linux-kernel - Re: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead of 0 as initial sum

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Date:   Wed, 22 Jul 2020 15:42:13 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     David Laight <David.Laight@...lab.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [PATCH 04/18] csum_and_copy_..._user(): pass 0xffffffff instead
 of 0 as initial sum

On Wed, Jul 22, 2020 at 09:27:32AM +0000, David Laight wrote:
> From: Al Viro
> > Sent: 21 July 2020 21:26
> > Preparation for the change of calling conventions; right now all
> > callers pass 0 as initial sum.  Passing 0xffffffff instead yields
> > the values comparable mod 0xffff and guarantees that 0 will not
> > be returned on success.
> > 
> > Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
> > ---
> >  lib/iov_iter.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> > index 7405922caaec..d5b7e204fea6 100644
> > --- a/lib/iov_iter.c
> > +++ b/lib/iov_iter.c
> > @@ -1451,7 +1451,7 @@ size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum,
> >  		int err = 0;
> >  		next = csum_and_copy_from_user(v.iov_base,
> >  					       (to += v.iov_len) - v.iov_len,
> > -					       v.iov_len, 0, &err);
> > +					       v.iov_len, ~0U, &err);
> >  		if (!err) {
> >  			sum = csum_block_add(sum, next, off);
> >  			off += v.iov_len;
> 
> Can't you remove the csum_block_add() by passing the
> old 'sum' in instead of the ~0U ?
> You'll need to keep track of whether the buffer fragment
> is odd/even aligned.
> After an odd length fragment a bswap32() or 8 bit rotate will
> fix things (and maybe one right at the end).

And the benefit of that would be...?  It wouldn't be any simpler,
it almost certainly would not even be a valid microoptimization
(nevermind that this is an arch-independent code)...