Re: commit 0790303ec869 leads to cpu stall without CONFIG_FANOTIFY_ACCESS_PERMISSIONS=y

Jan Kara
2024-12-09T12:26:55Z

On Mon 09-12-24 13:11:04, Jan Kara wrote:
> > Then I took a closer look at the function called in the problematic code
> > and noticed that fsnotify_file_area_perm(), is a NOOP when
> > CONFIG_FANOTIFY_ACCESS_PERMISSIONS is not set (which was the case in my
> > .config). This also explains why this was not found before, as
> > distributional .config file have this option enabled. Setting the option
> > to y solves the issue, too
>
> Well, I agree with you on all the points but the real question is, how come
> the test FMODE_FSNOTIFY_HSM(file->f_mode) was true on our kernel when you
> clearly don't run HSM software, even more so with
> CONFIG_FANOTIFY_ACCESS_PERMISSIONS disabled. That's the real cause of this
> problem. Something fishy is going on here... checking...
>
> Ah, because I've botched out file_set_fsnotify_mode() in case
> CONFIG_FANOTIFY_ACCESS_PERMISSIONS is disabled. This should fix the
> problem:
>
> index 1a9ef8f6784d..778a88fcfddc 100644
> --- a/include/linux/fsnotify.h
> +++ b/include/linux/fsnotify.h
> @@ -215,6 +215,7 @@ static inline int fsnotify_open_perm(struct file *file)
> #else
> static inline void file_set_fsnotify_mode(struct file *file)
> {
> + file->f_mode |= FMODE_NONOTIFY_PERM;
> }
>
> I'm going to test this with CONFIG_FANOTIFY_ACCESS_PERMISSIONS disabled and
> push out a fixed version. Thanks again for the report and analysis!

So this was not enough, What we need is:
index 1a9ef8f6784d..778a88fcfddc 100644
--- a/include/linux/fsnotify.h
+++ b/include/linux/fsnotify.h
@@ -215,6 +215,10 @@ static inline int fsnotify_open_perm(struct file *file)
#else
static inline void file_set_fsnotify_mode(struct file *file)
{
+ /* Is it a file opened by fanotify? */
+ if (FMODE_FSNOTIFY_NONE(file->f_mode))
+ return;
+ file->f_mode |= FMODE_NONOTIFY_PERM;
}

This passes testing for me so I've pushed it out and the next linux-next
build should have this fix.