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 18:50:41 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     Jan Kara <jack@...e.cz>, 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 6:14 PM Gabriel Krisman Bertazi
<krisman@...labora.com> wrote:
>
> Jan Kara <jack@...e.cz> writes:
>
> > 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?
>
> Jan,
>
> Looking back at the email from Amir, I realize I misunderstood his
> original suggestion.  Amir suggested it be FILEID_INVALID with 0-len OR
> FILEID_INO32_GEN with zeroed fields.  I mixed the two suggestions.
>

That was from a discussion about UAPI and agree it doesn't make much sense
to report non-zero handle_bytes and invalid handle_type.

But specifically, Jan's question above was directly referring to the internal
representation of the event.

Since you are going to allocate a file handle buffer of size MAX_HANDLE_SZ
(right?), then there is no caveat in declaring the NULL_FH with positive size
internally, which I think, simplifies the implementation.
NULL_FH internal length could be 4 instead of 8 though.

You will not have to special case fanotify_fid_info_len() and the info record
hdr.len will include a 4 bytes zero padding after the fid info record
with NULL_FH.

You will only special case this line in copy_fid_info_to_user():
    handle.handle_type = fh->type;
    if (fh->type != FILEID_INVALID)
        handle.handle_bytes = fh_len;

Thanks,
Amir.