(&s.enabledPoints[word])&(uint32(1)<<bit) != 0
+}
+
+func (s *state) getCheckers() []Checker {
+	return SeqAtomicLoadCheckerSlice(&s.registrationSeq, &s.checkers)
+}
+
+// Preconditions: s.registrationMu must be locked.
+func (s *state) appendCheckerLocked(c Checker) {
+	s.registrationSeq.BeginWrite()
+	s.checkers = append(s.checkers, c)
+	s.registrationSeq.EndWrite()
+}
@@ -0,0 +1,157 @@
+// Copyright 2021 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 seccheck
+
+import (
+	"errors"
+	"testing"
+
+	"gvisor.dev/gvisor/pkg/context"
+)
+
+type testChecker struct {
+	CheckerDefaults
+
+	onClone func(ctx context.Context, mask CloneFieldSet, info CloneInfo) error
+}
+
+// Clone implements Checker.Clone.
+func (c *testChecker) Clone(ctx context.Context, mask CloneFieldSet, info CloneInfo) error {
+	if c.onClone == nil {
+		return nil
+	}
+	return c.onClone(ctx, mask, info)
+}
+
+func TestNoChecker(t *testing.T) {
+	var s state
+	if s.Enabled(PointClone) {
+		t.Errorf("Enabled(PointClone): got true, wanted false")
+	}
+}
+
+func TestCheckerNotRegisteredForPoint(t *testing.T) {
+	var s state
+	s.AppendChecker(&testChecker{}, &CheckerReq{})
+	if s.Enabled(PointClone) {
+		t.Errorf("Enabled(PointClone): got true, wanted false")
+	}
+}
+
+func TestCheckerRegistered(t *testing.T) {
+	var s state
+	checkerCalled := false
+	s.AppendChecker(&testChecker{onClone: func(ctx context.Context, mask CloneFieldSet, info CloneInfo) error {
+		checkerCalled = true
+		return nil
+	}}, &CheckerReq{
+		Points: []Point{PointClone},
+		Clone: CloneFields{
+			Credentials: true,
+		},
+	})
+
+	if !s.Enabled(PointClone) {
+		t.Errorf("Enabled(PointClone): got false, wanted true")
+	}
+	if !s.CloneReq().Contains(CloneFieldCredentials) {
+		t.Errorf("CloneReq().Contains(CloneFieldCredentials): got false, wanted true")
+	}
+	if err := s.Clone(context.Background(), CloneFieldSet{}, &CloneInfo{}); err != nil {
+		t.Errorf("Clone(): got %v, wanted nil", err)
+	}
+	if !checkerCalled {
+		t.Errorf("Clone() did not call Checker.Clone()")
+	}
+}
+
+func TestMultipleCheckersRegistered(t *testing.T) {
+	var s state
+	checkersCalled := [2]bool{}
+	s.AppendChecker(&testChecker{onClone: func(ctx context.Context, mask CloneFieldSet, info CloneInfo) error {
+		checkersCalled[0] = true
+		return nil
+	}}, &CheckerReq{
+		Points: []Point{PointClone},
+		Clone: CloneFields{
+			Args: true,
+		},
+	})
+	s.AppendChecker(&testChecker{onClone: func(ctx context.Context, mask CloneFieldSet, info CloneInfo) error {
+		checkersCalled[1] = true
+		return nil
+	}}, &CheckerReq{
+		Points: []Point{PointClone},
+		Clone: CloneFields{
+			Created: TaskFields{
+				ThreadID: true,
+			},
+		},
+	})
+
+	if !s.Enabled(PointClone) {
+		t.Errorf("Enabled(PointClone): got false, wanted true")
+	}
+	// CloneReq() should return the union of requested fields from all calls to
+	// AppendChecker.
