[PATCH v5 04/18] fanotify: introduce FAN_PRE_ACCESS permission event

Josef Bacik
2024-09-04T20:29:21Z

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>
Reviewed-by: Christian Brauner <brauner@kernel.org>
---
fs/notify/fanotify/fanotify.c | 3 ++-
fs/notify/fanotify/fanotify_user.c | 17 ++++++++++++++---
include/linux/fanotify.h | 14 ++++++++++----
include/uapi/linux/fanotify.h | 2 ++
4 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/fs/notify/fanotify/fanotify.c b/fs/notify/fanotify/fanotify.c
index 224bccaab4cc..7dac8e4486df 100644
--- a/fs/notify/fanotify/fanotify.c
+++ b/fs/notify/fanotify/fanotify.c
@@ -910,8 +910,9 @@ 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 2e2fba8a9d20..c294849e474f 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -1628,6 +1628,7 @@ 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) ||
@@ -1665,9 +1666,15 @@ 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;

+ /* Pre-content events are only supported on regular files and dirs */
+ if (mask & FANOTIFY_PRE_CONTENT_EVENTS) {
+ if (!is_dir && !d_is_reg(path->dentry))
+ return -EINVAL;
+ }
+
return 0;
}

@@ -1769,11 +1776,15 @@ 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 4f1c4f603118..5c811baf44d2 100644
--- a/include/linux/fanotify.h
+++ b/include/linux/fanotify.h
@@ -88,6 +88,16 @@
#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)

@@ -103,10 +113,6 @@
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 a37de58ca571..bcada21a3a2e 100644
--- a/include/uapi/linux/fanotify.h
+++ b/include/uapi/linux/fanotify.h
@@ -26,6 +26,8 @@
#define FAN_ACCESS_PERM 0x00020000 /* File accessed in perm check */
#define FAN_OPEN_EXEC_PERM 0x00040000 /* File open/exec in perm check */

+#define FAN_PRE_ACCESS 0x00080000 /* Pre-content access hook */
+
#define FAN_EVENT_ON_CHILD 0x08000000 /* Interested in child events */

#define FAN_RENAME 0x10000000 /* File was renamed */
--
2.43.0