From: Al Viro <viro@zeniv.linux.org.uk>
Date: Sat, 26 Apr 2025 02:40:48 +0000 (-0400)
Subject: attach_recursive_mnt(): unify the mnt_change_mountpoint() logics
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=96f5d2e051653f2cfe9137c5d3c7794c358ffb03;p=thirdparty%2Fkernel%2Flinux.git

attach_recursive_mnt(): unify the mnt_change_mountpoint() logics

The logics used for tucking under existing mount differs for original
and copies; copies do a mount hash lookup to see if mountpoint to be is
already overmounted, while the original is told explicitly.

But the same logics that is used for copies works for the original,
at which point the only place where we get very close to eliminating
the need of passing 'beneath' flag to attach_recursive_mnt().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---

diff --git a/fs/namespace.c b/fs/namespace.c
index 937c2a1825f2f..9b8d07df4aa5d 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2643,7 +2643,7 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 	HLIST_HEAD(tree_list);
 	struct mnt_namespace *ns = top_mnt->mnt_ns;
 	struct mountpoint *smp;
-	struct mountpoint *secondary = NULL;
+	struct mountpoint *shorter = NULL;
 	struct mount *child, *dest_mnt, *p;
 	struct mount *top;
 	struct hlist_node *n;
@@ -2655,14 +2655,12 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 	 * mounted beneath mounts on the same mountpoint.
 	 */
 	for (top = source_mnt; unlikely(top->overmount); top = top->overmount) {
-		if (!secondary && is_mnt_ns_file(top->mnt.mnt_root))
-			secondary = top->mnt_mp;
+		if (!shorter && is_mnt_ns_file(top->mnt.mnt_root))
+			shorter = top->mnt_mp;
 	}
 	smp = get_mountpoint(top->mnt.mnt_root);
 	if (IS_ERR(smp))
 		return PTR_ERR(smp);
-	if (!secondary)
-		secondary = smp;
 
 	/* Is there space to add these mounts to the mount namespace? */
 	if (!moving) {
@@ -2706,9 +2704,14 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 	}
 
 	mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt);
-	if (beneath)
-		mnt_change_mountpoint(top, smp, top_mnt);
-	commit_tree(source_mnt);
+	/*
+	 * Now the original copy is in the same state as the secondaries -
+	 * its root attached to mountpoint, but not hashed and all mounts
+	 * in it are either in our namespace or in no namespace at all.
+	 * Add the original to the list of copies and deal with the
+	 * rest of work for all of them uniformly.
+	 */
+	hlist_add_head(&source_mnt->mnt_hash, &tree_list);
 
 	hlist_for_each_entry_safe(child, n, &tree_list, mnt_hash) {
 		struct mount *q;
@@ -2719,10 +2722,13 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 		q = __lookup_mnt(&child->mnt_parent->mnt,
 				 child->mnt_mountpoint);
 		if (q) {
+			struct mountpoint *mp = smp;
 			struct mount *r = child;
 			while (unlikely(r->overmount))
 				r = r->overmount;
-			mnt_change_mountpoint(r, secondary, q);
+			if (unlikely(shorter) && child != source_mnt)
+				mp = shorter;
+			mnt_change_mountpoint(r, mp, q);
 		}
 		commit_tree(child);
 	}