[PATCH 13/19] vfs: Add a mount-notification facility [ver #16]

@@ -478,3 +478,4 @@
 547	common	openat2				sys_openat2
 548	common	pidfd_getfd			sys_pidfd_getfd
 549	common	fsinfo				sys_fsinfo
+550	common	watch_mount			sys_watch_mount

@@ -452,3 +452,4 @@
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	fsinfo				sys_fsinfo
+440	common	watch_mount			sys_watch_mount

@@ -359,3 +359,4 @@
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	fsinfo				sys_fsinfo
+440	common	watch_mount			sys_watch_mount

@@ -439,3 +439,4 @@
 438	common	pidfd_getfd			sys_pidfd_getfd
 # 435 reserved for clone3
 439	common	fsinfo				sys_fsinfo
+440	common	watch_mount			sys_watch_mount

@@ -444,3 +444,4 @@
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	fsinfo				sys_fsinfo
+440	common	watch_mount			sys_watch_mount

@@ -377,3 +377,4 @@
 437	n32	openat2				sys_openat2
 438	n32	pidfd_getfd			sys_pidfd_getfd
 439	n32	fsinfo				sys_fsinfo
+440	n32	watch_mount			sys_watch_mount

@@ -353,3 +353,4 @@
 437	n64	openat2				sys_openat2
 438	n64	pidfd_getfd			sys_pidfd_getfd
 439	n64	fsinfo				sys_fsinfo
+440	n64	watch_mount			sys_watch_mount

@@ -426,3 +426,4 @@
 437	o32	openat2				sys_openat2
 438	o32	pidfd_getfd			sys_pidfd_getfd
 439	o32	fsinfo				sys_fsinfo
+440	o32	watch_mount			sys_watch_mount

@@ -436,3 +436,4 @@
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	fsinfo				sys_fsinfo
+440	common	watch_mount			sys_watch_mount

@@ -520,3 +520,4 @@
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	fsinfo				sys_fsinfo
+440	common	watch_mount			sys_watch_mount

@@ -441,3 +441,4 @@
 437  common	openat2			sys_openat2			sys_openat2
 438  common	pidfd_getfd		sys_pidfd_getfd			sys_pidfd_getfd
 439  common	fsinfo			sys_fsinfo			sys_fsinfo
+440	common	watch_mount		sys_watch_mount			sys_watch_mount

@@ -441,3 +441,4 @@
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	fsinfo				sys_fsinfo
+440	common	watch_mount			sys_watch_mount

@@ -484,3 +484,4 @@
 437	common	openat2			sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	fsinfo				sys_fsinfo
+440	common	watch_mount			sys_watch_mount

@@ -443,3 +443,4 @@
 437	i386	openat2			sys_openat2			__ia32_sys_openat2
 438	i386	pidfd_getfd		sys_pidfd_getfd			__ia32_sys_pidfd_getfd
 439	i386	fsinfo			sys_fsinfo			__ia32_sys_fsinfo
+440	i386	watch_mount		sys_watch_mount			__ia32_sys_watch_mount

@@ -360,6 +360,7 @@
 437	common	openat2			__x64_sys_openat2
 438	common	pidfd_getfd		__x64_sys_pidfd_getfd
 439	common	fsinfo			__x64_sys_fsinfo
+440	common	watch_mount		__x64_sys_watch_mount
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact

@@ -409,3 +409,4 @@
 437	common	openat2				sys_openat2
 438	common	pidfd_getfd			sys_pidfd_getfd
 439	common	fsinfo				sys_fsinfo
+440	common	watch_mount			sys_watch_mount

@@ -124,6 +124,15 @@ source "fs/verity/Kconfig"
 
 source "fs/notify/Kconfig"
 
+config MOUNT_NOTIFICATIONS
+	bool "Mount topology change notifications"
+	select WATCH_QUEUE
+	help
+	  This option provides support for getting change notifications on the
+	  mount tree topology.  This makes use of the /dev/watch_queue misc
+	  device to handle the notification buffer and provides the
+	  mount_notify() system call to enable/disable watchpoints.
+
 source "fs/quota/Kconfig"
 
 source "fs/autofs/Kconfig"

