fsnotify: introduce pre-content permission events

The new FS_PRE_ACCESS permission event is similar to FS_ACCESS_PERM,
but it meant for a different use case of filling file content before
access to a file range, so it has slightly different semantics.

Generate FS_PRE_ACCESS/FS_ACCESS_PERM as two seperate events, so content
scanners could inspect the content filled by pre-content event handler.

Unlike FS_ACCESS_PERM, FS_PRE_ACCESS is also called before a file is
modified by syscalls as write() and fallocate().

FS_ACCESS_PERM is reported also on blockdev and pipes, but the new
pre-content events are only reported for regular files and dirs.

The pre-content events are meant to be used by hierarchical storage
managers that want to fill the content of files on first access.

There are some specific requirements from filesystems that could
be used with pre-content events, so add a flag for fs to opt-in
for pre-content events explicitly before they can be used.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/b934c5e3af205abc4e0e4709f6486815937ddfdf.1731684329.git.josef@toxicpanda.com

1 files changed, 18 insertions, 1 deletions

diff --git a/include/linux/fsnotify.h b/include/linux/fsnotify.h
index 8d1849137a96..d91aa064f0e4 100644
--- a/include/linux/fsnotify.h
+++ b/include/linux/fsnotify.h
@@ -144,12 +144,29 @@ static inline int fsnotify_file_area_perm(struct file *file, int perm_mask,
 	 */
 	lockdep_assert_once(file_write_not_started(file));
 
-	if (!(perm_mask & MAY_READ))
+	if (!(perm_mask & (MAY_READ | MAY_WRITE | MAY_ACCESS)))
 		return 0;
 
 	if (likely(!FMODE_FSNOTIFY_PERM(file->f_mode)))
 		return 0;
 
+	/*
+	 * read()/write() and other types of access generate pre-content events.
+	 */
+	if (unlikely(FMODE_FSNOTIFY_HSM(file->f_mode))) {
+		int ret = fsnotify_path(&file->f_path, FS_PRE_ACCESS);
+
+		if (ret)
+			return ret;
+	}
+
+	if (!(perm_mask & MAY_READ))
+		return 0;
+
+	/*
+	 * read() also generates the legacy FS_ACCESS_PERM event, so content
+	 * scanners can inspect the content filled by pre-content event.
+	 */
 	return fsnotify_path(&file->f_path, FS_ACCESS_PERM);
 }