+	req := s.CloneReq()
+	if !req.Contains(CloneFieldArgs) {
+		t.Errorf("req.Contains(CloneFieldArgs): got false, wanted true")
+	}
+	if !req.Created.Contains(TaskFieldThreadID) {
+		t.Errorf("req.Created.Contains(TaskFieldThreadID): got false, wanted true")
+	}
+	if err := s.Clone(context.Background(), CloneFieldSet{}, &CloneInfo{}); err != nil {
+		t.Errorf("Clone(): got %v, wanted nil", err)
+	}
+	for i := range checkersCalled {
+		if !checkersCalled[i] {
+			t.Errorf("Clone() did not call Checker.Clone() index %d", i)
+		}
+	}
+}
+
+func TestCheckpointReturnsFirstCheckerError(t *testing.T) {
+	errFirstChecker := errors.New("first Checker error")
+	errSecondChecker := errors.New("second Checker error")
+
+	var s state
+	checkersCalled := [2]bool{}
+	s.AppendChecker(&testChecker{onClone: func(ctx context.Context, mask CloneFieldSet, info CloneInfo) error {
+		checkersCalled[0] = true
+		return errFirstChecker
+	}}, &CheckerReq{
+		Points: []Point{PointClone},
+	})
+	s.AppendChecker(&testChecker{onClone: func(ctx context.Context, mask CloneFieldSet, info CloneInfo) error {
+		checkersCalled[1] = true
+		return errSecondChecker
+	}}, &CheckerReq{
+		Points: []Point{PointClone},
+	})
+
+	if !s.Enabled(PointClone) {
+		t.Errorf("Enabled(PointClone): got false, wanted true")
+	}
+	if err := s.Clone(context.Background(), CloneFieldSet{}, &CloneInfo{}); err != errFirstChecker {
+		t.Errorf("Clone(): got %v, wanted %v", err, errFirstChecker)
+	}
+	if !checkersCalled[0] {
+		t.Errorf("Clone() did not call first Checker")
+	}
+	if checkersCalled[1] {
+		t.Errorf("Clone() called second Checker")
+	}
+}
@@ -0,0 +1,39 @@
+// Copyright 2021 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 seccheck
+
+import (
+	ktime "gvisor.dev/gvisor/pkg/sentry/kernel/time"
+)
+
+// TaskInfo contains information unambiguously identifying a single thread
+// and/or its containing process.
+//
+// +fieldenum Task
+type TaskInfo struct {
+	// ThreadID is the thread's ID in the root PID namespace.
+	ThreadID int32
+
+	// ThreadStartTime is the thread's CLOCK_REALTIME start time.
+	ThreadStartTime ktime.Time
+
+	// ThreadGroupID is the thread's group leader's ID in the root PID
+	// namespace.
+	ThreadGroupID int32
+
+	// ThreadGroupStartTime is the thread's group leader's CLOCK_REALTIME start
+	// time.
+	ThreadGroupStartTime ktime.Time
+}
@@ -0,0 +1,15 @@
+load("//tools:defs.bzl", "bzl_library", "go_binary")
+
+licenses(["notice"])
+
+go_binary(
+    name = "fieldenum",
+    srcs = ["main.go"],
+    visibility = ["//:sandbox"],
+)
+
+bzl_library(
+    name = "defs_bzl",
+    srcs = ["defs.bzl"],
+    visibility = ["//visibility:private"],
+)
@@ -0,0 +1,29 @@
+"""The go_fieldenum target infers Field, Fields, and FieldSet types for each
+struct in an input source file marked +fieldenum.
+"""
+
+def _go_fieldenum_impl(ctx):
+    output = ctx.outputs.out
+
+    args = ["-pkg=%s" % ctx.attr.package, "-out=%s" % output.path]
+    for src in ctx.attr.srcs:
+        args += [f.path for f in src.files.to_list()]
+
+    ctx.actions.run(
+        inputs = ctx.files.srcs,
+        outputs = [output],
+        mnemonic = "GoFieldenum",
+        progress_message = "Generating Go field enumerators %s" % ctx.label,
+        arguments = args,
+        executable = ctx.executable._tool,
+    )
+
+go_fieldenum = rule(
+    implementation = _go_fieldenum_impl,
+    attrs = {
+        "srcs": attr.label_list(doc = "input source files", mandatory = True, allow_files = True),
+        "package": attr.string(doc = "the package for the generated source file", mandatory = True),
+        "out": attr.output(doc = "output file", mandatory = True),
+        "_tool": attr.label(executable = True, cfg = "host", default = Label("//tools/go_fieldenum:fieldenum")),
+    },
+)
@@ -0,0 +1,310 @@
+// Copyright 2021 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.
+
+// Binary fieldenum emits field bitmasks for all structs in a package marked
+// "+fieldenum".
+package main
+
+import (
+	"flag"
+	"fmt"
+	"go/ast"
+	"go/parser"
+	"go/token"
+	"log"
+	"os"
+	"strings"
+)
+
+var (
+	outputPkg      = flag.String("pkg", "", "output package")
+	outputFilename = flag.String("out", "-", "output filename")
+)
+
+func main() {
+	// Parse command line arguments.