@@ -22,6 +22,7 @@ obj-y +=	no-block.o
 endif
 
 obj-$(CONFIG_PROC_FS) += proc_namespace.o
+obj-$(CONFIG_MOUNT_NOTIFICATIONS) += mount_notify.o
 
 obj-y				+= notify/
 obj-$(CONFIG_EPOLL)		+= eventpoll.o

@@ -4,6 +4,7 @@
 #include <linux/poll.h>
 #include <linux/ns_common.h>
 #include <linux/fs_pin.h>
+#include <linux/watch_queue.h>
 
 struct mnt_namespace {
 	atomic_t		count;

@@ -70,9 +71,13 @@ struct mount {
 	int mnt_id;			/* mount identifier */
 	int mnt_group_id;		/* peer group identifier */
 	int mnt_expiry_mark;		/* true if marked for expiry */
+	int mnt_nr_watchers;		/* The number of subtree watches tracking this */
 	struct hlist_head mnt_pins;
 	struct hlist_head mnt_stuck_children;
 	atomic_t mnt_change_counter;	/* Number of changed applied */
+#ifdef CONFIG_MOUNT_NOTIFICATIONS
+	struct watch_list *mnt_watchers; /* Watches on dentries within this mount */
+#endif
 } __randomize_layout;
 
 #define MNT_NS_INTERNAL ERR_PTR(-EINVAL) /* distinct from any mnt_namespace */

@@ -155,18 +160,8 @@ static inline bool is_anon_ns(struct mnt_namespace *ns)
 	return ns->seq == 0;
 }
 
-/*
- * Type of mount topology change notification.
- */
-enum mount_notification_subtype {
-	NOTIFY_MOUNT_NEW_MOUNT	= 0, /* New mount added */
-	NOTIFY_MOUNT_UNMOUNT	= 1, /* Mount removed manually */
-	NOTIFY_MOUNT_EXPIRY	= 2, /* Automount expired */
-	NOTIFY_MOUNT_READONLY	= 3, /* Mount R/O state changed */
-	NOTIFY_MOUNT_SETATTR	= 4, /* Mount attributes changed */
-	NOTIFY_MOUNT_MOVE_FROM	= 5, /* Mount moved from here */
-	NOTIFY_MOUNT_MOVE_TO	= 6, /* Mount moved to here (compare op_id) */
-};
+extern void post_mount_notification(struct mount *changed,
+				    struct mount_notification *notify);
 
 static inline void notify_mount(struct mount *changed,
 				struct mount *aux,

@@ -174,4 +169,18 @@ static inline void notify_mount(struct mount *changed,
 				u32 info_flags)
 {
 	atomic_inc(&changed->mnt_change_counter);
+
+#ifdef CONFIG_MOUNT_NOTIFICATIONS
+	{
+		struct mount_notification n = {
+			.watch.type	= WATCH_TYPE_MOUNT_NOTIFY,
+			.watch.subtype	= subtype,
+			.watch.info	= info_flags | watch_sizeof(n),
+			.triggered_on	= changed->mnt_id,
+			.changed_mount	= aux ? aux->mnt_id : 0,
+		};
+
+		post_mount_notification(changed, &n);
+	}
+#endif
 }

