On Wed 13-11-24 19:49:31, Amir Goldstein wrote: > From 7a2cd74654a53684d545b96c57c9091e420b3add Mon Sep 17 00:00:00 2001 > From: Amir Goldstein <amir73il@xxxxxxxxx> > Date: Tue, 12 Nov 2024 13:46:08 +0100 > Subject: [PATCH] fsnotify: opt-in for permission events at file open time > > 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. > > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Link: https://lore.kernel.org/linux-fsdevel/CAHk-=wj8L=mtcRTi=NECHMGfZQgXOp_uix1YVh04fEmrKaMnXA@xxxxxxxxxxxxxx/ > Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx> Couple of notes below. > diff --git a/fs/open.c b/fs/open.c > index 226aca8c7909..194c2c8d8cd4 100644 > --- a/fs/open.c > +++ b/fs/open.c > @@ -901,7 +901,7 @@ static int do_dentry_open(struct file *f, > f->f_sb_err = file_sample_sb_err(f); > > if (unlikely(f->f_flags & O_PATH)) { > - f->f_mode = FMODE_PATH | FMODE_OPENED; > + f->f_mode = FMODE_PATH | FMODE_OPENED | FMODE_NONOTIFY; > f->f_op = &empty_fops; > return 0; > } > @@ -929,6 +929,12 @@ static int do_dentry_open(struct file *f, > if (error) > goto cleanup_all; > > + /* > + * Set FMODE_NONOTIFY_* bits according to existing permission watches. > + * If FMODE_NONOTIFY was already set for an fanotify fd, this doesn't > + * change anything. > + */ > + f->f_mode |= fsnotify_file_mode(f); Maybe it would be obvious to do this like: file_set_fsnotify_mode(f); Because currently this depends on the details of how exactly FMODE_NONOTIFY is encoded. > diff --git a/include/linux/fs.h b/include/linux/fs.h > index 70359dd669ff..dd583ce7dba8 100644 > --- a/include/linux/fs.h > +++ b/include/linux/fs.h > @@ -173,13 +173,14 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset, > > #define FMODE_NOREUSE ((__force fmode_t)(1 << 23)) > > -/* FMODE_* bit 24 */ > - > /* File is embedded in backing_file object */ > -#define FMODE_BACKING ((__force fmode_t)(1 << 25)) > +#define FMODE_BACKING ((__force fmode_t)(1 << 24)) > + > +/* File shouldn't generate fanotify pre-content events */ > +#define FMODE_NONOTIFY_HSM ((__force fmode_t)(1 << 25)) > > -/* File was opened by fanotify and shouldn't generate fanotify events */ > -#define FMODE_NONOTIFY ((__force fmode_t)(1 << 26)) > +/* File shouldn't generate fanotify permission events */ > +#define FMODE_NONOTIFY_PERM ((__force fmode_t)(1 << 26)) > > /* File is capable of returning -EAGAIN if I/O will block */ > #define FMODE_NOWAIT ((__force fmode_t)(1 << 27)) > @@ -190,6 +191,21 @@ typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset, > /* File does not contribute to nr_files count */ > #define FMODE_NOACCOUNT ((__force fmode_t)(1 << 29)) > > +/* > + * The two FMODE_NONOTIFY_ bits used together have a special meaning of > + * not reporting any events at all including non-permission events. > + * These are the possible values of FMODE_NOTIFY(f->f_mode) and their meaning: > + * > + * FMODE_NONOTIFY_HSM - suppress only pre-content events. > + * FMODE_NONOTIFY_PERM - suppress permission (incl. pre-content) events. > + * FMODE_NONOTIFY - suppress all (incl. non-permission) events. > + */ > +#define FMODE_NONOTIFY_MASK \ > + (FMODE_NONOTIFY_HSM | FMODE_NONOTIFY_PERM) > +#define FMODE_NONOTIFY FMODE_NONOTIFY_MASK > +#define FMODE_NOTIFY(mode) \ > + ((mode) & FMODE_NONOTIFY_MASK) This looks a bit error-prone to me (FMODE_NONOTIFY looks like another FMODE flag but in fact it is not which is an invitation for subtle bugs) and the tests below which are sometimes done as (FMODE_NOTIFY(mode) == xxx) and sometimes as (file->f_mode & xxx) are inconsistent and confusing (unless you understand what's happening under the hood). So how about defining macros like FMODE_FSNOTIFY_NORMAL(), FMODE_FSNOTIFY_CONTENT() and FMODE_FSNOTIFY_PRE_CONTENT() which evaluate to true if we should be sending normal/content/pre-content events to the file. With appropriate comments this should make things more obvious. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
