cmd/hakurei: document stable behaviour

These are undocumented anywhere else and is required by tools invoking hakurei.

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

cmd/hakurei/main.go

@@ -1,3 +1,40 @@
+// Hakurei runs user-specified containers as subordinate users.
+//
+// This program is generally invoked by another, higher level program, which
+// creates container configuration via package [hst] or an implementation of it.
+//
+// The parent may leave files open and specify their file descriptor for various
+// uses. In these cases, standard streams and netpoll files are treated as
+// invalid file descriptors and rejected. All string representations must be in
+// decimal.
+//
+// When specifying a [hst.Config] JSON stream or file to the run subcommand, the
+// argument "-" is equivalent to stdin. Otherwise, file descriptor rules
+// described above applies. Invalid file descriptors are treated as file names
+// in their string representation, with the exception that if a netpoll file
+// descriptor is attempted, the program fails.
+//
+// The flag --identifier-fd can be optionally specified to the run subcommand to
+// receive the identifier of the newly started instance. File descriptor rules
+// described above applies, and the file must be writable. This is sent after
+// its state is made available, so the client must not attempt to poll for it.
+// This uses the internal binary format of [hst.ID].
+//
+// For the show and ps subcommands, the flag --json can be applied to the main
+// hakurei command to serialise output in JSON when applicable. Additionally,
+// the flag --short targeting each subcommand is used to omit some information
+// in both JSON and user-facing output. Only JSON-encoded output is covered
+// under the compatibility promise.
+//
+// A template for [hst.Config] demonstrating all available configuration fields
+// is returned by [hst.Template]. The JSON-encoded equivalent of this can be
+// obtained via the template subcommand. Fields left unpopulated in the template
+// (the direct_* family of fields, which are insecure under any configuration if
+// enabled) are unsupported.
+//
+// For simple (but insecure) testing scenarios, the exec subcommand can be used
+// to generate a simple, permissive configuration in-memory. See its help
+// message for all available options.
 package main
 
 import (
@@ -25,8 +62,8 @@ func main() {
 	// early init path, skips root check and duplicate PR_SET_DUMPABLE
 	container.TryArgv0(nil)
 
-	log.SetPrefix("hakurei: ")
 	log.SetFlags(0)
+	log.SetPrefix("hakurei: ")
 	msg := message.New(log.Default())
 
 	early := earlyHardeningErrs{

cmd/hakurei/parse.go

@@ -17,8 +17,9 @@ import (
 )
 
 // tryPath attempts to read [hst.Config] from multiple sources.
-// tryPath reads from [os.Stdin] if name has value "-".
-// Otherwise, name is passed to tryFd, and if that returns nil, name is passed to [os.Open].
+//
+// tryPath reads from [os.Stdin] if name has value "-". Otherwise, name is
+// passed to tryFd, and if that returns nil, name is passed to [os.Open].
 func tryPath(msg message.Msg, name string) (config *hst.Config) {
 	var r io.ReadCloser
 	config = new(hst.Config)
@@ -46,7 +47,8 @@ func tryPath(msg message.Msg, name string) (config *hst.Config) {
 	return
 }
 
-// tryFd returns a [io.ReadCloser] if name represents an integer corresponding to a valid file descriptor.
+// tryFd returns a [io.ReadCloser] if name represents an integer corresponding
+// to a valid file descriptor.
 func tryFd(msg message.Msg, name string) io.ReadCloser {
 	if v, err := strconv.Atoi(name); err != nil {
 		if !errors.Is(err, strconv.ErrSyntax) {
@@ -60,7 +62,12 @@ func tryFd(msg message.Msg, name string) io.ReadCloser {
 
 		msg.Verbosef("trying config stream from %d", v)
 		fd := uintptr(v)
-		if _, _, errno := syscall.Syscall(syscall.SYS_FCNTL, fd, syscall.F_GETFD, 0); errno != 0 {
+		if _, _, errno := syscall.Syscall(
+			syscall.SYS_FCNTL,
+			fd,
+			syscall.F_GETFD,
+			0,
+		); errno != 0 {
 			if errors.Is(errno, syscall.EBADF) { // reject bad fd
 				return nil
 			}
@@ -75,10 +82,12 @@ func tryFd(msg message.Msg, name string) io.ReadCloser {
 	}
 }
 
-// shortLengthMin is the minimum length a short form identifier can have and still be interpreted as an identifier.
+// shortLengthMin is the minimum length a short form identifier can have and
+// still be interpreted as an identifier.
 const shortLengthMin = 1 << 3
 
-// shortIdentifier returns an eight character short representation of [hst.ID] from its random bytes.
+// shortIdentifier returns an eight character short representation of [hst.ID]
+// from its random bytes.
 func shortIdentifier(id *hst.ID) string {
 	return shortIdentifierString(id.String())
 }
@@ -88,7 +97,8 @@ func shortIdentifierString(s string) string {
 	return s[len(hst.ID{}) : len(hst.ID{})+shortLengthMin]
 }
 
-// tryIdentifier attempts to match [hst.State] from a [hex] representation of [hst.ID] or a prefix of its lower half.
+// tryIdentifier attempts to match [hst.State] from a [hex] representation of
+// [hst.ID] or a prefix of its lower half.
 func tryIdentifier(msg message.Msg, name string, s *store.Store) *hst.State {
 	const (
 		likeShort = 1 << iota
@@ -96,7 +106,8 @@ func tryIdentifier(msg message.Msg, name string, s *store.Store) *hst.State {
 	)
 
 	var likely uintptr
-	if len(name) >= shortLengthMin && len(name) <= len(hst.ID{}) { // half the hex representation
+	// half the hex representation
+	if len(name) >= shortLengthMin && len(name) <= len(hst.ID{}) {
 		// cannot safely decode here due to unknown alignment
 		for _, c := range name {
 			if c >= '0' && c <= '9' {