@@ -0,0 +1,188 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Provide mount topology/attribute change notifications.
+ *
+ * Copyright (C) 2019 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/fs.h>
+#include <linux/namei.h>
+#include <linux/syscalls.h>
+#include <linux/slab.h>
+#include <linux/security.h>
+#include "mount.h"
+
+/*
+ * Post mount notifications to all watches going rootwards along the tree.
+ *
+ * Must be called with the mount_lock held.
+ */
+void post_mount_notification(struct mount *changed,
+			     struct mount_notification *notify)
+{
+	const struct cred *cred = current_cred();
+	struct path cursor;
+	struct mount *mnt;
+	unsigned seq;
+
+	seq = 0;
+	rcu_read_lock();
+restart:
+	cursor.mnt = &changed->mnt;
+	cursor.dentry = changed->mnt.mnt_root;
+	mnt = real_mount(cursor.mnt);
+	notify->watch.info &= ~NOTIFY_MOUNT_IN_SUBTREE;
+
+	read_seqbegin_or_lock(&rename_lock, &seq);
+	for (;;) {
+		if (mnt->mnt_watchers &&
+		    !hlist_empty(&mnt->mnt_watchers->watchers)) {
+			if (cursor.dentry->d_flags & DCACHE_MOUNT_WATCH)
+				post_watch_notification(mnt->mnt_watchers,
+							&notify->watch, cred,
+							(unsigned long)cursor.dentry);
+		} else {
+			cursor.dentry = mnt->mnt.mnt_root;
+		}
+		notify->watch.info |= NOTIFY_MOUNT_IN_SUBTREE;
+
+		if (cursor.dentry == cursor.mnt->mnt_root ||
+		    IS_ROOT(cursor.dentry)) {
+			struct mount *parent = READ_ONCE(mnt->mnt_parent);
+
+			/* Escaped? */
+			if (cursor.dentry != cursor.mnt->mnt_root)
+				break;
+
+			/* Global root? */
+			if (mnt == parent)
+				break;
+
+			cursor.dentry = READ_ONCE(mnt->mnt_mountpoint);
+			mnt = parent;
+			cursor.mnt = &mnt->mnt;
+		} else {
+			cursor.dentry = cursor.dentry->d_parent;
+		}
+	}
+
+	if (need_seqretry(&rename_lock, seq)) {
+		seq = 1;
+		goto restart;
+	}
+
+	done_seqretry(&rename_lock, seq);
+	rcu_read_unlock();
+}
+
+static void release_mount_watch(struct watch *watch)
+{
+	struct dentry *dentry = (struct dentry *)(unsigned long)watch->id;
+
+	dput(dentry);
+}
+
+/**
+ * sys_watch_mount - Watch for mount topology/attribute changes
+ * @dfd: Base directory to pathwalk from or fd referring to mount.
+ * @filename: Path to mount to place the watch upon
+ * @at_flags: Pathwalk control flags
+ * @watch_fd: The watch queue to send notifications to.
+ * @watch_id: The watch ID to be placed in the notification (-1 to remove watch)
+ */
+SYSCALL_DEFINE5(watch_mount,
+		int, dfd,
+		const char __user *, filename,
+		unsigned int, at_flags,
+		int, watch_fd,
+		int, watch_id)
+{
+	struct watch_queue *wqueue;
+	struct watch_list *wlist = NULL;
+	struct watch *watch = NULL;
+	struct mount *m;
+	struct path path;
+	unsigned int lookup_flags =
+		LOOKUP_DIRECTORY | LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT;
+	int ret;
+
+	if (watch_id < -1 || watch_id > 0xff)
+		return -EINVAL;
+	if ((at_flags & ~(AT_NO_AUTOMOUNT | AT_EMPTY_PATH)) != 0)
+		return -EINVAL;
+	if (at_flags & AT_NO_AUTOMOUNT)
+		lookup_flags &= ~LOOKUP_AUTOMOUNT;
+	if (at_flags & AT_EMPTY_PATH)
+		lookup_flags |= LOOKUP_EMPTY;
+
+	ret = user_path_at(dfd, filename, lookup_flags, &path);
+	if (ret)
+		return ret;
+
+	ret = inode_permission(path.dentry->d_inode, MAY_EXEC);
+	if (ret)
+		goto err_path;
+
+	wqueue = get_watch_queue(watch_fd);
+	if (IS_ERR(wqueue))
+		goto err_path;
+
+	m = real_mount(path.mnt);
+
+	if (watch_id >= 0) {
+		ret = -ENOMEM;
+		if (!m->mnt_watchers) {
+			wlist = kzalloc(sizeof(*wlist), GFP_KERNEL);
+			if (!wlist)
+				goto err_wqueue;
+			init_watch_list(wlist, release_mount_watch);
+		}
+
+		watch = kzalloc(sizeof(*watch), GFP_KERNEL);
+		if (!watch)
+			goto err_wlist;
+
+		init_watch(watch, wqueue);
+		watch->id		= (unsigned long)path.dentry;
+		watch->info_id		= (u32)watch_id << 24;
+
+		ret = security_watch_mount(watch, &path);
+		if (ret < 0)
+			goto err_watch;
+
+		down_write(&m->mnt.mnt_sb->s_umount);
+		if (!m->mnt_watchers) {
+			m->mnt_watchers = wlist;
+			wlist = NULL;
+		}
+
+		ret = add_watch_to_object(watch, m->mnt_watchers);
+		if (ret == 0) {
+			spin_lock(&path.dentry->d_lock);
+			path.dentry->d_flags |= DCACHE_MOUNT_WATCH;
+			spin_unlock(&path.dentry->d_lock);
+			dget(path.dentry);
+			watch = NULL;
+		}
+		up_write(&m->mnt.mnt_sb->s_umount);
+	} else {
+		ret = -EBADSLT;
+		if (m->mnt_watchers) {
+			down_write(&m->mnt.mnt_sb->s_umount);
+			ret = remove_watch_from_object(m->mnt_watchers, wqueue,
+						       (unsigned long)path.dentry,
+						       false);
+			up_write(&m->mnt.mnt_sb->s_umount);
+		}
+	}
+
+err_watch:
+	kfree(watch);
+err_wlist:
+	kfree(wlist);
+err_wqueue:
+	put_watch_queue(wqueue);
+err_path:
+	path_put(&path);
+	return ret;
+}

