container: improve documentation

This change removes inconsistencies collected over time in this package.

Signed-off-by: Ophestra <cat@gensokyo.uk>

23 files changed, 222 insertions, 97 deletions

diff --git a/container/autoetc.go b/container/autoetc.go
index 3fd9727e..a2be68b9 100644
--- a/container/autoetc.go
+++ b/container/autoetc.go
@@ -10,8 +10,7 @@ import (
 
 func init() { gob.Register(new(AutoEtcOp)) }
 
-// Etc appends an [Op] that expands host /etc into a toplevel symlink mirror with /etc semantics.
-// This is not a generic setup op. It is implemented here to reduce ipc overhead.
+// Etc is a helper for appending [AutoEtcOp] to [Ops].
 func (f *Ops) Etc(host *check.Absolute, prefix string) *Ops {
 	e := &AutoEtcOp{prefix}
 	f.Mkdir(fhs.AbsEtc, 0755)
@@ -20,6 +19,9 @@ func (f *Ops) Etc(host *check.Absolute, prefix string) *Ops {
 	return f
 }
 
+// AutoEtcOp expands host /etc into a toplevel symlink mirror with /etc semantics.
+//
+// This is not a generic setup op. It is implemented here to reduce ipc overhead.
 type AutoEtcOp struct{ Prefix string }
 
 func (e *AutoEtcOp) Valid() bool                                { return e != nil }
diff --git a/container/autoroot.go b/container/autoroot.go
index 09e059d9..82ca9e29 100644
--- a/container/autoroot.go
+++ b/container/autoroot.go
@@ -11,13 +11,15 @@ import (
 
 func init() { gob.Register(new(AutoRootOp)) }
 
-// Root appends an [Op] that expands a directory into a toplevel bind mount mirror on container root.
-// This is not a generic setup op. It is implemented here to reduce ipc overhead.
+// Root is a helper for appending [AutoRootOp] to [Ops].
 func (f *Ops) Root(host *check.Absolute, flags int) *Ops {
 	*f = append(*f, &AutoRootOp{host, flags, nil})
 	return f
 }
 
+// AutoRootOp expands a directory into a toplevel bind mount mirror on container root.
+//
+// This is not a generic setup op. It is implemented here to reduce ipc overhead.
 type AutoRootOp struct {
 	Host *check.Absolute
 	// passed through to bindMount
diff --git a/container/capability.go b/container/capability.go
index 9f526e65..3ea9d028 100644
--- a/container/capability.go
+++ b/container/capability.go
@@ -50,10 +50,16 @@ func capset(hdrp *capHeader, datap *[2]capData) error {
 }
 
 // capBoundingSetDrop drops a capability from the calling thread's capability bounding set.
-func capBoundingSetDrop(cap uintptr) error { return Prctl(syscall.PR_CAPBSET_DROP, cap, 0) }
+func capBoundingSetDrop(cap uintptr) error {
+	return Prctl(syscall.PR_CAPBSET_DROP, cap, 0)
+}
 
 // capAmbientClearAll clears the ambient capability set of the calling thread.
-func capAmbientClearAll() error { return Prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_CLEAR_ALL, 0) }
+func capAmbientClearAll() error {
+	return Prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_CLEAR_ALL, 0)
+}
 
 // capAmbientRaise adds to the ambient capability set of the calling thread.
-func capAmbientRaise(cap uintptr) error { return Prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, cap) }
+func capAmbientRaise(cap uintptr) error {
+	return Prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, cap)
+}
diff --git a/container/check/overlay.go b/container/check/overlay.go
index ab907fbd..fbbcd1e6 100644
--- a/container/check/overlay.go
+++ b/container/check/overlay.go
@@ -11,7 +11,8 @@ const (
 	SpecialOverlayPath = ":"
 )
 
-// EscapeOverlayDataSegment escapes a string for formatting into the data argument of an overlay mount call.
+// EscapeOverlayDataSegment escapes a string for formatting into the data
+// argument of an overlay mount system call.
 func EscapeOverlayDataSegment(s string) string {
 	if s == "" {
 		return ""
diff --git a/container/container.go b/container/container.go
index 2a6363a5..a6457714 100644
--- a/container/container.go
+++ b/container/container.go
@@ -1,4 +1,5 @@
-// Package container implements unprivileged Linux containers with built-in support for syscall filtering.
+// Package container implements unprivileged Linux containers with built-in
+// support for syscall filtering.
 package container
 
 import (
@@ -42,22 +43,25 @@ type (
 		SchedPolicy int
 		// Cgroup fd, nil to disable.
 		Cgroup *int
-		// ExtraFiles passed through to initial process in the container,
-		// with behaviour identical to its [exec.Cmd] counterpart.
+		// ExtraFiles passed through to initial process in the container, with
+		// behaviour identical to its [exec.Cmd] counterpart.
 		ExtraFiles []*os.File
 
-		// param pipe for shim and init
+		// Write end of a pipe connected to the init to deliver [Params].
 		setup *os.File
-		// cancels cmd
+		// Cancels the context passed to the underlying cmd.
 		cancel context.CancelFunc
-		// closed after Wait returns
+		// Closed after Wait returns. Keeps the spawning thread alive.
 		wait chan struct{}
 
 		Stdin  io.Reader
 		Stdout io.Writer
 		Stderr io.Writer
 
-		Cancel    func(cmd *exec.Cmd) error
+		// Custom cancellation behaviour for the underlying [exec.Cmd]. Must
+		// deliver [CancelSignal] before returning.
+		Cancel func(cmd *exec.Cmd) error
+		// Copied to the underlying [exec.Cmd].
 		WaitDelay time.Duration
 
 		cmd *exec.Cmd
@@ -286,7 +290,11 @@ func (p *Container) Start() error {
 
 	// place setup pipe before user supplied extra files, this is later restored by init
 	if fd, f, err := Setup(&p.cmd.ExtraFiles); err != nil {
-		return &StartError{true, "set up params stream", err, false, false}
+		return &StartError{
+			Fatal: true,
+			Step:  "set up params stream",
+			Err:   err,
+		}
 	} else {
 		p.setup = f
 		p.cmd.Env = []string{setupEnv + "=" + strconv.Itoa(fd)}
@@ -298,10 +306,16 @@ func (p *Container) Start() error {
 		runtime.LockOSThread()
 		p.wait = make(chan struct{})
 
-		done <- func() error { // setup depending on per-thread state must happen here
-			// PR_SET_NO_NEW_PRIVS: depends on per-thread state but acts on all processes created from that thread
+		// setup depending on per-thread state must happen here
+		done <- func() error {
+			// PR_SET_NO_NEW_PRIVS: thread-directed but acts on all processes
+			// created from the calling thread
 			if err := SetNoNewPrivs(); err != nil {
-				return &StartError{true, "prctl(PR_SET_NO_NEW_PRIVS)", err, false, false}
+				return &StartError{
+					Fatal: true,
+					Step:  "prctl(PR_SET_NO_NEW_PRIVS)",
+					Err:   err,
+				}
 			}
 
 			// landlock: depends on per-thread state but acts on a process group
@@ -313,28 +327,40 @@ func (p *Container) Start() error {
 
 				if abi, err := LandlockGetABI(); err != nil {
 					if p.HostAbstract {
-						// landlock can be skipped here as it restricts access to resources
-						// already covered by namespaces (pid)
+						// landlock can be skipped here as it restricts access
+						// to resources already covered by namespaces (pid)
 						goto landlockOut
 					}
-					return &StartError{false, "get landlock ABI", err, false, false}
+					return &StartError{Step: "get landlock ABI", Err: err}
 				} else if abi < 6 {
 					if p.HostAbstract {
 						// see above comment
 						goto landlockOut
 					}
-					return &StartError{false, "kernel version too old for LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET", ENOSYS, true, false}
+					return &StartError{
+						Step:   "kernel too old for LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET",
+						Err:    ENOSYS,
+						Origin: true,
+					}
 				} else {
 					p.msg.Verbosef("landlock abi version %d", abi)
 				}
 
 				if rulesetFd, err := rulesetAttr.Create(0); err != nil {
-					return &StartError{true, "create landlock ruleset", err, false, false}
+					return &StartError{
+						Fatal: true,
+						Step:  "create landlock ruleset",
+						Err:   err,
+					}
 				} else {
 					p.msg.Verbosef("enforcing landlock ruleset %s", rulesetAttr)
 					if err = LandlockRestrictSelf(rulesetFd, 0); err != nil {
 						_ = Close(rulesetFd)
-						return &StartError{true, "enforce landlock ruleset", err, false, false}
+						return &StartError{
+							Fatal: true,
+							Step:  "enforce landlock ruleset",
+							Err:   err,
+						}
 					}
 					if err = Close(rulesetFd); err != nil {
 						p.msg.Verbosef("cannot close landlock ruleset: %v", err)
@@ -346,7 +372,7 @@ func (p *Container) Start() error {
 			}
 
 			// sched_setscheduler: thread-directed but acts on all processes
-			// created from that thread
+			// created from the calling thread
 			if p.SchedPolicy > 0 {
 				p.msg.Verbosef("setting scheduling policy %d", p.SchedPolicy)
 				if err := schedSetscheduler(
@@ -364,7 +390,11 @@ func (p *Container) Start() error {
 
 			p.msg.Verbose("starting container init")
 			if err := p.cmd.Start(); err != nil {
-				return &StartError{false, "start container init", err, false, true}
+				return &StartError{
+					Step:        "start container init",
+					Err:         err,
+					Passthrough: true,
+				}
 			}
 			return nil
 		}()
@@ -376,6 +406,7 @@ func (p *Container) Start() error {
 }
 
