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

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Date:   Thu, 12 Aug 2021 18:17:10 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Gabriel Krisman Bertazi <krisman@...labora.com>,
        Jan Kara <jack@...e.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Theodore Tso <tytso@....edu>,
        Dave Chinner <david@...morbit.com>,
        David Howells <dhowells@...hat.com>,
        Khazhismel Kumykov <khazhy@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Ext4 <linux-ext4@...r.kernel.org>,
        Linux API <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 Thu, Aug 12, 2021 at 5:20 PM Jan Kara <jack@...e.cz> wrote:
>
> 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?

With current code, zero fh->len means we are not reporting an FID info
record (e.g. due to encode error), see copy_info_records_to_user().

This is because fh->len plays a dual role for indicating the length of the
file handle and the existence of FID info.

I figured that keeping a positive length for the special NULL_FH is an
easy way to workaround this ambiguity and keep the code simpler.
We don't really need to pay any cost for keeping the 8 bytes zero buffer.

Thanks,
Amir.