On Thu, Aug 12, 2021 at 5:20 PM Jan Kara <jack@xxxxxxx> wrote:
>
> On Wed 11-08-21 17:12:05, Gabriel Krisman Bertazi wrote:
> > Jan Kara <jack@xxxxxxx> 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.
