hst: conditionally skip root remount

This enables the writable root overlay use case.

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

hst/fs.go

@@ -56,8 +56,10 @@ type Ops interface {
 
 // ApplyState holds the address of [Ops] and any relevant application state.
 type ApplyState struct {
-	// AutoEtcPrefix is the prefix for [FSBind] in autoetc [FSBind.Special] condition.
+	// Prefix for [FSBind] in autoetc [FSBind.Special] condition.
 	AutoEtcPrefix string
+	// Whether to skip remounting root.
+	NoRemountRoot bool
 
 	Ops
 }

hst/fsoverlay.go

@@ -5,6 +5,7 @@ import (
 	"strings"
 
 	"hakurei.app/check"
+	"hakurei.app/fhs"
 )
 
 func init() { gob.Register(new(FSOverlay)) }
@@ -69,9 +70,12 @@ func (o *FSOverlay) Apply(z *ApplyState) {
 		return
 	}
 
-	if o.Upper != nil && o.Work != nil { // rw
+	if o.Upper != nil && o.Work != nil {
 		z.Overlay(o.Target, o.Upper, o.Work, o.Lower...)
-	} else { // ro
+		if o.Target.Is(fhs.AbsRoot) {
+			z.NoRemountRoot = true
+		}
+	} else {
 		z.OverlayReadonly(o.Target, o.Lower...)
 	}
 }

hst/fsoverlay_test.go

@@ -49,5 +49,18 @@ func TestFSOverlay(t *testing.T) {
 			Lower:  ms("/tmp/.src0", "/tmp/.src1"),
 		}}, m("/mnt/src"), ms("/tmp/.src0", "/tmp/.src1"),
 			"*/mnt/src:/tmp/.src0:/tmp/.src1"},
+
+		{"no remount root", &hst.FSOverlay{
+			Target: m("/"),
+			Lower:  ms("/tmp/.src0", "/tmp/.src1"),
+			Upper:  m("/tmp/upper"),
+			Work:   m("/tmp/work"),
+		}, true, container.Ops{&container.MountOverlayOp{
+			Target: m("/"),
+			Lower:  ms("/tmp/.src0", "/tmp/.src1"),
+			Upper:  m("/tmp/upper"),
+			Work:   m("/tmp/work"),
+		}}, m("/"), ms("/tmp/upper", "/tmp/work", "/tmp/.src0", "/tmp/.src1"),
+			"w*/:/tmp/upper:/tmp/work:/tmp/.src0:/tmp/.src1"},
 	})
 }

internal/outcome/spcontainer.go

@@ -290,7 +290,9 @@ func (s *spFilesystemOp) toContainer(state *outcomeStateParams) error {
 	if state.Container.Flags&hst.FDevice == 0 {
 		state.params.Remount(fhs.AbsDev, syscall.MS_RDONLY)
 	}
+	if !state.as.NoRemountRoot {
 		state.params.Remount(fhs.AbsRoot, syscall.MS_RDONLY)
+	}
 
 	state.params.Env = make([]string, 0, len(state.env))
 	for key, value := range state.env {