 // Serve serves [Container.Params] to the container init.
+//
 // Serve must only be called once.
 func (p *Container) Serve() error {
 	if p.setup == nil {
@@ -385,12 +416,21 @@ func (p *Container) Serve() error {
 	setup := p.setup
 	p.setup = nil
 	if err := setup.SetDeadline(time.Now().Add(initSetupTimeout)); err != nil {
-		return &StartError{true, "set init pipe deadline", err, false, true}
+		return &StartError{
+			Fatal:       true,
+			Step:        "set init pipe deadline",
+			Err:         err,
+			Passthrough: true,
+		}
 	}
 
 	if p.Path == nil {
 		p.cancel()
-		return &StartError{false, "invalid executable pathname", EINVAL, true, false}
+		return &StartError{
+			Step:   "invalid executable pathname",
+			Err:    EINVAL,
+			Origin: true,
+		}
 	}
 
 	// do not transmit nil
@@ -415,7 +455,8 @@ func (p *Container) Serve() error {
 	return err
 }
 
-// Wait waits for the container init process to exit and releases any resources associated with the [Container].
+// Wait blocks until the container init process to exit and releases any
+// resources associated with the [Container].
 func (p *Container) Wait() error {
 	if p.cmd == nil || p.cmd.Process == nil {
 		return EINVAL
@@ -460,11 +501,13 @@ func (p *Container) StderrPipe() (r io.ReadCloser, err error) {
 }
 
 func (p *Container) String() string {
-	return fmt.Sprintf("argv: %q, filter: %v, rules: %d, flags: %#x, presets: %#x",
-		p.Args, !p.SeccompDisable, len(p.SeccompRules), int(p.SeccompFlags), int(p.SeccompPresets))
+	return fmt.Sprintf(
+		"argv: %q, filter: %v, rules: %d, flags: %#x, presets: %#x",
+		p.Args, !p.SeccompDisable, len(p.SeccompRules), int(p.SeccompFlags), int(p.SeccompPresets),
+	)
 }
 
-// ProcessState returns the address to os.ProcessState held by the underlying [exec.Cmd].
+// ProcessState returns the address of os.ProcessState held by the underlying [exec.Cmd].
 func (p *Container) ProcessState() *os.ProcessState {
 	if p.cmd == nil {
 		return nil
@@ -472,7 +515,8 @@ func (p *Container) ProcessState() *os.ProcessState {
 	return p.cmd.ProcessState
 }
 
-// New returns the address to a new instance of [Container] that requires further initialisation before use.
+// New returns the address to a new instance of [Container]. This value requires
+// further initialisation before use.
 func New(ctx context.Context, msg message.Msg) *Container {
 	if msg == nil {
 		msg = message.New(nil)
@@ -486,7 +530,13 @@ func New(ctx context.Context, msg message.Msg) *Container {
 }
 
 // NewCommand calls [New] and initialises the [Params.Path] and [Params.Args] fields.
-func NewCommand(ctx context.Context, msg message.Msg, pathname *check.Absolute, name string, args ...string) *Container {
+func NewCommand(
+	ctx context.Context,
+	msg message.Msg,
+	pathname *check.Absolute,
+	name string,
+	args ...string,
+) *Container {
 	z := New(ctx, msg)
 	z.Path = pathname
 	z.Args = append([]string{name}, args...)
diff --git a/container/dispatcher.go b/container/dispatcher.go
index 7231d56d..23786b5b 100644
--- a/container/dispatcher.go
+++ b/container/dispatcher.go
@@ -21,7 +21,8 @@ type osFile interface {
 	fs.File
 }
 
-// syscallDispatcher provides methods that make state-dependent system calls as part of their behaviour.
+// syscallDispatcher provides methods that make state-dependent system calls as
+// part of their behaviour.
 type syscallDispatcher interface {
 	// new starts a goroutine with a new instance of syscallDispatcher.
 	// A syscallDispatcher must never be used in any goroutine other than the one owning it,
diff --git a/container/errors.go b/container/errors.go
index d7651aa9..150dcd13 100644
--- a/container/errors.go
+++ b/container/errors.go
@@ -43,7 +43,8 @@ func messageFromError(err error) (m string, ok bool) {
 }
 
 // messagePrefix checks and prefixes the error message of a non-pointer error.
-// While this is usable for pointer errors, such use should be avoided as nil check is omitted.
+// While this is usable for pointer errors, such use should be avoided as nil
+// check is omitted.
 func messagePrefix[T error](prefix string, err error) (string, bool) {
 	var targetError T
 	if errors.As(err, &targetError) {
diff --git a/container/fhs/fhs.go b/container/fhs/fhs.go
index 7e0d36c7..e0c1d861 100644
--- a/container/fhs/fhs.go
+++ b/container/fhs/fhs.go
@@ -9,7 +9,8 @@ const (
 	// Tmp points to the place for small temporary files.
 	Tmp = "/tmp/"
 
-	// Run points to a "tmpfs" file system for system packages to place runtime data, socket files, and similar.
+	// Run points to a "tmpfs" file system for system packages to place runtime
+	// data, socket files, and similar.
 	Run = "/run/"
 	// RunUser points to a directory containing per-user runtime directories,
 	// each usually individually mounted "tmpfs" instances.
@@ -17,10 +18,12 @@ const (
 