@@ -510,7 +510,8 @@ static int mnt_make_readonly(struct mount *mnt)
 	mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
 	unlock_mount_hash();
 	if (ret == 0)
-		notify_mount(mnt, NULL, NOTIFY_MOUNT_READONLY, 0x10000);
+		notify_mount(mnt, NULL, NOTIFY_MOUNT_READONLY,
+			     NOTIFY_MOUNT_IS_NOW_RO);
 	return ret;
 }
 

@@ -1176,6 +1177,11 @@ static void mntput_no_expire(struct mount *mnt)
 	mnt->mnt.mnt_flags |= MNT_DOOMED;
 	rcu_read_unlock();
 
+#ifdef CONFIG_MOUNT_NOTIFICATIONS
+	if (mnt->mnt_watchers)
+		remove_watch_list(mnt->mnt_watchers, mnt->mnt_id);
+#endif
+
 	list_del(&mnt->mnt_instance);
 
 	if (unlikely(!list_empty(&mnt->mnt_mounts))) {

@@ -2108,7 +2114,11 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 			list_del_init(&source_mnt->mnt_ns->list);
 		}
 		mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt);
-		notify_mount(dest_mnt, source_mnt, NOTIFY_MOUNT_NEW_MOUNT, 0);
+		notify_mount(dest_mnt, source_mnt, NOTIFY_MOUNT_NEW_MOUNT,
+			     (source_mnt->mnt.mnt_sb->s_flags & SB_RDONLY ?
+			      NOTIFY_MOUNT_IS_NOW_RO : 0) |
+			     (source_mnt->mnt.mnt_sb->s_flags & SB_SUBMOUNT ?
+			      NOTIFY_MOUNT_IS_SUBMOUNT : 0));
 		commit_tree(source_mnt);
 	}
 

@@ -2486,7 +2496,8 @@ static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags)
 	mnt->mnt.mnt_flags = mnt_flags;
 	touch_mnt_namespace(mnt->mnt_ns);
 	unlock_mount_hash();
-	notify_mount(mnt, NULL, NOTIFY_MOUNT_SETATTR, 0);
+	notify_mount(mnt, NULL, NOTIFY_MOUNT_SETATTR,
+		     (mnt_flags & SB_RDONLY ? NOTIFY_MOUNT_IS_NOW_RO : 0));
 }
 
 static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *mnt)

