linux-kernel - Re: [git pull] vfs.git pile 2

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Message-ID: <20161218234031.GA7311@birch.djwong.org>
Date:   Sun, 18 Dec 2016 15:40:31 -0800
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [git pull] vfs.git pile 2

On Sun, Dec 18, 2016 at 04:06:06AM +0000, Al Viro wrote:
> On Sat, Dec 17, 2016 at 07:34:45PM -0800, Linus Torvalds wrote:
> > > What else am I missing there?
> > 
> > I absolutely *abhor* this part:
> > 
> >                 *len = isize - pos_in;
> > 
> > because the whole code then depends on the overflow checking a few
> > lines down, and it's not at all obvious. We have not tested that
> > "pos_in" is smaller than "isize", even though the comment above the
> > "isize == 0" test inplies we did some kind of "past the end check" (we
> > did not).
> > 
> > The whole "depend on overflow checking" being nasty is particularly
> > true when that checking itself is damn subtle, and depends deeply on
> > the type of "*len" being unsigned and larger than "loff_t". Which in
> > turn is true, but it's all really nasty, and it's subtle. "loff_t" is
> > "long long", while "*len" is u64, and it's almost just luck that the
> > comparison does in fact end up unsigned.
> 
> I agree, but that one is a straight move - exact same thing is there in
> xfs_reflink.c counterpart in the current mainline.

Ok, fair enough.  I thought it was ok but then I've spent so much time
staring at the reflink code it's good to have a fresh set of eyes. :)
I'll add a if (pos_in > isize) return -EINVAL there to make it more
explicit.

> > So I think that code really needs a fair amount of loving.
> 
> Indeed.  Darrick, would you add a followup cleaning that up?  It can be
> done after the move to fs/read_write.c - no need to reorder/rebase that
> thing.  While we are at it, it might be better to turn the return value
> into -E.../0/1, 0 being "no error, but nothing to do" and 1 - the normal
> success case.  That would get rid of using *len = 0 as signalling mechanism -
> the caller would simply do
> 	ret = vfs_..._inodes(.....);
> 	if (ret <= 0)
> 		goto out_unlock;
> 	/* returned positive, we have work to do */

Sounds good.  I'll post a cleanup patch once it goes through the
xfstests wringer.

--D