cmd/snap-update-ns: allow detaches to propagate

When snap-update-ns chooses not to reuse a mount entry it attempts to unmount
it by detaching the mount entry. Before that is done the mount entry used to be
switch to private propagation, so that the unmount operation was isolated.

Disable this logic so that unmounts (and detaches) propagate normally. The
mount namespace of the snap is mostly switched to slaved propagation, with the
sole exception of /media or /run/media, which uses bidirectional propagation by
design.

This patch is experimental and is not ready for being merged yet.

Jira: https://warthogs.atlassian.net/browse/SNAPDENG-31645

Signed-off-by: Zygmunt Krynicki <[email protected]>

cmd/snap-update-ns/change.go

@@ -405,15 +405,6 @@ func (c *Change) lowLevelPerform(as *Assumptions) error {
 			flags := umountNoFollow
 			if c.Entry.XSnapdDetach() {
 				flags |= syscall.MNT_DETACH
-				// If we are detaching something then before performing the actual detach
-				// switch the entire hierarchy to private event propagation (that is,
-				// none). This works around a bit of peculiar kernel behavior when the
-				// kernel reports EBUSY during a detach operation, because the changes
-				// propagate in a way that conflicts with itself. This is also documented
-				// in umount(2).
-				err = sysMount("none", c.Entry.Dir, "", syscall.MS_REC|syscall.MS_PRIVATE, "")
-				logger.Debugf("mount --make-rprivate %q (error: %v)", c.Entry.Dir, err)
-				err = clearMissingMountError(err)
 			}
 
 			// Perform the raw unmount operation.

cmd/snap-update-ns/change_test.go

@@ -812,7 +812,6 @@ func (s *changeSuite) TestPerformFilesystemMountWithoutMountPointAndReadOnlyBase
 
 		{C: `lstat "/rofs"`, R: testutil.FileInfoDir},
 		{C: `mount "tmpfs" "/rofs" "tmpfs" 0 "mode=0755,uid=0,gid=0"`},
-		{C: `mount "none" "/tmp/.snap/rofs" "" MS_REC|MS_PRIVATE ""`},
 		{C: `unmount "/tmp/.snap/rofs" UMOUNT_NOFOLLOW|MNT_DETACH`},
 
 		// Perform clean up after the unmount operation.
@@ -976,7 +975,6 @@ func (s *changeSuite) TestPerformFilesystemDetch(c *C) {
 	synth, err := chg.Perform(s.as)
 	c.Assert(err, IsNil)
 	c.Assert(s.sys.RCalls(), testutil.SyscallsEqual, []testutil.CallResultError{
-		{C: `mount "none" "/target" "" MS_REC|MS_PRIVATE ""`},
 		{C: `unmount "/target" UMOUNT_NOFOLLOW|MNT_DETACH`},
 
 		// Perform clean up after the unmount operation.
@@ -1323,7 +1321,6 @@ func (s *changeSuite) TestPerformDirectoryBindMountWithoutMountPointAndReadOnlyB
 
 		{C: `lstat "/rofs"`, R: testutil.FileInfoDir},
 		{C: `mount "tmpfs" "/rofs" "tmpfs" 0 "mode=0755,uid=0,gid=0"`},
-		{C: `mount "none" "/tmp/.snap/rofs" "" MS_REC|MS_PRIVATE ""`},
 		{C: `unmount "/tmp/.snap/rofs" UMOUNT_NOFOLLOW|MNT_DETACH`},
 
 		// Perform clean up after the unmount operation.
@@ -1471,7 +1468,6 @@ func (s *changeSuite) TestPerformDirectoryBindMountWithoutMountSourceAndReadOnly
 		{C: `close 4`},
 		{C: `lstat "/rofs"`, R: testutil.FileInfoDir},
 		{C: `mount "tmpfs" "/rofs" "tmpfs" 0 "mode=0755,uid=0,gid=0"`},
-		{C: `mount "none" "/tmp/.snap/rofs" "" MS_REC|MS_PRIVATE ""`},
 		{C: `unmount "/tmp/.snap/rofs" UMOUNT_NOFOLLOW|MNT_DETACH`},
 
 		// Perform clean up after the unmount operation.
@@ -1864,7 +1860,6 @@ func (s *changeSuite) TestPerformFileBindMountWithoutMountPointAndReadOnlyBase(c
 
 		{C: `lstat "/rofs"`, R: testutil.FileInfoDir},
 		{C: `mount "tmpfs" "/rofs" "tmpfs" 0 "mode=0755,uid=0,gid=0"`},
-		{C: `mount "none" "/tmp/.snap/rofs" "" MS_REC|MS_PRIVATE ""`},
 		{C: `unmount "/tmp/.snap/rofs" UMOUNT_NOFOLLOW|MNT_DETACH`},
 
 		// Perform clean up after the unmount operation.
@@ -2259,7 +2254,6 @@ func (s *changeSuite) TestPerformCreateSymlinkWithoutBaseDirAndReadOnlyBase(c *C
 
 		{C: `lstat "/rofs"`, R: testutil.FileInfoDir},
 		{C: `mount "tmpfs" "/rofs" "tmpfs" 0 "mode=0755,uid=0,gid=0"`},
-		{C: `mount "none" "/tmp/.snap/rofs" "" MS_REC|MS_PRIVATE ""`},
 		{C: `unmount "/tmp/.snap/rofs" UMOUNT_NOFOLLOW|MNT_DETACH`},
 
 		// Perform clean up after the unmount operation.
@@ -2520,7 +2514,6 @@ func (s *changeSuite) TestPerformCreateSymlinkWithAvoidedTrespassing(c *C) {
 		{C: `close 7`},
 
 		// We're done restoring now.
-		{C: `mount "none" "/tmp/.snap/etc" "" MS_REC|MS_PRIVATE ""`},
 		{C: `unmount "/tmp/.snap/etc" UMOUNT_NOFOLLOW|MNT_DETACH`},
 
 		// Perform clean up after the unmount operation.