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Message-ID: <CAOQ4uxjy2FOt6r5=x9FO3YXs8_FWwew055ZfrumDjSz0HCgz3w@mail.gmail.com> 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.
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