interfaces/builtin: always use /snap for the content interface

dirs/dirs.go

@@ -34,6 +34,7 @@ var (
 
 	SnapMountDir              string
 	BaseSnapMountDir          string
+	CoreSnapMountDir          string
 	SnapBlobDir               string
 	SnapDataDir               string
 	SnapDataHomeGlob          string
@@ -185,6 +186,7 @@ func SetRootDir(rootdir string) {
 	}
 
 	CoreLibExecDir = "/usr/lib/snapd"
+	CoreSnapMountDir = "/snap"
 
 	XdgRuntimeDirBase = filepath.Join(rootdir, "/run/user")
 	XdgRuntimeDirGlob = filepath.Join(rootdir, XdgRuntimeDirBase, "*/")

interfaces/builtin/content.go

@@ -25,6 +25,7 @@ import (
 	"path/filepath"
 	"strings"
 
+	"github.com/snapcore/snapd/dirs"
 	"github.com/snapcore/snapd/interfaces"
 	"github.com/snapcore/snapd/interfaces/apparmor"
 	"github.com/snapcore/snapd/interfaces/mount"
@@ -116,13 +117,23 @@ func (iface *contentInterface) path(slot *interfaces.Slot, name string) []string
 	return out
 }
 
+const (
+	SnapMountDir = iota
+	SnapDataDir
+	SnapCommonDir
+)
+
 // resolveSpecialVariable resolves one of the three $SNAP* variables at the
 // beginning of a given path.  The variables are $SNAP, $SNAP_DATA and
 // $SNAP_COMMON. If there are no variables then $SNAP is implicitly assumed
 // (this is the behavior that was used before the variables were supporter).
 func resolveSpecialVariable(path string, snapInfo *snap.Info) string {
 	if strings.HasPrefix(path, "$SNAP/") || path == "$SNAP" {
-		return strings.Replace(path, "$SNAP", snapInfo.MountDir(), 1)
+		// NOTE: We use dirs.CoreSnapMountDir here as the path used will be always
+		// inside the mount namespace snap-confine creates and there we will
+		// always have a /snap directory available regardless if the system
+		// we're running on supports this or not.
+		return strings.Replace(path, "$SNAP", snap.MountDirWithBasePath(dirs.CoreSnapMountDir, snapInfo.Name(), snapInfo.Revision), 1)
 	}
 	if strings.HasPrefix(path, "$SNAP_DATA/") || path == "$SNAP_DATA" {
 		return strings.Replace(path, "$SNAP_DATA", snapInfo.DataDir(), 1)

interfaces/builtin/content_test.go

@@ -200,11 +200,11 @@ apps:
 
 func (s *ContentSuite) TestResolveSpecialVariable(c *C) {
 	info := snaptest.MockInfo(c, "name: name", &snap.SideInfo{Revision: snap.R(42)})
-	c.Check(builtin.ResolveSpecialVariable("foo", info), Equals, filepath.Join(dirs.BaseSnapMountDir, "name/42/foo"))
-	c.Check(builtin.ResolveSpecialVariable("$SNAP/foo", info), Equals, filepath.Join(dirs.BaseSnapMountDir, "name/42/foo"))
+	c.Check(builtin.ResolveSpecialVariable("foo", info), Equals, filepath.Join(dirs.CoreSnapMountDir, "name/42/foo"))
+	c.Check(builtin.ResolveSpecialVariable("$SNAP/foo", info), Equals, filepath.Join(dirs.CoreSnapMountDir, "name/42/foo"))
 	c.Check(builtin.ResolveSpecialVariable("$SNAP_DATA/foo", info), Equals, "/var/snap/name/42/foo")
 	c.Check(builtin.ResolveSpecialVariable("$SNAP_COMMON/foo", info), Equals, "/var/snap/name/common/foo")
-	c.Check(builtin.ResolveSpecialVariable("$SNAP", info), Equals, filepath.Join(dirs.BaseSnapMountDir, "name/42"))
+	c.Check(builtin.ResolveSpecialVariable("$SNAP", info), Equals, filepath.Join(dirs.CoreSnapMountDir, "name/42"))
 	c.Check(builtin.ResolveSpecialVariable("$SNAP_DATA", info), Equals, "/var/snap/name/42")
 	c.Check(builtin.ResolveSpecialVariable("$SNAP_COMMON", info), Equals, "/var/snap/name/common")
 }
@@ -230,8 +230,8 @@ slots:
 	spec := &mount.Specification{}
 	c.Assert(spec.AddConnectedPlug(s.iface, plug, nil, slot, nil), IsNil)
 	expectedMnt := []mount.Entry{{
-		Name:    filepath.Join(dirs.BaseSnapMountDir, "producer/5/export"),
-		Dir:     filepath.Join(dirs.BaseSnapMountDir, "consumer/7/import"),
+		Name:    filepath.Join(dirs.CoreSnapMountDir, "producer/5/export"),
+		Dir:     filepath.Join(dirs.CoreSnapMountDir, "consumer/7/import"),
 		Options: []string{"bind", "ro"},
 	}}
 	c.Assert(spec.MountEntries(), DeepEquals, expectedMnt)
@@ -261,8 +261,8 @@ slots:
 	spec := &mount.Specification{}
 	c.Assert(spec.AddConnectedPlug(s.iface, plug, nil, slot, nil), IsNil)
 	expectedMnt := []mount.Entry{{
-		Name:    filepath.Join(dirs.BaseSnapMountDir, "producer/5/export"),
-		Dir:     filepath.Join(dirs.BaseSnapMountDir, "consumer/7/import"),
+		Name:    filepath.Join(dirs.CoreSnapMountDir, "producer/5/export"),
+		Dir:     filepath.Join(dirs.CoreSnapMountDir, "consumer/7/import"),
 		Options: []string{"bind", "ro"},
 	}}
 	c.Assert(spec.MountEntries(), DeepEquals, expectedMnt)
@@ -276,7 +276,7 @@ slots:
 # snaps may directly access the slot implementation's files
 # read-only.
 %s/producer/5/export/** mrkix,
-`, dirs.BaseSnapMountDir)
+`, dirs.CoreSnapMountDir)
 	c.Assert(apparmorSpec.SnippetForTag("snap.consumer.app"), Equals, expected)
 }
 

snap/info.go

@@ -70,7 +70,12 @@ func MinimalPlaceInfo(name string, revision Revision) PlaceInfo {
 
 // MountDir returns the base directory where it gets mounted of the snap with the given name and revision.
 func MountDir(name string, revision Revision) string {
-	return filepath.Join(dirs.SnapMountDir, name, revision.String())
+	return MountDirWithBasePath(dirs.SnapMountDir, name, revision)
+}
+
+// MountDirWithBasePath returns the base directory where it gets mounted of the snap with the given name and revision.
+func MountDirWithBasePath(basePath, name string, revision Revision) string {
+	return filepath.Join(basePath, name, revision.String())
 }
 
 // MountFile returns the path where the snap file that is mounted is installed.