@@ -217,6 +217,7 @@ struct dentry_operations {
 #define DCACHE_PAR_LOOKUP		0x10000000 /* being looked up (with parent locked shared) */
 #define DCACHE_DENTRY_CURSOR		0x20000000
 #define DCACHE_NORCU			0x40000000 /* No RCU delay for freeing */
+#define DCACHE_MOUNT_WATCH		0x80000000 /* There's a mount watch here */
 
 extern seqlock_t rename_lock;
 

@@ -1007,6 +1007,8 @@ asmlinkage long sys_pidfd_getfd(int pidfd, int fd, unsigned int flags);
 asmlinkage long sys_fsinfo(int dfd, const char __user *pathname,
 			   struct fsinfo_params __user *params,
 			   void __user *buffer, size_t buf_size);
+asmlinkage long sys_watch_mount(int dfd, const char __user *path,
+				unsigned int at_flags, int watch_fd, int watch_id);
 
 /*
  * Architecture-specific system calls

@@ -857,9 +857,11 @@ __SYSCALL(__NR_openat2, sys_openat2)
 __SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd)
 #define __NR_fsinfo 439
 __SYSCALL(__NR_fsinfo, sys_fsinfo)
+#define __NR_watch_mount 440
+__SYSCALL(__NR_watch_mount, sys_watch_mount)
 
 #undef __NR_syscalls
-#define __NR_syscalls 440
+#define __NR_syscalls 441
 
 /*
  * 32 bit systems traditionally used different

@@ -14,7 +14,8 @@
 enum watch_notification_type {
 	WATCH_TYPE_META		= 0,	/* Special record */
 	WATCH_TYPE_KEY_NOTIFY	= 1,	/* Key change event notification */
-	WATCH_TYPE__NR		= 2
+	WATCH_TYPE_MOUNT_NOTIFY	= 2,	/* Mount topology change notification */
+	WATCH_TYPE___NR		= 3
 };
 
 enum watch_meta_notification_subtype {

@@ -101,4 +102,33 @@ struct key_notification {
 	__u32	aux;		/* Per-type auxiliary data */
 };
 
+/*
+ * Type of mount topology change notification.
+ */
+enum mount_notification_subtype {
+	NOTIFY_MOUNT_NEW_MOUNT	= 0, /* New mount added */
+	NOTIFY_MOUNT_UNMOUNT	= 1, /* Mount removed manually */
+	NOTIFY_MOUNT_EXPIRY	= 2, /* Automount expired */
+	NOTIFY_MOUNT_READONLY	= 3, /* Mount R/O state changed */
+	NOTIFY_MOUNT_SETATTR	= 4, /* Mount attributes changed */
+	NOTIFY_MOUNT_MOVE_FROM	= 5, /* Mount moved from here */
+	NOTIFY_MOUNT_MOVE_TO	= 6, /* Mount moved to here (compare op_id) */
+};
+
+#define NOTIFY_MOUNT_IN_SUBTREE		WATCH_INFO_FLAG_0 /* Event not actually at watched dentry */
+#define NOTIFY_MOUNT_IS_RECURSIVE	WATCH_INFO_FLAG_1 /* Change applied recursively */
+#define NOTIFY_MOUNT_IS_NOW_RO		WATCH_INFO_FLAG_2 /* Mount changed to R/O */
+#define NOTIFY_MOUNT_IS_SUBMOUNT	WATCH_INFO_FLAG_3 /* New mount is submount */
+
+/*
+ * Mount topology/configuration change notification record.
+ * - watch.type = WATCH_TYPE_MOUNT_NOTIFY
+ * - watch.subtype = enum mount_notification_subtype
+ */
+struct mount_notification {
+	struct watch_notification watch; /* WATCH_TYPE_MOUNT_NOTIFY */
+	__u32	triggered_on;		/* The mount that the notify was on */
+	__u32	changed_mount;		/* The mount that got changed */
+};
+
 #endif /* _UAPI_LINUX_WATCH_QUEUE_H */

@@ -52,6 +52,7 @@ COND_SYSCALL(io_uring_setup);
 COND_SYSCALL(io_uring_enter);
 COND_SYSCALL(io_uring_register);
 COND_SYSCALL(fsinfo);
+COND_SYSCALL(watch_mount);
 
 /* fs/xattr.c */