 	// Usr points to vendor-supplied operating system resources.
 	Usr = "/usr/"
-	// UsrBin points to binaries and executables for user commands that shall appear in the $PATH search path.
+	// UsrBin points to binaries and executables for user commands that shall
+	// appear in the $PATH search path.
 	UsrBin = Usr + "bin/"
 
-	// Var points to persistent, variable system data. Writable during normal system operation.
+	// Var points to persistent, variable system data. Writable during normal
+	// system operation.
 	Var = "/var/"
 	// VarLib points to persistent system data.
 	VarLib = Var + "lib/"
@@ -29,12 +32,16 @@ const (
 
 	// Dev points to the root directory for device nodes.
 	Dev = "/dev/"
-	// DevShm is the place for POSIX shared memory segments, as created via shm_open(3).
+	// DevShm is the place for POSIX shared memory segments, as created via
+	// shm_open(3).
 	DevShm = "/dev/shm/"
-	// Proc points to a virtual kernel file system exposing the process list and other functionality.
+	// Proc points to a virtual kernel file system exposing the process list and
+	// other functionality.
 	Proc = "/proc/"
-	// ProcSys points to a hierarchy below /proc/ that exposes a number of kernel tunables.
+	// ProcSys points to a hierarchy below /proc/ that exposes a number of
+	// kernel tunables.
 	ProcSys = Proc + "sys/"
-	// Sys points to a virtual kernel file system exposing discovered devices and other functionality.
+	// Sys points to a virtual kernel file system exposing discovered devices
+	// and other functionality.
 	Sys = "/sys/"
 )
diff --git a/container/init.go b/container/init.go
index 5516c422..8fbf757e 100644
--- a/container/init.go
+++ b/container/init.go
@@ -33,12 +33,12 @@ const (
 	- This path is only accessible by init and root:
 	  The container init sets SUID_DUMP_DISABLE and terminates if that fails.
 
-	It should be noted that none of this should become relevant at any point since the resulting
-	intermediate root tmpfs should be effectively anonymous. */
+	It should be noted that none of this should become relevant at any point
+	since the resulting intermediate root tmpfs should be effectively anonymous. */
 	intermediateHostPath = fhs.Proc + "self/fd"
 
-	// setupEnv is the name of the environment variable holding the string representation of
-	// the read end file descriptor of the setup params pipe.
+	// setupEnv is the name of the environment variable holding the string
+	// representation of the read end file descriptor of the setup params pipe.
 	setupEnv = "HAKUREI_SETUP"
 
 	// exitUnexpectedWait4 is the exit code if wait4 returns an unexpected errno.
@@ -59,7 +59,8 @@ type (
 		// late is called right before starting the initial process.
 		late(state *setupState, k syscallDispatcher) error
 
-		// prefix returns a log message prefix, and whether this Op prints no identifying message on its own.
+		// prefix returns a log message prefix, and whether this Op prints no
+		// identifying message on its own.
 		prefix() (string, bool)
 
 		Is(op Op) bool
@@ -71,9 +72,11 @@ type (
 	setupState struct {
 		nonrepeatable uintptr
 
-		// Whether early reaping has concluded. Must only be accessed in the wait4 loop.
+		// Whether early reaping has concluded. Must only be accessed in the
+		// wait4 loop.
 		processConcluded bool
-		// Process to syscall.WaitStatus populated in the wait4 loop. Freed after early reaping concludes.
+		// Process to syscall.WaitStatus populated in the wait4 loop. Freed
+		// after early reaping concludes.
 		process map[int]WaitStatus
 		// Synchronises access to process.
 		processMu sync.RWMutex
@@ -216,9 +219,10 @@ func initEntrypoint(k syscallDispatcher, msg message.Msg) {
 	defer cancel()
 
 	/* early is called right before pivot_root into intermediate root;
-	this step is mostly for gathering information that would otherwise be difficult to obtain
-	via library functions after pivot_root, and implementations are expected to avoid changing
-	the state of the mount namespace */
+	this step is mostly for gathering information that would otherwise be
+	difficult to obtain via library functions after pivot_root, and
+	implementations are expected to avoid changing the state of the mount
+	namespace */
 	for i, op := range *params.Ops {
 		if op == nil || !op.Valid() {
 			k.fatalf(msg, "invalid op at index %d", i)
@@ -258,10 +262,10 @@ func initEntrypoint(k syscallDispatcher, msg message.Msg) {
 		k.fatalf(msg, "cannot enter intermediate root: %v", err)
 	}
 
