[PATCH] libmount: provide tree fd even when a mount helper is used

[Linus Heckemann <git@sphalerite.org>]

Previously, the X-mount.subdir option would fail (mount exited with
code 0, but the target was not mounted) when a helper was used.

In addition to fixing X-mount.subdir, this allows dropping the
fallback behaviour previously implemented specifically by the
set_vfsflags and set_propagation hooks.

I realise this patch is not acceptable as is, since I just exported
the previously private open_mount_tree symbol from hook_mount.c
without even adding it to a header (so builds with sensible warning
configurations, with -Werror=implicit-function-declaration, won't even
compile this); but I'm submitting this as is in hopes that someone who
knows their way around libmount will suggest a sensible way to shuffle
the code around.
---
 libmount/src/context_mount.c | 15 ++++++++++++++-
 libmount/src/hook_mount.c    | 20 +-------------------
 2 files changed, 15 insertions(+), 20 deletions(-)

diff --git a/libmount/src/context_mount.c b/libmount/src/context_mount.c
index 676287733..ef66f47af 100644
--- a/libmount/src/context_mount.c
+++ b/libmount/src/context_mount.c
@@ -482,8 +482,21 @@ static int exec_helper(struct libmnt_context *cxt)
 			DBG(CXT, ul_debugobj(cxt, "%s forked [status=%d, rc=%d]",
 				cxt->helper,
 				cxt->helper_status, rc));
+
+			struct libmnt_sysapi *api = mnt_context_get_sysapi(cxt);
+			assert(api);
+
+			/* a mount helper, unlike an internal mount, won't provide the mount fd.
+			 * This is, however, needed by hooks like subdir -- so let's open the tree
+			 * that was mounted by our helper child. */
+			if (api->fd_tree < 0 && mnt_fs_get_target(cxt->fs)) {
+				api->fd_tree = open_mount_tree(cxt, NULL, (unsigned long) -1);
+				if (api->fd_tree > 0) {
+					rc = 0;
+				}
+			}
+			break;
 		}
-		break;
 	}
 
 	case -1:
diff --git a/libmount/src/hook_mount.c b/libmount/src/hook_mount.c
index 6b7caff85..b80fe4839 100644
--- a/libmount/src/hook_mount.c
+++ b/libmount/src/hook_mount.c
@@ -255,7 +255,7 @@ static int open_fs_configuration_context(struct libmnt_context *cxt,
 	return api->fd_fs;
 }
 
-static int open_mount_tree(struct libmnt_context *cxt, const char *path, unsigned long mflg)
+int open_mount_tree(struct libmnt_context *cxt, const char *path, unsigned long mflg)
 {
 	unsigned long oflg = OPEN_TREE_CLOEXEC;
 	int rc = 0, fd = -1;
@@ -416,15 +416,6 @@ static int set_vfsflags(struct libmnt_context *cxt,
 	api = get_sysapi(cxt);
 	assert(api);
 
-	/* fallback only; necessary when init_sysapi() during preparation
-	 * cannot open the tree -- for example when we call /sbin/mount.<type> */
-	if (api->fd_tree < 0 && mnt_fs_get_target(cxt->fs)) {
-		rc = api->fd_tree = open_mount_tree(cxt, NULL, (unsigned long) -1);
-		if (rc < 0)
-			return rc;
-		rc = 0;
-	}
-
 	if (recursive)
 		callflags |= AT_RECURSIVE;
 
@@ -494,15 +485,6 @@ static int hook_set_propagation(struct libmnt_context *cxt,
 	api = get_sysapi(cxt);
 	assert(api);
 
-	/* fallback only; necessary when init_sysapi() during preparation
-	 * cannot open the tree -- for example when we call /sbin/mount.<type> */
-	if (api->fd_tree < 0 && mnt_fs_get_target(cxt->fs)) {
-		rc = api->fd_tree = open_mount_tree(cxt, NULL, (unsigned long) -1);
-		if (rc < 0)
-			goto done;
-		rc = 0;
-	}
-
 	mnt_reset_iter(&itr, MNT_ITER_FORWARD);
 
 	while (mnt_optlist_next_opt(ol, &itr, &opt) == 0) {
-- 
2.42.0