Re: [git pull] vfs.git pile 2

Darrick J. Wong
2016-12-18T23:40:41Z

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