linux-ext4 - Re: [PATCH 06/31] e2p: Fix f[gs]etflags argument size mismatch

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Message-ID: <20131008002807.GH6860@birch.djwong.org>
Date:	Mon, 7 Oct 2013 17:28:07 -0700
From:	"Darrick J. Wong" <darrick.wong@...cle.com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 06/31] e2p: Fix f[gs]etflags argument size mismatch

On Mon, Oct 07, 2013 at 08:06:02PM -0400, Theodore Ts'o wrote:
> On Mon, Oct 07, 2013 at 04:23:58PM -0700, Darrick J. Wong wrote:
> > Yuck.  FUSE assumes an interface contract (the data size encoded in the ioctl
> > number) that neither userspace nor kernel actually abide.  This has gone on for
> > years with no problems, since both components made the same implicit assumption
> > about data size in the same way.  Unfortunately, userspace breaks only on FUSE,
> > so I don't know what to do.
> 
> I suspect we've never noticed because traditionally, FUSE has never
> been used to front-end a file system that supports chattr/lsattr ---
> most of thsoe file systems are available as native Linux file systems,
> so it's probably not a common use case for FUSE.
> 
> Can we make the FUSE ioctl handler in fs/fuse/ioctl.c special case
> handle the EXT2_IOC_[SG]ETFLAG ioctls.  That would it be consistent
> with the other file systms.

I can try changing fuse_do_ioctl() to add in the special case.  There don't
seem to be any definitions of _IOR('f', 1, long) that aren't *_IOC_GETFLAGS
(same for _IOW_('f', 2, long) for *_IOC_SETFLAGS) so a hack to bring FUSE in
line with xfs/btrfs/ext*/hfs+/reiser3/nilfs/gfs2/logfs/f2fs/ubifs/cifs might be
doable.

If not I suppose new ioctls are a slow-moving plan B.

--D
> 
> > Long term I guess we could define a new pair of ioctls that work with pointers
> > to 64-bit values and deprecate the old ones.  Or perhaps there's a better
> > suggestion than "don't run chattr/lsattr on a FUSE"?
> 
> Well we can create a new pair of ioctls, and then have the userspace
> code try the new ioctl, and if the kernel doesn't support it, try the
> new ioctl.  But then we would have to fix up all of the file systems
> in Linux, and it would take a while before users have a new kernel and
> a new userspace which supports the new ioctl.
> 
> If we put the hack in fs/fuse/file.c's ioctl handler, then it only
> requires a kernel upgrade....
> 
> 						- Ted
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