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

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Date:   Sun, 18 Dec 2016 04:06:06 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>
Subject: Re: [git pull] vfs.git pile 2

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.

> 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 */