linux-ext4 - Re: [PATCH v6 12/21] fanotify: Encode invalid file handle when no inode is provided

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Date:   Mon, 16 Aug 2021 18:11:43 +0200
From:   Jan Kara <jack@...e.cz>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Jan Kara <jack@...e.cz>,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        Jan Kara <jack@...e.com>,
        Linux API <linux-api@...r.kernel.org>,
        Ext4 <linux-ext4@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Khazhismel Kumykov <khazhy@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Dave Chinner <david@...morbit.com>,
        Theodore Tso <tytso@....edu>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Matthew Bobrowski <repnop@...gle.com>, kernel@...labora.com
Subject: Re: [PATCH v6 12/21] fanotify: Encode invalid file handle when no
 inode is provided

On Mon 16-08-21 18:54:58, Amir Goldstein wrote:
> On Mon, Aug 16, 2021 at 5:07 PM Jan Kara <jack@...e.cz> wrote:
> > Dunno, it still seems like quite some complications (simple ones but
> > non-trivial amount of them) for what is rather a corner case. What if we
> > *internally* propagated the information that there's no inode info with
> > FILEID_ROOT fh? That means: No changes to fanotify_encode_fh_len(),
> > fanotify_encode_fh(), or fanotify_alloc_name_event(). In
> > copy_info_to_user() we just mangle FILEID_ROOT to FILEID_INVALID and that's
> > all. No useless padding, no specialcasing of copying etc. Am I missing
> > something?
> 
> I am perfectly fine with encoding "no inode" with FILEID_ROOT internally.
> It's already the value used by fanotify_encode_fh() in upstream.
> 
> However, if we use zero len internally, we need to pass fh_type to
> fanotify_fid_info_len() and special case FILEID_ROOT in order to
> take FANOTIFY_FID_INFO_HDR_LEN into account.
> 
> And special case fanotify_event_object_fh_len() in
>  fanotify_event_info_len() and in copy_info_records_to_user().

Right, this will need some tweaking. I would actually leave
fanotify_fid_info_len() alone, just have in fanotify_event_info_len()
something like:

-	if (fh_len)
+	if (fh_len || fanotify_event_needs_fsid(event))

and similarly in copy_info_records_to_user():

-	if (fanotify_event_object_fh_len(event)) {
+	if (fanotify_event_object_fh_len(event) ||
+	    fanotify_event_needs_fsid(event)) {

And that should be all that's needed as far as I'm reading the code.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR