Re: [PATCH v7 05/18] fsnotify: introduce pre-content permission events

[Al Viro <viro@zeniv.linux.org.uk>]

On Tue, Nov 12, 2024 at 04:38:42PM -0800, Linus Torvalds wrote:
> Looking at that locking code in fadvise() just for the f_mode use does
> make me think this would be a really good cleanup.
> 
> I note that our fcntl code seems buggy as-is, because while it does
> use f_lock for assignments (good), it clearly does *not* use them for
> reading.
> 
> So it looks like you can actually read inconsistent values.
> 
> I get the feeling that f_flags would want WRITE_ONCE/READ_ONCE in
> _addition_ to the f_lock use it has.

AFAICS, fasync logics is the fishy part - the rest should be sane.

> The f_mode thing with fadvise() smells like the same bug. Just because
> the modifications are serialized wrt each other doesn't mean that
> readers are then automatically ok.

Reads are also under ->f_lock in there, AFAICS...

Another thing in the vicinity is ->f_mode modifications after the calls
of anon_inode_getfile() in several callers - probably ought to switch
those to anon_inode_getfile_fmode().  That had been discussed back in
April when the function got merged, but "convert to using it" followup
series hadn't materialized...