+	flag.Parse()
+	if len(*outputPkg) == 0 {
+		log.Fatalf("-pkg must be provided")
+	}
+	if len(flag.Args()) == 0 {
+		log.Fatalf("Input files must be provided")
+	}
+
+	// Parse input files.
+	inputFiles := make([]*ast.File, 0, len(flag.Args()))
+	fset := token.NewFileSet()
+	for _, filename := range flag.Args() {
+		f, err := parser.ParseFile(fset, filename, nil, parser.ParseComments)
+		if err != nil {
+			log.Fatalf("Failed to parse input file %q: %v", filename, err)
+		}
+		inputFiles = append(inputFiles, f)
+	}
+
+	// Determine which types are marked "+fieldenum" and will consequently have
+	// code generated.
+	fieldEnumTypes := make(map[string]fieldEnumTypeInfo)
+	for _, f := range inputFiles {
+		for _, decl := range f.Decls {
+			d, ok := decl.(*ast.GenDecl)
+			if !ok || d.Tok != token.TYPE || d.Doc == nil || len(d.Specs) == 0 {
+				continue
+			}
+			for _, l := range d.Doc.List {
+				const fieldenumPrefixWithSpace = "// +fieldenum "
+				if l.Text == "// +fieldenum" || strings.HasPrefix(l.Text, fieldenumPrefixWithSpace) {
+					spec := d.Specs[0].(*ast.TypeSpec)
+					name := spec.Name.Name
+					prefix := name
+					if len(l.Text) > len(fieldenumPrefixWithSpace) {
+						prefix = strings.TrimSpace(l.Text[len(fieldenumPrefixWithSpace):])
+					}
+					st, ok := spec.Type.(*ast.StructType)
+					if !ok {
+						log.Fatalf("Type %s is marked +fieldenum, but is not a struct", name)
+					}
+					fieldEnumTypes[name] = fieldEnumTypeInfo{
+						prefix:     prefix,
+						structType: st,
+					}
+					break
+				}
+			}
+		}
+	}
+
+	// Collect information for each type for which code is being generated.
+	structInfos := make([]structInfo, 0, len(fieldEnumTypes))
+	needSyncAtomic := false
+	for typeName, typeInfo := range fieldEnumTypes {
+		var si structInfo
+		si.name = typeName
+		si.prefix = typeInfo.prefix
+		for _, field := range typeInfo.structType.Fields.List {
+			name := structFieldName(field)
+			// If the field's type is a type that is also marked +fieldenum,
+			// include a FieldSet for that type in this one's. The field must
+			// be a struct by value, since if it's a pointer then that struct
+			// might also point to or include this one (which would make
+			// FieldSet inclusion circular). It must also be a type defined in
+			// this package, since otherwise we don't know whether it's marked
+			// +fieldenum. Thus, field.Type must be an identifier (rather than
+			// an ast.StarExpr or SelectorExpr).
+			if tident, ok := field.Type.(*ast.Ident); ok {
+				if fieldTypeInfo, ok := fieldEnumTypes[tident.Name]; ok {
+					fsf := fieldSetField{
+						fieldName:  name,
+						typePrefix: fieldTypeInfo.prefix,
+					}
+					si.reprByFieldSet = append(si.reprByFieldSet, fsf)
+					si.allFields = append(si.allFields, fsf)
+					continue
+				}
+			}
+			si.reprByBit = append(si.reprByBit, name)
+			si.allFields = append(si.allFields, fieldSetField{
+				fieldName: name,
+			})
+			// sync/atomic import will be needed for FieldSet.Load().
+			needSyncAtomic = true
+		}
+		structInfos = append(structInfos, si)
+	}
+
+	// Build the output file.
+	var b strings.Builder
+	fmt.Fprintf(&b, "// Generated by go_fieldenum.\n\n")
+	fmt.Fprintf(&b, "package %s\n\n", *outputPkg)
+	if needSyncAtomic {
+		fmt.Fprintf(&b, "import \"sync/atomic\"\n\n")
+	}
+	for _, si := range structInfos {
+		si.writeTo(&b)
+	}
+
+	if *outputFilename == "-" {
+		// Write output to stdout.
+		fmt.Printf("%s", b.String())
+	} else {
+		// Write output to file.
