[PATCH 10/19] fanotify: introduce FAN_PRE_ACCESS permission event

[Jan Kara <jack@suse.cz>]

From: Amir Goldstein <amir73il@gmail.com>

Similar to FAN_ACCESS_PERM permission event, but it is only allowed with
class FAN_CLASS_PRE_CONTENT and only allowed on regular files and dirs.

Unlike FAN_ACCESS_PERM, it is safe to write to the file being accessed
in the context of the event handler.

This pre-content event is meant to be used by hierarchical storage
managers that want to fill the content of files on first read access.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/b80986f8d5b860acea2c9a73c0acd93587be5fe4.1731684329.git.josef@toxicpanda.com
---
 fs/notify/fanotify/fanotify.c      |  3 ++-
 fs/notify/fanotify/fanotify_user.c | 22 +++++++++++++++++++---
 include/linux/fanotify.h           | 14 ++++++++++----
 include/uapi/linux/fanotify.h      |  2 ++
 4 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index c1e4ae221093..5e05410ddb9f 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -917,8 +917,9 @@ static int fanotify_handle_event(struct fsnotify_group *group, u32 mask,
 	BUILD_BUG_ON(FAN_OPEN_EXEC_PERM != FS_OPEN_EXEC_PERM);
 	BUILD_BUG_ON(FAN_FS_ERROR != FS_ERROR);
 	BUILD_BUG_ON(FAN_RENAME != FS_RENAME);
+	BUILD_BUG_ON(FAN_PRE_ACCESS != FS_PRE_ACCESS);
 
-	BUILD_BUG_ON(HWEIGHT32(ALL_FANOTIFY_EVENT_BITS) != 21);
+	BUILD_BUG_ON(HWEIGHT32(ALL_FANOTIFY_EVENT_BITS) != 22);
 
 	mask = fanotify_group_event_mask(group, iter_info, &match_mask,
 					 mask, data, data_type, dir);
diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 8d8afa09560b..b4dfc8f0d037 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -1646,11 +1646,23 @@ static int fanotify_events_supported(struct fsnotify_group *group,
 				     unsigned int flags)
 {
 	unsigned int mark_type = flags & FANOTIFY_MARK_TYPE_BITS;
+	bool is_dir = d_is_dir(path->dentry);
 	/* Strict validation of events in non-dir inode mask with v5.17+ APIs */
 	bool strict_dir_events = FAN_GROUP_FLAG(group, FAN_REPORT_TARGET_FID) ||
 				 (mask & FAN_RENAME) ||
 				 (flags & FAN_MARK_IGNORE);
 
+	/*
+	 * Filesystems need to opt-into pre-content evnets (a.k.a HSM)
+	 * and they are only supported on regular files and directories.
+	 */
+	if (mask & FANOTIFY_PRE_CONTENT_EVENTS) {
+		if (!(path->mnt->mnt_sb->s_iflags & SB_I_ALLOW_HSM))
+			return -EOPNOTSUPP;
+		if (!is_dir && !d_is_reg(path->dentry))
+			return -EINVAL;
+	}
+
 	/*
 	 * Some filesystems such as 'proc' acquire unusual locks when opening
 	 * files. For them fanotify permission events have high chances of
@@ -1683,7 +1695,7 @@ static int fanotify_events_supported(struct fsnotify_group *group,
 	 * but because we always allowed it, error only when using new APIs.
 	 */
 	if (strict_dir_events && mark_type == FAN_MARK_INODE &&
-	    !d_is_dir(path->dentry) && (mask & FANOTIFY_DIRONLY_EVENT_BITS))
+	    !is_dir && (mask & FANOTIFY_DIRONLY_EVENT_BITS))
 		return -ENOTDIR;
 
 	return 0;
@@ -1787,11 +1799,15 @@ static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
 		goto fput_and_out;
 
 	/*
-	 * Permission events require minimum priority FAN_CLASS_CONTENT.
+	 * Permission events are not allowed for FAN_CLASS_NOTIF.
+	 * Pre-content permission events are not allowed for FAN_CLASS_CONTENT.
 	 */
 	ret = -EINVAL;
 	if (mask & FANOTIFY_PERM_EVENTS &&
-	    group->priority < FSNOTIFY_PRIO_CONTENT)
+	    group->priority == FSNOTIFY_PRIO_NORMAL)
+		goto fput_and_out;
+	else if (mask & FANOTIFY_PRE_CONTENT_EVENTS &&
+		 group->priority == FSNOTIFY_PRIO_CONTENT)
 		goto fput_and_out;
 
 	if (mask & FAN_FS_ERROR &&
diff --git a/include/linux/fanotify.h b/include/linux/fanotify.h
index 89ff45bd6f01..c747af064d2c 100644
--- a/include/linux/fanotify.h
+++ b/include/linux/fanotify.h
@@ -89,6 +89,16 @@
 #define FANOTIFY_DIRENT_EVENTS	(FAN_MOVE | FAN_CREATE | FAN_DELETE | \
 				 FAN_RENAME)
 
+/* Content events can be used to inspect file content */
+#define FANOTIFY_CONTENT_PERM_EVENTS (FAN_OPEN_PERM | FAN_OPEN_EXEC_PERM | \
+				      FAN_ACCESS_PERM)
+/* Pre-content events can be used to fill file content */
+#define FANOTIFY_PRE_CONTENT_EVENTS  (FAN_PRE_ACCESS)
+
+/* Events that require a permission response from user */
+#define FANOTIFY_PERM_EVENTS	(FANOTIFY_CONTENT_PERM_EVENTS | \
+				 FANOTIFY_PRE_CONTENT_EVENTS)
+
 /* Events that can be reported with event->fd */
 #define FANOTIFY_FD_EVENTS (FANOTIFY_PATH_EVENTS | FANOTIFY_PERM_EVENTS)
 
@@ -104,10 +114,6 @@
 				 FANOTIFY_INODE_EVENTS | \
 				 FANOTIFY_ERROR_EVENTS)
 
-/* Events that require a permission response from user */
-#define FANOTIFY_PERM_EVENTS	(FAN_OPEN_PERM | FAN_ACCESS_PERM | \
-				 FAN_OPEN_EXEC_PERM)
-
 /* Extra flags that may be reported with event or control handling of events */
 #define FANOTIFY_EVENT_FLAGS	(FAN_EVENT_ON_CHILD | FAN_ONDIR)
 
diff --git a/include/uapi/linux/fanotify.h b/include/uapi/linux/fanotify.h
index 79072b6894f2..7596168c80eb 100644
--- a/include/uapi/linux/fanotify.h
+++ b/include/uapi/linux/fanotify.h
@@ -27,6 +27,8 @@
 #define FAN_OPEN_EXEC_PERM	0x00040000	/* File open/exec in perm check */
 /* #define FAN_DIR_MODIFY	0x00080000 */	/* Deprecated (reserved) */
 
+#define FAN_PRE_ACCESS		0x00100000	/* Pre-content access hook */
+
 #define FAN_EVENT_ON_CHILD	0x08000000	/* Interested in child events */
 
 #define FAN_RENAME		0x10000000	/* File was renamed */
-- 
2.35.3