[PATCH 6.1.y 1/3] fsnotify: opt-in for permission events at file open time

From: Amir Goldstein <[email protected]>

[ Upstream commit a94204f4d48e28a711b7ed10399f749286c433e3 ]

Legacy inotify/fanotify listeners can add watches for events on inode,
parent or mount and expect to get events (e.g. FS_MODIFY) on files that
were already open at the time of setting up the watches.

fanotify permission events are typically used by Anti-malware sofware,
that is watching the entire mount and it is not common to have more that
one Anti-malware engine installed on a system.

To reduce the overhead of the fsnotify_file_perm() hooks on every file
access, relax the semantics of the legacy FAN_ACCESS_PERM event to generate
events only if there were *any* permission event listeners on the
filesystem at the time that the file was opened.

The new semantic is implemented by extending the FMODE_NONOTIFY bit into
two FMODE_NONOTIFY_* bits, that are used to store a mode for which of the
events types to report.

This is going to apply to the new fanotify pre-content events in order
to reduce the cost of the new pre-content event vfs hooks.

[Thanks to Bert Karwatzki <[email protected]> for reporting a bug in this
code with CONFIG_FANOTIFY_ACCESS_PERMISSIONS disabled]

Suggested-by: Linus Torvalds <[email protected]>
Link: https://lore.kernel.org/linux-fsdevel/CAHk-=wj8L=mtcRTi=NECHMGfZQgXOp_uix1YVh04fEmrKaMnXA@mail.gmail.com/
Signed-off-by: Amir Goldstein <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Link: https://patch.msgid.link/5ea5f8e283d1edb55aa79c35187bfe344056af14.1731684329.git.josef@toxicpanda.com
Stable-dep-of: db1856ea9196 ("binfmt_misc: restore write access when removing an entry")
Signed-off-by: Sasha Levin <[email protected]>
---
 include/linux/fs.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 02c83cd07d4f2..c1bd804b86153 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -184,6 +184,12 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 /* File supports async nowait buffered writes */
 #define FMODE_BUF_WASYNC	((__force fmode_t)0x80000000)
 
+/*
+ * fsnotify pre-content events do not exist in this kernel, so a file is never
+ * watched by a pre-content event listener.
+ */
+#define FMODE_FSNOTIFY_HSM(mode)	0
+
 /*
  * Attribute flags.  These should be or-ed together to figure out what
  * has been changed!
-- 
2.53.0