+		f, err := os.OpenFile(*outputFilename, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0644)
+		if err != nil {
+			log.Fatalf("Failed to open output file %q: %v", *outputFilename, err)
+		}
+		if _, err := f.WriteString(b.String()); err != nil {
+			log.Fatalf("Failed to write output file %q: %v", *outputFilename, err)
+		}
+		f.Close()
+	}
+}
+
+type fieldEnumTypeInfo struct {
+	prefix     string
+	structType *ast.StructType
+}
+
+// structInfo contains information about the code generated for a given struct.
+type structInfo struct {
+	// name is the name of the represented struct.
+	name string
+
+	// prefix is the prefix X applied to the name of each generated type and
+	// constant, referred to as X in the comments below for convenience.
+	prefix string
+
+	// reprByBit contains the names of fields in X that should be represented
+	// by a bit in the bit mask XFieldSet.fields, and by a bool in XFields.
+	reprByBit []string
+
+	// reprByFieldSet contains fields in X whose type is a named struct (e.g.
+	// Y) that has a corresponding FieldSet type YFieldSet, and which should
+	// therefore be represented by including a value of type YFieldSet in
+	// XFieldSet, and a value of type YFields in XFields.
+	reprByFieldSet []fieldSetField
+
+	// allFields contains all fields in X in order of declaration. Fields in
+	// reprByBit have fieldSetField.typePrefix == "".
+	allFields []fieldSetField
+}
+
+type fieldSetField struct {
+	fieldName  string
+	typePrefix string
+}
+
+func structFieldName(f *ast.Field) string {
+	if len(f.Names) != 0 {
+		return f.Names[0].Name
+	}
+	// For embedded struct fields, the field name is the unqualified type name.
+	texpr := f.Type
+	for {
+		switch t := texpr.(type) {
+		case *ast.StarExpr:
+			texpr = t.X
+		case *ast.SelectorExpr:
+			texpr = t.Sel
+		case *ast.Ident:
+			return t.Name
+		default:
+			panic(fmt.Sprintf("unexpected %T", texpr))
+		}
+	}
+}
+
+// Workaround for Go defect (map membership test isn't usable in an
+// expression).
+func fetContains(xs map[string]*ast.StructType, x string) bool {
+	_, ok := xs[x]
+	return ok
+}
+
+func (si *structInfo) writeTo(b *strings.Builder) {
+	fmt.Fprintf(b, "// A %sField represents a field in %s.\n", si.prefix, si.name)
+	fmt.Fprintf(b, "type %sField uint\n\n", si.prefix)
+	if len(si.reprByBit) != 0 {
+		fmt.Fprintf(b, "// %sFieldX represents %s field X.\n", si.prefix, si.name)
+		fmt.Fprintf(b, "const (\n")
+		fmt.Fprintf(b, "\t%sField%s %sField = iota\n", si.prefix, si.reprByBit[0], si.prefix)
+		for _, fieldName := range si.reprByBit[1:] {
+			fmt.Fprintf(b, "\t%sField%s\n", si.prefix, fieldName)
+		}
+		fmt.Fprintf(b, ")\n\n")
+	}
+
+	fmt.Fprintf(b, "// %sFields represents a set of fields in %s in a literal-friendly form.\n", si.prefix, si.name)
+	fmt.Fprintf(b, "// The zero value of %sFields represents an empty set.\n", si.prefix)
+	fmt.Fprintf(b, "type %sFields struct {\n", si.prefix)
+	for _, fieldSetField := range si.allFields {
+		if fieldSetField.typePrefix == "" {
+			fmt.Fprintf(b, "\t%s bool\n", fieldSetField.fieldName)
+		} else {
+			fmt.Fprintf(b, "\t%s %sFields\n", fieldSetField.fieldName, fieldSetField.typePrefix)
+		}
+	}
+	fmt.Fprintf(b, "}\n\n")
+
+	fmt.Fprintf(b, "// %sFieldSet represents a set of fields in %s in a compact form.\n", si.prefix, si.name)
