linux-ext4 - Re: [PATCH v5 14/23] fanotify: Encode invalid file handler when no inode is provided

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Message-ID: <20210812142047.GG14675@quack2.suse.cz>
Date:   Thu, 12 Aug 2021 16:20:47 +0200
From:   Jan Kara <jack@...e.cz>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     Jan Kara <jack@...e.cz>, jack@...e.com, amir73il@...il.com,
        djwong@...nel.org, tytso@....edu, david@...morbit.com,
        dhowells@...hat.com, khazhy@...gle.com,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-api@...r.kernel.org, kernel@...labora.com
Subject: Re: [PATCH v5 14/23] fanotify: Encode invalid file handler when no
 inode is provided

On Wed 11-08-21 17:12:05, Gabriel Krisman Bertazi wrote:
> Jan Kara <jack@...e.cz> writes:
> >> @@ -376,14 +371,24 @@ static int fanotify_encode_fh(struct fanotify_fh *fh, struct inode *inode,
> >>  		fh->flags |= FANOTIFY_FH_FLAG_EXT_BUF;
> >>  	}
> >>  
> >> -	dwords = fh_len >> 2;
> >> -	type = exportfs_encode_inode_fh(inode, buf, &dwords, NULL);
> >> -	err = -EINVAL;
> >> -	if (!type || type == FILEID_INVALID || fh_len != dwords << 2)
> >> -		goto out_err;
> >> -
> >> -	fh->type = type;
> >> -	fh->len = fh_len;
> >> +	if (inode) {
> >> +		dwords = fh_len >> 2;
> >> +		type = exportfs_encode_inode_fh(inode, buf, &dwords, NULL);
> >> +		err = -EINVAL;
> >> +		if (!type || type == FILEID_INVALID || fh_len != dwords << 2)
> >> +			goto out_err;
> >> +		fh->type = type;
> >> +		fh->len = fh_len;
> >> +	} else {
> >> +		/*
> >> +		 * Invalid FHs are used on FAN_FS_ERROR for errors not
> >> +		 * linked to any inode. Caller needs to guarantee the fh
> >> +		 * has at least FANOTIFY_NULL_FH_LEN bytes of space.
> >> +		 */
> >> +		fh->type = FILEID_INVALID;
> >> +		fh->len = FANOTIFY_NULL_FH_LEN;
> >> +		memset(buf, 0, FANOTIFY_NULL_FH_LEN);
> >> +	}
> >
> > Maybe it will become clearer later during the series but why do you set
> > fh->len to FANOTIFY_NULL_FH_LEN and not 0?
> 
> Jan,
> 
> That is how we encode a NULL file handle (i.e. superblock error).  Amir
> suggested it would be an invalid FILEID_INVALID, with a zeroed handle of
> size 8.  I will improve the comment on the next iteration.

Thanks for info. Then I have a question for Amir I guess :) Amir, what's
the advantage of zeroed handle of size 8 instead of just 0 length file
handle?

								Honza

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