-	/* apply is called right after pivot_root and entering the new root;
-	this step sets up the container filesystem, and implementations are expected to keep the host root
-	and sysroot mount points intact but otherwise can do whatever they need to;
-	chdir is allowed but discouraged */
+	/* apply is called right after pivot_root and entering the new root. This
+	step sets up the container filesystem, and implementations are expected to
+	keep the host root and sysroot mount points intact but otherwise can do
+	whatever they need to. Calling chdir is allowed but discouraged. */
 	for i, op := range *params.Ops {
 		// ops already checked during early setup
 		if prefix, ok := op.prefix(); ok {
diff --git a/container/initbind.go b/container/initbind.go
index f91d5086..7f240a37 100644
--- a/container/initbind.go
+++ b/container/initbind.go
@@ -12,14 +12,16 @@ import (
 
 func init() { gob.Register(new(BindMountOp)) }
 
-// Bind appends an [Op] that bind mounts host path [BindMountOp.Source] on container path [BindMountOp.Target].
+// Bind is a helper for appending [BindMountOp] to [Ops].
 func (f *Ops) Bind(source, target *check.Absolute, flags int) *Ops {
 	*f = append(*f, &BindMountOp{nil, source, target, flags})
 	return f
 }
 
-// BindMountOp bind mounts host path Source on container path Target.
-// Note that Flags uses bits declared in this package and should not be set with constants in [syscall].
+// BindMountOp creates a bind mount from host path Source to container path Target.
+//
+// Note that Flags uses bits declared in the [std] package and should not be set
+// with constants in [syscall].
 type BindMountOp struct {
 	sourceFinal, Source, Target *check.Absolute
 
diff --git a/container/initdaemon.go b/container/initdaemon.go
index 0e631b25..1e58e2ab 100644
--- a/container/initdaemon.go
+++ b/container/initdaemon.go
@@ -24,8 +24,7 @@ const (
 	daemonTimeout = 5 * time.Second
 )
 
-// Daemon appends an [Op] that starts a daemon in the container and blocks until
-// [DaemonOp.Target] appears.
+// Daemon is a helper for appending [DaemonOp] to [Ops].
 func (f *Ops) Daemon(target, path *check.Absolute, args ...string) *Ops {
 	*f = append(*f, &DaemonOp{target, path, args})
 	return f
diff --git a/container/initdev.go b/container/initdev.go
index 50df680b..1ce722cb 100644
--- a/container/initdev.go
+++ b/container/initdev.go
@@ -19,7 +19,9 @@ func (f *Ops) Dev(target *check.Absolute, mqueue bool) *Ops {
 }
 
 // DevWritable appends an [Op] that mounts a writable subset of host /dev.
-// There is usually no good reason to write to /dev, so this should always be followed by a [RemountOp].
+//
+// There is usually no good reason to write to /dev, so this should always be
+// followed by a [RemountOp].
 func (f *Ops) DevWritable(target *check.Absolute, mqueue bool) *Ops {
 	*f = append(*f, &MountDevOp{target, mqueue, true})
 	return f
diff --git a/container/initmkdir.go b/container/initmkdir.go
index d3c4bbb7..d43c8b8f 100644
--- a/container/initmkdir.go
+++ b/container/initmkdir.go
@@ -10,7 +10,7 @@ import (
 
 func init() { gob.Register(new(MkdirOp)) }
 
-// Mkdir appends an [Op] that creates a directory in the container filesystem.
+// Mkdir is a helper for appending [MkdirOp] to [Ops].
 func (f *Ops) Mkdir(name *check.Absolute, perm os.FileMode) *Ops {
 	*f = append(*f, &MkdirOp{name, perm})
 	return f
diff --git a/container/initoverlay.go b/container/initoverlay.go
index 0c31f8fd..2fec6364 100644
--- a/container/initoverlay.go
+++ b/container/initoverlay.go
@@ -54,8 +54,11 @@ func (e *OverlayArgumentError) Error() string {
 	}
 }
 
-// Overlay appends an [Op] that mounts the overlay pseudo filesystem on [MountOverlayOp.Target].
-func (f *Ops) Overlay(target, state, work *check.Absolute, layers ...*check.Absolute) *Ops {
+// Overlay is a helper for appending [MountOverlayOp] to [Ops].
+func (f *Ops) Overlay(
+	target, state, work *check.Absolute,
+	layers ...*check.Absolute,
+) *Ops {
 	*f = append(*f, &MountOverlayOp{
 		Target: target,
 		Lower:  layers,
@@ -65,13 +68,12 @@ func (f *Ops) Overlay(target, state, work *check.Absolute, layers ...*check.Abso
 	return f
 }
 
-// OverlayEphemeral appends an [Op] that mounts the overlay pseudo filesystem on [MountOverlayOp.Target]
-// with an ephemeral upperdir and workdir.
+// OverlayEphemeral appends a [MountOverlayOp] with an ephemeral upperdir and workdir.
 func (f *Ops) OverlayEphemeral(target *check.Absolute, layers ...*check.Absolute) *Ops {
 	return f.Overlay(target, fhs.AbsRoot, nil, layers...)
 }
 
-// OverlayReadonly appends an [Op] that mounts the overlay pseudo filesystem readonly on [MountOverlayOp.Target]
+// OverlayReadonly appends a readonly [MountOverlayOp].
 func (f *Ops) OverlayReadonly(target *check.Absolute, layers ...*check.Absolute) *Ops {
 	return f.Overlay(target, nil, nil, layers...)
 }
@@ -82,25 +84,34 @@ type MountOverlayOp struct {
 
 	// Any filesystem, does not need to be on a writable filesystem.
 	Lower []*check.Absolute
-	// formatted for [OptionOverlayLowerdir], resolved, prefixed and escaped during early
+	// Formatted for [OptionOverlayLowerdir].
+	//
+	// Resolved, prefixed and escaped during early.
 	lower []string
+
 	// The upperdir is normally on a writable filesystem.
 	//
-	// If Work is nil and Upper holds the special value [fhs.AbsRoot],
-	// an ephemeral upperdir and workdir will be set up.
+	// If Work is nil and Upper holds the special value [fhs.AbsRoot], an
+	// ephemeral upperdir and workdir will be set up.
 	//
-	// If both Work and Upper are nil, upperdir and workdir is omitted and the overlay is mounted readonly.
+	// If both Work and Upper are nil, upperdir and workdir is omitted and the
+	// overlay is mounted readonly.
 	Upper *check.Absolute
-	// formatted for [OptionOverlayUpperdir], resolved, prefixed and escaped during early
+	// Formatted for [OptionOverlayUpperdir].
+	//
+	// Resolved, prefixed and escaped during early.
 	upper string
+
 	// The workdir needs to be an empty directory on the same filesystem as upperdir.
 	Work *check.Absolute
-	// formatted for [OptionOverlayWorkdir], resolved, prefixed and escaped during early
+	// Formatted for [OptionOverlayWorkdir].
+	//
+	// Resolved, prefixed and escaped during early.
 	work string
 
 	ephemeral bool
 
-	// used internally for mounting to the intermediate root
+	// Used internally for mounting to the intermediate root.
 	noPrefix bool
 }
 
diff --git a/container/initplace.go b/container/initplace.go
index 9974c58b..d0567ff6 100644
--- a/container/initplace.go
+++ b/container/initplace.go
@@ -16,7 +16,7 @@ const (
 
 func init() { gob.Register(new(TmpfileOp)) }
 
-// Place appends an [Op] that places a file in container path [TmpfileOp.Path] containing [TmpfileOp.Data].
+// Place is a helper for appending [TmpfileOp] to [Ops].
 func (f *Ops) Place(name *check.Absolute, data []byte) *Ops {
 	*f = append(*f, &TmpfileOp{name, data})
 	return f
diff --git a/container/initproc.go b/container/initproc.go
index 3ecb270a..42b519d7 100644
--- a/container/initproc.go
+++ b/container/initproc.go
@@ -10,7 +10,7 @@ import (
 
 func init() { gob.Register(new(MountProcOp)) }
 
-// Proc appends an [Op] that mounts a private instance of proc.
+// Proc is a helper for appending [MountProcOp] to [Ops].
 func (f *Ops) Proc(target *check.Absolute) *Ops {
 	*f = append(*f, &MountProcOp{target})
 	return f
diff --git a/container/initremount.go b/container/initremount.go
index b6d16970..64822914 100644
--- a/container/initremount.go
+++ b/container/initremount.go
@@ -9,7 +9,7 @@ import (
 
 func init() { gob.Register(new(RemountOp)) }
 
-// Remount appends an [Op] that applies [RemountOp.Flags] on container path [RemountOp.Target].
+// Remount is a helper for appending [RemountOp] to [Ops].
 func (f *Ops) Remount(target *check.Absolute, flags uintptr) *Ops {
 	*f = append(*f, &RemountOp{target, flags})
 	return f
diff --git a/container/landlock.go b/container/landlock.go
index 7f0a821f..acaab585 100644
--- a/container/landlock.go
+++ b/container/landlock.go
@@ -38,6 +38,7 @@ const (
 	_LANDLOCK_ACCESS_FS_DELIM
 )
 
+// String returns a space-separated string of [LandlockAccessFS] flags.
 func (f LandlockAccessFS) String() string {
 	switch f {
 	case LANDLOCK_ACCESS_FS_EXECUTE:
@@ -116,6 +117,7 @@ const (
 	_LANDLOCK_ACCESS_NET_DELIM
 )
 
+// String returns a space-separated string of [LandlockAccessNet] flags.
 func (f LandlockAccessNet) String() string {
 	switch f {
 	case LANDLOCK_ACCESS_NET_BIND_TCP:
@@ -152,6 +154,7 @@ const (
 	_LANDLOCK_SCOPE_DELIM
 )
 
+// String returns a space-separated string of [LandlockScope] flags.
 func (f LandlockScope) String() string {
 	switch f {
 	case LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET:
@@ -184,10 +187,12 @@ type RulesetAttr struct {
 	HandledAccessFS LandlockAccessFS
 	// Bitmask of handled network actions.
 	HandledAccessNet LandlockAccessNet
-	// Bitmask of scopes restricting a Landlock domain from accessing outside resources (e.g. IPCs).
+	// Bitmask of scopes restricting a Landlock domain from accessing outside
+	// resources (e.g. IPCs).
 	Scoped LandlockScope
 }
 