+	fmt.Fprintf(b, "// The zero value of %sFieldSet represents an empty set.\n", si.prefix)
+	fmt.Fprintf(b, "type %sFieldSet struct {\n", si.prefix)
+	numBitmaskUint32s := (len(si.reprByBit) + 31) / 32
+	for _, fieldSetField := range si.reprByFieldSet {
+		fmt.Fprintf(b, "\t%s %sFieldSet\n", fieldSetField.fieldName, fieldSetField.typePrefix)
+	}
+	if len(si.reprByBit) != 0 {
+		fmt.Fprintf(b, "\tfields [%d]uint32\n", numBitmaskUint32s)
+	}
+	fmt.Fprintf(b, "}\n\n")
+
+	if len(si.reprByBit) != 0 {
+		fmt.Fprintf(b, "// Contains returns true if f is present in the %sFieldSet.\n", si.prefix)
+		fmt.Fprintf(b, "func (fs %sFieldSet) Contains(f %sField) bool {\n", si.prefix, si.prefix)
+		if numBitmaskUint32s == 1 {
+			fmt.Fprintf(b, "\treturn fs.fields[0] & (uint32(1) << uint(f)) != 0\n")
+		} else {
+			fmt.Fprintf(b, "\treturn fs.fields[f/32] & (uint32(1) << (f%%32)) != 0\n")
+		}
+		fmt.Fprintf(b, "}\n\n")
+
+		fmt.Fprintf(b, "// Add adds f to the %sFieldSet.\n", si.prefix)
+		fmt.Fprintf(b, "func (fs *%sFieldSet) Add(f %sField) {\n", si.prefix, si.prefix)
+		if numBitmaskUint32s == 1 {
+			fmt.Fprintf(b, "\tfs.fields[0] |= uint32(1) << uint(f)\n")
+		} else {
+			fmt.Fprintf(b, "\tfs.fields[f/32] |= uint32(1) << (f%%32)\n")
+		}
+		fmt.Fprintf(b, "}\n\n")
+
+		fmt.Fprintf(b, "// Remove removes f from the %sFieldSet.\n", si.prefix)
+		fmt.Fprintf(b, "func (fs *%sFieldSet) Remove(f %sField) {\n", si.prefix, si.prefix)
+		if numBitmaskUint32s == 1 {
+			fmt.Fprintf(b, "\tfs.fields[0] &^= uint32(1) << uint(f)\n")
+		} else {
+			fmt.Fprintf(b, "\tfs.fields[f/32] &^= uint32(1) << (f%%32)\n")
+		}
+		fmt.Fprintf(b, "}\n\n")
+	}
+
+	fmt.Fprintf(b, "// Load returns a copy of the %sFieldSet.\n", si.prefix)
+	fmt.Fprintf(b, "// Load is safe to call concurrently with AddFieldsLoadable, but not Add or Remove.\n")
+	fmt.Fprintf(b, "func (fs *%sFieldSet) Load() (copied %sFieldSet) {\n", si.prefix, si.prefix)
+	for _, fieldSetField := range si.reprByFieldSet {
+		fmt.Fprintf(b, "\tcopied.%s = fs.%s.Load()\n", fieldSetField.fieldName, fieldSetField.fieldName)
+	}
+	for i := 0; i < numBitmaskUint32s; i++ {
+		fmt.Fprintf(b, "\tcopied.fields[%d] = atomic.LoadUint32(&fs.fields[%d])\n", i, i)
+	}
+	fmt.Fprintf(b, "\treturn\n")
+	fmt.Fprintf(b, "}\n\n")
+
+	fmt.Fprintf(b, "// AddFieldsLoadable adds the given fields to the %sFieldSet.\n", si.prefix)
+	fmt.Fprintf(b, "// AddFieldsLoadable is safe to call concurrently with Load, but not other methods (including other calls to AddFieldsLoadable).\n")
+	fmt.Fprintf(b, "func (fs *%sFieldSet) AddFieldsLoadable(fields %sFields) {\n", si.prefix, si.prefix)
+	for _, fieldSetField := range si.reprByFieldSet {
+		fmt.Fprintf(b, "\tfs.%s.AddFieldsLoadable(fields.%s)\n", fieldSetField.fieldName, fieldSetField.fieldName)
+	}
+	for _, fieldName := range si.reprByBit {
+		fieldConstName := fmt.Sprintf("%sField%s", si.prefix, fieldName)
+		fmt.Fprintf(b, "\tif fields.%s {\n", fieldName)
+		if numBitmaskUint32s == 1 {
+			fmt.Fprintf(b, "\t\tatomic.StoreUint32(&fs.fields[0], fs.fields[0] | (uint32(1) << uint(%s)))\n", fieldConstName)
+		} else {
+			fmt.Fprintf(b, "\t\tword, bit := %s/32, %s%%32\n", fieldConstName, fieldConstName)
+			fmt.Fprintf(b, "\t\tatomic.StoreUint32(&fs.fields[word], fs.fields[word] | (uint32(1) << bit))\n")
+		}
+		fmt.Fprintf(b, "\t}\n")
+	}
+	fmt.Fprintf(b, "}\n\n")
+}