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Hallyn" , Martin KaFai Lau , Eduard Zingerman , Yonghong Song , John Fastabend , Kumar Kartikeya Dwivedi , Jiri Olsa , Jeff Layton , Amir Goldstein , Mateusz Guzik , Shuah Khan , Tingmao Wang , Justin Suess Subject: [RFC PATCH bpf-next 08/12] fs: add mnt_undo_legitimize() Date: Mon, 5 Oct 2026 20:20:15 -0400 Message-ID: <20261006002020.2890858-9-utilityemal77@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20261006002020.2890858-1-utilityemal77@gmail.com> References: <20261006002020.2890858-1-utilityemal77@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit A failed __legitimize_mnt() can oblige its caller to mntput() the mount, which may be the final put and so may sleep: legitimize_mnt() leaves the RCU read-side critical section it ran under to do it. A caller that cannot leave its critical section - a BPF program driving a lockless walk, which only the verifier ends - needs that put to be callable from inside it. Add mnt_undo_legitimize(), which drops the count and, when the count turns out to be the mount's last, keeps it and hands the mount to delayed_mntput() for the sleepable final put. delayed_mntput() then tells the two queue populations apart by MNT_DOOMED rather than by an unlocked mnt_get_count() read, which a concurrent failed __legitimize_mnt() could transiently inflate. legitimize_mnt() switches to it as well, retiring its rcu_read_unlock/mntput/rcu_read_lock dance. Unlike mntput(), mnt_expiry_mark is left alone: a walker that failed to legitimize a mount never used it. Signed-off-by: Justin Suess --- fs/mount.h | 1 + fs/namespace.c | 63 ++++++++++++++++++++++++++++++++++++++++++++------ 2 files changed, 57 insertions(+), 7 deletions(-) diff --git a/fs/mount.h b/fs/mount.h index 94fcc306d21e..e714040f8e8f 100644 --- a/fs/mount.h +++ b/fs/mount.h @@ -140,6 +140,7 @@ static inline int is_mounted(struct vfsmount *mnt) extern struct mount *__lookup_mnt(struct vfsmount *, struct dentry *); extern int __legitimize_mnt(struct vfsmount *, unsigned); +void mnt_undo_legitimize(struct mount *mnt); static inline bool __path_is_mountpoint(const struct path *path) { diff --git a/fs/namespace.c b/fs/namespace.c index 580877e46b1a..920b8a85f9fa 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -767,11 +767,8 @@ static bool legitimize_mnt(struct vfsmount *bastard, unsigned seq) int res = __legitimize_mnt(bastard, seq); if (likely(!res)) return true; - if (unlikely(res < 0)) { - rcu_read_unlock(); - mntput(bastard); - rcu_read_lock(); - } + if (unlikely(res < 0)) + mnt_undo_legitimize(real_mount(bastard)); return false; } @@ -1330,8 +1327,19 @@ static void delayed_mntput(struct work_struct *unused) struct llist_node *node = llist_del_all(&delayed_mntput_list); struct mount *m, *t; - llist_for_each_entry_safe(m, t, node, mnt_llist) - cleanup_mnt(m); + llist_for_each_entry_safe(m, t, node, mnt_llist) { + /* + * MNT_DOOMED is only ever set on the mounts + * mntput_no_expire_slowpath() queues here, never on the + * kept-count ones mnt_undo_legitimize() queues. Route on it + * rather than on mnt_get_count(), which a concurrent failed + * __legitimize_mnt() can transiently inflate. + */ + if (m->mnt.mnt_flags & MNT_DOOMED) + cleanup_mnt(m); + else + mntput(&m->mnt); /* kept by mnt_undo_legitimize() */ + } } static DECLARE_DELAYED_WORK(delayed_mntput_work, delayed_mntput); @@ -1423,6 +1431,47 @@ void mntput(struct vfsmount *mnt) } EXPORT_SYMBOL(mntput); +/** + * mnt_undo_legitimize - drop a count __legitimize_mnt() asked us to put + * @mnt: the mount __legitimize_mnt() failed on + * + * Like the mntput() that a failed __legitimize_mnt() normally obliges, + * but callable from the RCU read-side critical section the legitimization + * ran under: when the count turns out to be the mount's last, it is kept + * and the mount handed to delayed_mntput() for the sleepable final put. + * Unlike mntput(), mnt_expiry_mark is left alone: a walker that failed + * to legitimize the mount never used it. + */ +void mnt_undo_legitimize(struct mount *mnt) +{ + if (likely(READ_ONCE(mnt->mnt_ns))) { + /* Not the final count, as in mntput_no_expire(). */ + mnt_add_count(mnt, -1); + return; + } + lock_mount_hash(); + /* + * As in mntput_no_expire(): make sure that if a concurrent + * __legitimize_mnt() has not seen us grab mount_lock, we'll see + * its refcount increment here. + */ + smp_mb(); + if (likely(mnt_get_count(mnt) > 1)) { + mnt_add_count(mnt, -1); + unlock_mount_hash(); + return; + } + unlock_mount_hash(); + /* + * Ours is the last count: nothing else can reach the mount any + * more, which makes its mnt_llist ours to use. delayed_mntput() + * tells our kept-count mount from the doomed ones by MNT_DOOMED. + */ + WARN_ON_ONCE(mnt->mnt.mnt_flags & MNT_DOOMED); + if (llist_add(&mnt->mnt_llist, &delayed_mntput_list)) + schedule_delayed_work(&delayed_mntput_work, 1); +} + struct vfsmount *mntget(struct vfsmount *mnt) { if (mnt) -- 2.55.0