linux-kernel - Re: [man-pages RFC PATCH v4] statx, inode: document the new STATX_INO

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-id: <166259786233.30452.5417306132987966849@noble.neil.brown.name>
Date:   Thu, 08 Sep 2022 10:44:22 +1000
From:   "NeilBrown" <neilb@...e.de>
To:     "Jan Kara" <jack@...e.cz>
Cc:     "Jeff Layton" <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>, tytso@....edu,
        adilger.kernel@...ger.ca, djwong@...nel.org, david@...morbit.com,
        trondmy@...merspace.com, viro@...iv.linux.org.uk,
        zohar@...ux.ibm.com, xiubli@...hat.com, chuck.lever@...cle.com,
        lczerner@...hat.com, jack@...e.cz, brauner@...nel.org,
        fweimer@...hat.com, linux-man@...r.kernel.org,
        linux-api@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        ceph-devel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-nfs@...r.kernel.org, linux-xfs@...r.kernel.org
Subject: Re: [man-pages RFC PATCH v4] statx, inode: document the new
 STATX_INO_VERSION field

On Wed, 07 Sep 2022, Jan Kara wrote:
> On Wed 07-09-22 09:12:34, Jeff Layton wrote:
> > On Wed, 2022-09-07 at 08:52 -0400, J. Bruce Fields wrote:
> > > On Wed, Sep 07, 2022 at 08:47:20AM -0400, Jeff Layton wrote:
> > > > On Wed, 2022-09-07 at 21:37 +1000, NeilBrown wrote:
> > > > > On Wed, 07 Sep 2022, Jeff Layton wrote:
> > > > > > +The change to \fIstatx.stx_ino_version\fP is not atomic with respect to the
> > > > > > +other changes in the inode. On a write, for instance, the i_version it usually
> > > > > > +incremented before the data is copied into the pagecache. Therefore it is
> > > > > > +possible to see a new i_version value while a read still shows the old data.
> > > > > 
> > > > > Doesn't that make the value useless?
> > > > > 
> > > > 
> > > > No, I don't think so. It's only really useful for comparing to an older
> > > > sample anyway. If you do "statx; read; statx" and the value hasn't
> > > > changed, then you know that things are stable. 
> > > 
> > > I don't see how that helps.  It's still possible to get:
> > > 
> > > 		reader		writer
> > > 		------		------
> > > 				i_version++
> > > 		statx
> > > 		read
> > > 		statx
> > > 				update page cache
> > > 
> > > right?
> > > 
> > 
> > Yeah, I suppose so -- the statx wouldn't necessitate any locking. In
> > that case, maybe this is useless then other than for testing purposes
> > and userland NFS servers.
> > 
> > Would it be better to not consume a statx field with this if so? What
> > could we use as an alternate interface? ioctl? Some sort of global
> > virtual xattr? It does need to be something per-inode.
> 
> I was thinking how hard would it be to increment i_version after updating
> data but it will be rather hairy. In particular because of stuff like
> IOCB_NOWAIT support which needs to bail if i_version update is needed. So
> yeah, I don't think there's an easy way how to provide useful i_version for
> general purpose use.
> 

Why cannot IOCB_NOWAIT update i_version?  Do we not want to wait on the
cmp_xchg loop in inode_maybe_inc_iversion(), or do we not want to
trigger an inode update?

The first seems unlikely, but the second seems unreasonable.  We already
acknowledge that after a crash iversion might go backwards and/or miss
changes.

Thanks,
NeilBrown