+// String returns a user-facing description of [RulesetAttr].
 func (rulesetAttr *RulesetAttr) String() string {
 	if rulesetAttr == nil {
 		return "NULL"
@@ -208,6 +213,7 @@ func (rulesetAttr *RulesetAttr) String() string {
 	return strings.Join(elems, ", ")
 }
 
+// Create loads the ruleset into the kernel.
 func (rulesetAttr *RulesetAttr) Create(flags uintptr) (fd int, err error) {
 	var pointer, size uintptr
 	// NULL needed for abi version
@@ -216,10 +222,13 @@ func (rulesetAttr *RulesetAttr) Create(flags uintptr) (fd int, err error) {
 		size = unsafe.Sizeof(*rulesetAttr)
 	}
 
-	rulesetFd, _, errno := syscall.Syscall(std.SYS_LANDLOCK_CREATE_RULESET, pointer, size, flags)
+	rulesetFd, _, errno := syscall.Syscall(
+		std.SYS_LANDLOCK_CREATE_RULESET,
+		pointer, size,
+		flags,
+	)
 	fd = int(rulesetFd)
 	err = errno
-
 	if fd < 0 {
 		return
 	}
@@ -230,12 +239,19 @@ func (rulesetAttr *RulesetAttr) Create(flags uintptr) (fd int, err error) {
 	return fd, nil
 }
 
+// LandlockGetABI returns the ABI version supported by the kernel.
 func LandlockGetABI() (int, error) {
 	return (*RulesetAttr)(nil).Create(LANDLOCK_CREATE_RULESET_VERSION)
 }
 
+// LandlockRestrictSelf applies a loaded ruleset to the calling thread.
 func LandlockRestrictSelf(rulesetFd int, flags uintptr) error {
-	r, _, errno := syscall.Syscall(std.SYS_LANDLOCK_RESTRICT_SELF, uintptr(rulesetFd), flags, 0)
+	r, _, errno := syscall.Syscall(
+		std.SYS_LANDLOCK_RESTRICT_SELF,
+		uintptr(rulesetFd),
+		flags,
+		0,
+	)
 	if r != 0 {
 		return errno
 	}
diff --git a/container/path.go b/container/path.go
index bc0000ad..965ad494 100644
--- a/container/path.go
+++ b/container/path.go
@@ -15,7 +15,10 @@ import (
 
 const (
 	// Nonexistent is a path that cannot exist.
-	// /proc is chosen because a system with covered /proc is unsupported by this package.
+	//
+	// This path can never be presented by the kernel if proc is mounted on
+	// /proc/. This can only exist if parts of /proc/ is covered, or proc is not
+	// mounted at all. Neither configuration is supported by this package.
 	Nonexistent = fhs.Proc + "nonexistent"
 
 	hostPath    = fhs.Root + hostDir
diff --git a/container/seccomp/libseccomp.go b/container/seccomp/libseccomp.go
index 4684be26..afb1848b 100644
--- a/container/seccomp/libseccomp.go
+++ b/container/seccomp/libseccomp.go
@@ -88,18 +88,22 @@ var resPrefix = [...]string{
 	7: "seccomp_load failed",
 }
 
-// cbAllocateBuffer is the function signature for the function handle passed to hakurei_export_filter
-// which allocates the buffer that the resulting bpf program is copied into, and writes its slice header
-// to a value held by the caller.
+// cbAllocateBuffer is the function signature for the function handle passed to
+// hakurei_scmp_make_filter which allocates the buffer that the resulting bpf
+// program is copied into, and writes its slice header to a value held by the caller.
 type cbAllocateBuffer = func(len C.size_t) (buf unsafe.Pointer)
 
+// hakurei_scmp_allocate allocates a buffer of specified size known to the
+// runtime through a callback passed in a [cgo.Handle].
+//
 //export hakurei_scmp_allocate
 func hakurei_scmp_allocate(f C.uintptr_t, len C.size_t) (buf unsafe.Pointer) {
 	return cgo.Handle(f).Value().(cbAllocateBuffer)(len)
 }
 
-// makeFilter generates a bpf program from a slice of [std.NativeRule] and writes the resulting byte slice to p.
-// The filter is installed to the current process if p is nil.
+// makeFilter generates a bpf program from a slice of [std.NativeRule] and
+// writes the resulting byte slice to p. The filter is installed to the current
+// process if p is nil.
 func makeFilter(rules []std.NativeRule, flags ExportFlag, p *[]byte) error {
 	if len(rules) == 0 {
 		return ErrInvalidRules
@@ -170,8 +174,8 @@ func Export(rules []std.NativeRule, flags ExportFlag) (data []byte, err error) {
 	return
 }
 
-// Load generates a bpf program from a slice of [std.NativeRule] and enforces it on the current process.
-// Errors returned by libseccomp is wrapped in [LibraryError].
+// Load generates a bpf program from a slice of [std.NativeRule] and enforces it
+// on the current process. Errors returned by libseccomp is wrapped in [LibraryError].
 func Load(rules []std.NativeRule, flags ExportFlag) error { return makeFilter(rules, flags, nil) }
 
 type (
diff --git a/container/vfs/mangle.go b/container/vfs/mangle.go
index 83aba589..af7a0463 100644
--- a/container/vfs/mangle.go
+++ b/container/vfs/mangle.go
@@ -2,6 +2,8 @@ package vfs
 
 import "strings"
 
