[PATCH v5 10/18] fs: add a flag to indicate the fs supports pre-content events

[Josef Bacik <josef@toxicpanda.com>]

The pre-content events require some extra thinking, especially around
page faults.  In order to make sure we don't advertise a feature working
that doesn't actually work, add a flag to allow file systems to opt-in
to this behavior.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 fs/notify/fanotify/fanotify_user.c | 2 ++
 include/linux/fs.h                 | 1 +
 include/linux/fsnotify.h           | 4 ++++
 3 files changed, 7 insertions(+)

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 53eee8af34a0..936e9f9e0cbc 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -1736,6 +1736,8 @@ static int fanotify_events_supported(struct fsnotify_group *group,
 
 	/* Pre-content events are only supported on regular files and dirs */
 	if (mask & FANOTIFY_PRE_CONTENT_EVENTS) {
+		if (!(path->mnt->mnt_sb->s_type->fs_flags & FS_ALLOW_HSM))
+			return -EINVAL;
 		if (!is_dir && !d_is_reg(path->dentry))
 			return -EINVAL;
 		if (is_dir && mask & FAN_PRE_MODIFY)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index fd34b5755c0b..5708e91d3625 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2494,6 +2494,7 @@ struct file_system_type {
 #define FS_USERNS_MOUNT		8	/* Can be mounted by userns root */
 #define FS_DISALLOW_NOTIFY_PERM	16	/* Disable fanotify permission events */
 #define FS_ALLOW_IDMAP         32      /* FS has been updated to handle vfs idmappings. */
+#define FS_ALLOW_HSM		64	/* FS can handle fanotify pre-content events. */
 #define FS_RENAME_DOES_D_MOVE	32768	/* FS will handle d_move() during rename() internally. */
 	int (*init_fs_context)(struct fs_context *);
 	const struct fs_parameter_spec *parameters;
diff --git a/include/linux/fsnotify.h b/include/linux/fsnotify.h
index 9d001d328619..27992b548f0c 100644
--- a/include/linux/fsnotify.h
+++ b/include/linux/fsnotify.h
@@ -179,6 +179,10 @@ static inline int fsnotify_file_area_perm(struct file *file, int perm_mask,
 	if (!S_ISDIR(inode->i_mode) && !S_ISREG(inode->i_mode))
 		return 0;
 
+	/* The fs doesn't support pre-content events. */
+	if (!(inode->i_sb->s_type->fs_flags & FS_ALLOW_HSM))
+		return 0;
+
 	if (perm_mask & MAY_WRITE)
 		fsnotify_mask = FS_PRE_MODIFY;
 	else if (perm_mask & (MAY_READ | MAY_ACCESS))
-- 
2.43.0