+// Unmangle reverses mangling of strings done by the kernel. Its behaviour is
+// consistent with the equivalent function in util-linux.
 func Unmangle(s string) string {
 	if !strings.ContainsRune(s, '\\') {
 		return s
diff --git a/container/vfs/mountinfo.go b/container/vfs/mountinfo.go
index 5f9f2257..77f6a35d 100644
--- a/container/vfs/mountinfo.go
+++ b/container/vfs/mountinfo.go
@@ -24,6 +24,7 @@ var (
 	ErrMountInfoSep    = errors.New("bad optional fields separator")
 )
 
+// A DecoderError describes a nonrecoverable error decoding a mountinfo stream.
 type DecoderError struct {
 	Op   string
 	Line int
@@ -51,7 +52,8 @@ func (e *DecoderError) Error() string {
 }
 
 type (
-	// A MountInfoDecoder reads and decodes proc_pid_mountinfo(5) entries from an input stream.
+	// A MountInfoDecoder reads and decodes proc_pid_mountinfo(5) entries from
+	// an input stream.
 	MountInfoDecoder struct {
 		s *bufio.Scanner
 		m *MountInfo
@@ -72,13 +74,16 @@ type (
 	MountInfoEntry struct {
 		// mount ID: a unique ID for the mount (may be reused after umount(2)).
 		ID int `json:"id"`
-		// parent ID: the ID of the parent mount (or of self for the root of this mount namespace's mount tree).
+		// parent ID: the ID of the parent mount (or of self for the root of
+		// this mount namespace's mount tree).
 		Parent int `json:"parent"`
 		// major:minor: the value of st_dev for files on this filesystem (see stat(2)).
 		Devno DevT `json:"devno"`
-		// root: the pathname of the directory in the filesystem which forms the root of this mount.
+		// root: the pathname of the directory in the filesystem which forms the
+		// root of this mount.
 		Root string `json:"root"`
-		// mount point: the pathname of the mount point relative to the process's root directory.
+		// mount point: the pathname of the mount point relative to the
+		// process's root directory.
 		Target string `json:"target"`
 		// mount options: per-mount options (see mount(2)).
 		VfsOptstr string `json:"vfs_optstr"`
@@ -126,7 +131,8 @@ func (e *MountInfoEntry) Flags() (flags uintptr, unmatched []string) {
 
 // NewMountInfoDecoder returns a new decoder that reads from r.
 //
-// The decoder introduces its own buffering and may read data from r beyond the mountinfo entries requested.
+// The decoder introduces its own buffering and may read data from r beyond the
+// mountinfo entries requested.
 func NewMountInfoDecoder(r io.Reader) *MountInfoDecoder {
 	return &MountInfoDecoder{s: bufio.NewScanner(r)}
 }
@@ -271,6 +277,8 @@ func parseMountInfoLine(s string, ent *MountInfoEntry) error {
 	return nil
 }
 
+// EqualWithIgnore compares to [MountInfoEntry] values, ignoring fields that
+// compare equal to ignore.
 func (e *MountInfoEntry) EqualWithIgnore(want *MountInfoEntry, ignore string) bool {
 	return (e.ID == want.ID || want.ID == -1) &&
 		(e.Parent == want.Parent || want.Parent == -1) &&
@@ -284,6 +292,8 @@ func (e *MountInfoEntry) EqualWithIgnore(want *MountInfoEntry, ignore string) bo
 		(e.FsOptstr == want.FsOptstr || want.FsOptstr == ignore)
 }
 
+// String returns a user-facing representation of a [MountInfoEntry]. It fits
+// roughly into the mountinfo format, but without mangling.
 func (e *MountInfoEntry) String() string {
 	return fmt.Sprintf("%d %d %d:%d %s %s %s %s %s %s %s",
 		e.ID, e.Parent, e.Devno[0], e.Devno[1], e.Root, e.Target, e.VfsOptstr,
diff --git a/container/vfs/unfold.go b/container/vfs/unfold.go
index d63f0a9f..37c4cef3 100644
--- a/container/vfs/unfold.go
+++ b/container/vfs/unfold.go
@@ -6,6 +6,7 @@ import (
 	"strings"
 )
 
+// UnfoldTargetError is a pathname that never appeared in a mount hierarchy.
 type UnfoldTargetError string
 
 func (e UnfoldTargetError) Error() string {
@@ -27,6 +28,7 @@ func (n *MountInfoNode) Collective() iter.Seq[*MountInfoNode] {
 	return func(yield func(*MountInfoNode) bool) { n.visit(yield) }
 }
 
+// visit recursively visits all visible mountinfo nodes.
 func (n *MountInfoNode) visit(yield func(*MountInfoNode) bool) bool {
 	if !n.Covered && !yield(n) {
 		return false